LTC2986/LTC2986-1
1
29861fa
For more information www.linear.com/LTC2986
TYPICAL APPLICATION
FEATURES DESCRIPTION
Multi-Sensor High
Accuracy Digital Temperature
Measurement System with EEPROM
The LTC
®
2986 measures a wide variety of temperature
sensors and digitally outputs the result, in °C or °F, with
0.1°C accuracy and 0.001°C resolution. The LTC2986 can
measure the temperature of virtually all standard (Type B,
E, J, K, N, S, R, T) or custom thermocouples, automatically
compensate for cold junction temperatures and linearize
the results. The device can also measure temperature
with standard 2-, 3- or 4-wire RTDs, thermistors, and
diodes. The LTC2986 includes excitation current sources
and fault detection circuitry appropriate for each type of
temperature sensor.
The LTC2986/LTC2986-1 are 10-channel software and pin-
compatible versions of the 20-channel LTC2983/LTC2984.
Additional features include special modes that enable easy
protection in universal multi-sensor applications, custom
tables for generic ADC readings, and direct temperature
readout from active analog temperature sensors. The
LTC2986-1 is the EEPROM version of the LTC2986.
Thermocouple Measurement with Automatic Cold Junction Compensation Typical Temperature Error
Contribution
APPLICATIONS
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Patents Pending
n Directly Digitizes 2-, 3- or 4-Wire RTDs,
Thermocouples, Thermistors, and Diodes
n On-Chip EEPROM (LTC2986-1) Stores Channel
ConfigurationData and Custom Coefficients
n Single 2.85V to 5.25V Supply
n 10 Flexible Inputs Allow Interchanging Sensors
n Automatic Thermocouple Cold Junction Compensation
n Built-In Standard and User-Programmable Coefficients
for Thermocouples, RTDs and Thermistors
n Measures Negative Thermocouple Voltages
n Automatic Burn Out, Short-Circuit and Fault Detection
n Buffered Inputs Allow External Protection
n Simultaneous 50Hz/60Hz Rejection
n Includes 15ppm/°C (Max) Reference
n Includes Special Protection Modes
n Direct Thermocouple Measurements
n Direct RTD Measurements
n Direct Thermistor Measurements
n Custom Sensor Applications
TEMPERATURE (°C)
–200
–0.5
ERROR (°C)
0.3
0.2
0.1
–0.1
–0.2
–0.3
–0.4
0.5
200 600 800
29861 TA01b
0
0.4
0400 1000 14001200
THERMISTOR
THERMOCOUPLE
RTD
3904 DIODE
29861 TA01a
°C/°F
VREF (15ppm/°C) EEPROM
LTC2986-1
24-BIT
∆∑ ADC
24-BIT
∆∑ ADC
24-BIT
∆∑ ADC
RSENSE
2k
1
4
2
3
0.1µF
2.85V TO 5.25V
1k
1k
PT-100
RTD
LINEARIZATION/
FAULT DETECTION
SPI
INTERFACE
COM
LTC2986/LTC2986-1
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29861fa
For more information www.linear.com/LTC2986
Features ............................................................................................................................ 1
Applications ....................................................................................................................... 1
Typical Application ............................................................................................................... 1
Description......................................................................................................................... 1
Absolute Maximum Ratings ..................................................................................................... 3
Order Information ................................................................................................................. 3
Pin Configuration ................................................................................................................. 3
Complete System Electrical Characteristics .................................................................................. 4
ADC Electrical Characteristics .................................................................................................. 4
Reference Electrical Characteristics ........................................................................................... 5
Digital Inputs and Digital Outputs .............................................................................................. 5
LTC2986-1 EEPROM Characteristics ........................................................................................... 6
Typical Performance Characteristics .......................................................................................... 7
Pin Functions .....................................................................................................................10
Block Diagram ....................................................................................................................11
Test Circuits ......................................................................................................................12
Timing Diagram ..................................................................................................................12
Overview ..........................................................................................................................13
Applications Information .......................................................................................................17
EEPROM Overview (LTC2986-1) .......................................................................................................................... 23
EEPROM Read/Write Validation ............................................................................................................................ 23
EEPROM Write Operation ..................................................................................................................................... 23
EEPROM Read Operation (LTC2986-1) ................................................................................................................. 24
Thermocouple Measurements .............................................................................................................................. 25
Diode Measurements ............................................................................................................................................ 28
RTD Measurements .............................................................................................................................................. 32
Thermistor Measurements .................................................................................................................................... 51
Global Configuration Register ............................................................................................................................... 60
Input Overvoltage Protection – Overview.............................................................................................................. 60
Active Analog Temperature Sensors ..................................................................................................................... 66
Direct ADC Measurements .................................................................................................................................... 70
2- and 3-Cycle Conversion Modes ........................................................................................................................ 75
Running Conversions Consecutively on Multiple Channels ................................................................................... 75
Entering/Exiting Sleep Mode ................................................................................................................................. 76
MUX Configuration Delay ...................................................................................................................................... 76
Reference Considerations ..................................................................................................................................... 76
Custom Thermocouples ......................................................................................................... 77
Custom RTDs .....................................................................................................................80
Custom Thermistors .............................................................................................................83
Package Description ............................................................................................................88
Revision History .................................................................................................................89
Typical Application ..............................................................................................................90
Related Parts .....................................................................................................................90
TABLE OF CONTENTS
LTC2986/LTC2986-1
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For more information www.linear.com/LTC2986
PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS
Supply Voltage (VDD) ................................... –0.3V to 6V
Analog Input Pins (CH1 to
CH10, COM) ................................. –0.3V to (VDD + 0.3V)
Input Current (CH1 to CH10, COM) ...................... ±15mA
Digital Inputs (CS, SDI,
SCK, RESET) ................................ –0.3V to (VDD + 0.3V)
Digital Outputs (SDO, INTERRUPT) –0.3V to (VDD + 0.3V)
VREFP ........................................................ –0.3V to 2.8V
Q1, Q2, Q3, LDO, VREFOUT, VREF_BYP (Note 18)
Reference Short-Circuit Duration ..................... Indefinite
Operating Temperature Range
LTC2986C ................................................ 0°C to 70°C
LTC2986I .............................................–40°C to 85°C
LTC2986H .......................................... –40°C to 125°C
(Notes 1, 2)
ORDER INFORMATION
LEAD FREE FINISH TRAY PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC2986CLX#PBF LTC2986CLX#PBF LTC2986LX 48-Lead (7mm × 7mm) LQFP 0°C to 70°C
LTC2986ILX#PBF LTC2986ILX#PBF LTC2986LX 48-Lead (7mm × 7mm) LQFP –40°C to 85°C
LTC2986HLX#PBF LTC2986HLX#PBF LTC2986LX 48-Lead (7mm × 7mm) LQFP –40°C to 125°C
LTC2986CLX-1#PBF LTC2986CLX-1#PBF LTC2986LX-1 48-Lead (7mm × 7mm) LQFP 0°C to 70°C
LTC2986ILX-1#PBF LTC2986ILX-1#PBF LTC2986LX-1 48-Lead (7mm × 7mm) LQFP –40°C to 85°C
LTC2986HLX-1#PBF LTC2986HLX-1#PBF LTC2986LX-1 48-Lead (7mm × 7mm) LQFP –40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shippingcontainer.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
13
14
15
16
17
18
19
20
21
22
23
24
48
47
46
45
44
43
42
41
40
39
38
37
VREFOUT
VREFP
GND
CH1
CH2
CH3
CH4
CH5
CH6
CH7
CH8
CH9
25
26
27
28
29
30
31
32
33
34
35
36
CH10
GND
GND
GND
GND
GND
GND
GND
GND
GND
GND
COM
12
11
10
9
8
7
6
5
4
3
2
1
GND
VREF_BYP
NC
GND
VDD
GND
VDD
GND
VDD
GND
VDD
GND
Q1
Q2
Q3
VDD
GND
LDO
RESET
CS
SDI
SDO
SCK
INTERRUPT
TOP VIEW
LX PACKAGE
48-LEAD (7mm × 7mm) PLASTIC LQFP
TJMAX = 150°C, θJA = 57°C/W
http://www.linear.com/product/LTC2986#orderinfo
LTC2986/LTC2986-1
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ADC ELECTRICAL CHARACTERISTICS
PARAMETER CONDITIONS MIN TYP MAX UNITS
Resolution (No Missing Codes) –VREFOUT/2 ≤ VIN ≤ +VREFOUT/2 l24 Bits
Integral Nonlinearity VIN(CM) = 1.25V (Note 15) l2 30 ppm of VREF
Offset Error l0.5 2 µV
Offset Error Drift (Note 4) l10 20 nV/°C
Positive Full-Scale Error (Notes 3, 15) l100 ppm of VREF
Positive Full-Scale Drift (Notes 3, 15) l0.1 0.5 ppm of VREF/°C
Input Leakage (Note 19)
H-Grade
l
l
1
10
nA
nA
Negative Full-Scale Error (Notes 3, 15) l100 ppm of VREF
Negative Full-Scale Drift (Notes 3, 15) l0.1 0.5 ppm of VREF/°C
Input Referred Noise (Note 5)
H-Grade
l
l
0.8 1.5
2.0
µVRMS
µVRMS
Common Mode Input Range l–0.05 VDD – 0.3 V
RTD Excitation Current (Note 16) l–25 Table 33 25 %
RTD Excitation Current Matching Continuously Calibrated lError within Noise Level of ADC
Thermistor Excitation Current (Note 16) l–37.5 Table 57 37.5 %
The l denotes the specifications which apply over the full
operating temperature range, otherwise specifications are at TA = 25°C.
COMPLETE SYSTEM ELECTRICAL CHARACTERISTICS
The l denotes the specifications
which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
PARAMETER CONDITIONS MIN TYP MAX UNITS
Supply Voltage l2.85 5.25 V
Supply Current l15 20 mA
Sleep Current l25 60 µA
Input Range All Analog Input Channels l–0.05 VDD – 0.3 V
Output Rate Two Conversion Cycle Mode (Notes 6, 9) l150 164 170 ms
Output Rate Three Conversion Cycle Mode (Notes 6, 9) l225 246 255 ms
Input Common Mode Rejection 50Hz/60Hz (Note 4) l120 dB
Input Normal Mode Rejection 60Hz (Notes 4, 7) l120 dB
Input Normal Mode Rejection 50Hz (Notes 4, 8) l120 dB
Input Normal Mode Rejection 50Hz/60Hz (Notes 4, 6, 9) l75 dB
Power-On Reset Threshold 2.25 V
Analog Power-Up (Note 11) l100 ms
Digital Initialization (Note 12) l100 ms
LTC2986/LTC2986-1
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REFERENCE ELECTRICAL CHARACTERISTICS
PARAMETER CONDITIONS MIN TYP MAX UNITS
Output Voltage VREFOUT (Note 10) 2.49 2.51 V
Output Voltage Temperature Coefficient I-Grade, H-Grade l3 15 ppm/°C
Output Voltage Temperature Coefficient C-Grade l3 20 ppm/°C
Line Regulation l10 ppm/V
Load Regulation IOUT(SOURCE) = 100µA l5 mV/mA
IOUT(SINK) = 100µA l5 mV/mA
Output Voltage Noise 0.1Hz ≤ f ≤ 10Hz 4 µVP-P
10Hz ≤ f ≤ 1kHz 4.5 µVP-P
Output Short-Circuit Current Short VREFOUT to GND 40 mA
Short VREFOUT to VDD 30 mA
Turn-On Time 0.1% Setting, CLOAD = 1µF 115 µs
Long Term Drift of Output Voltage (Note 13) 60 ppm/√kHr
Hysteresis (Note 14) ∆T = 0°C to 70°C
∆T = –40°C to 85°C
30
70
ppm
ppm
The l denotes the specifications which apply over
the full operating temperature range, otherwise specifications are at TA = 25°C.
DIGITAL INPUTS AND DIGITAL OUTPUTS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
External SCK Frequency Range l0 2 MHz
External SCK LOW Period l250 ns
External SCK HIGH Period l250 ns
t1CS↓ to SDO Valid l0 200 ns
t2CS↑ to SDO Hi-Z l0 200 ns
t3CS↓ to SCK↑l100 ns
t4SCK↓ to SDO Valid l225 ns
t5SDO Hold After SCK↓l10 ns
t6SDI Setup Before SCK↑l100 ns
t7SDI HOLD After SCK↑l100 ns
High Level Input Voltage CS, SDI, SCK, RESET lVDD – 0.5 V
Low Level Input Voltage CS, SDI, SCK, RESET l0.5 V
Digital Input Current CS, SDI, SCK, RESET l–10 10 µA
Digital Input Capacitance CS, SDI, SCK, RESET 10 pF
LOW Level Output Voltage (SDO, INTERRUPT) IO = –800µA l0.4 V
High Level Output Voltage (SDO, INTERRUPT) IO = 1.6mA lVDD – 0.5 V
Hi-Z Output Leakage (SDO) l–10 10 µA
The l denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at TA = 25°C.
LTC2986/LTC2986-1
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For more information www.linear.com/LTC2986
LTC2986-1 EEPROM CHARACTERISTICS
The l denotes the specifications which apply over the full
operating temperature range, otherwise specifications are at TA = 25°C. These specifications apply only to LTC2986-1,
LTC2986 does not include EEPROM.
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All voltage values are with respect to GND.
Note 3: Full scale ADC error. Measurements do not include reference error.
Note 4: Guaranteed by design, not subject to test.
Note 5: The input referred noise includes the contribution of internal
calibration operations.
Note 6: MUX configuration delay = default 1ms.
Note 7: Global configuration set to 60Hz rejection.
Note 8: Global configuration set to 50Hz rejection.
Note 9: Global configuration default 50Hz/60Hz rejection.
Note 10: The exact value of VREF is stored in the LTC2986 and used
for all measurement calculations. Temperature coefficient is measured
by dividing the maximum change in output voltage by the specified
temperature range.
Note 11: Analog power-up. Command status register inaccessible during
this time.
Note 12: Digital initialization. Begins at the conclusion of Analog
Power-Up. Command status register is 0×80 at the beginning of digital
initialization and 0×40 at the conclusion.
Note 13: Long-term stability typically has a logarithmic characteristic
and therefore, changes after 1000 hours tend to be much smaller than
before that time. Total drift in the second thousand hours is normally less
than one third that of the first thousand hours with a continuing trend
toward reduced drift with time. Long-term stability will also be affected by
differential stresses between the IC and the board material created during
board assembly.
Note 14: Hysteresis in output voltage is created by package stress
that differs depending on whether the IC was previously at a higher or
lower temperature. Output voltage is always measured at 25°C, but
the IC is cycled to the hot or cold temperature limit before successive
measurements. Hysteresis measures the maximum output change for the
averages of three hot or cold temperature cycles. For instruments that
are stored at well controlled temperatures (within 20 or 30 degrees of
operational temperature), it is usually not a dominant error source. Typical
hysteresis is the worst-case of 25°C to cold to 25°C or 25°C to hot to
25°C, preconditioned by one thermal cycle.
Note 15: Differential Input Range is ±VREF/2.
Note 16: RTD and thermistor measurements are made ratiometrically.
As a result, current source excitation variation does not affect absolute
accuracy. Choose an excitation current such that largest sensor or RSENSE
resistance value, when driven by the nominal excitation current, will drop
1V or less. The extended ADC input range will accommodate variation in
excitation current and the ratiometric calculation will negate the absolute
value of the excitation current.
Note 17: 10-year data retention guaranteed for up to 1000 programcycles.
Note 18: Do not apply voltage or current sources to these pins. They must
be connected to capacitive loads only. Otherwise, permanent damage may
occur.
Note 19: Input leakage measured with VIN = –10mV and VIN = 2.5V.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Retention Notes 4 and 17 l10 Years
Endurance Note 4 l10000 Cycles
Programming Time Complete Transfer from RAM to EEPROM l2600 ms
Read Time Complete Transfer EEPROM to RAM l20 ms
LTC2986/LTC2986-1
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Type E Thermocouple Error and
RMS Noise vs Temperature
Type B Thermocouple Error and
RMS Noise vs Temperature
RTD PT-1000 Error and RMS
Noise vs Temperature
Type R Thermocouple Error and
RMS Noise vs Temperature
Type S Thermocouple Error and
RMS Noise vs Temperature
Type T Thermocouple Error and
RMS Noise vs Temperature
TYPICAL PERFORMANCE CHARACTERISTICS
Type J Thermocouple Error and
RMS Noise vs Temperature
Type K Thermocouple Error and
RMS Noise vs Temperature
Type N Thermocouple Error and
RMS Noise vs Temperature
THERMOCOUPLE TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G01
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–400 800 1200 16004000
RMS NOISE
ERROR
THERMOCOUPLE TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G02
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–400 800 1200 16004000
RMS NOISE
ERROR
THERMOCOUPLE TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G03
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–400 800 1200 16004000
RMS NOISE
ERROR
THERMOCOUPLE TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G04
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–400 800 1200 1600 20004000
RMS NOISE
ERROR
THERMOCOUPLE TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G05
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–400 800 1200 1600 20004000
RMS NOISE
ERROR
THERMOCOUPLE TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G06
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–400 200 400 6000–200
RMS NOISE
ERROR
THERMOCOUPLE TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G07
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–400 400 800 12000
RMS NOISE
ERROR
THERMOCOUPLE TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G08
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
400 1200 1600 2000800
RMS NOISE
ERROR
RTD TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G09
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–400 400 8000
RMS NOISE
ERROR
T
LTC2986/LTC2986-1
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TYPICAL PERFORMANCE CHARACTERISTICS
RTD PT-200 Error and RMS Noise
vs Temperature
RTD PT-100 Error and RMS Noise
vs Temperature
RTD NI-120 RTD Error and
RMS Noise vs Temperature
RTD TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G10
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–400 400 8000
RMS NOISE
ERROR
RTD TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G11
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–400 0 200 400 600 800 1000–200
RMS NOISE
ERROR
RTD TEMPERATURE (°C)
ERROR/RMS NOISE (°C)
29861 G12
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
–100 0 100 200 300
RMS NOISE
ERROR
5k Thermistor Error vs Temperature
10k Thermistor Error vs Temperature
3k Thermistor Error vs Temperature
30k Thermistor Error vs
Temperature
YSI-400 Thermistor Error vs
Temperature
THERMISTOR TEMPERATURE (°C)
ERROR (°C)
29861 G13
1.0
0.8
0.6
0.2
0.4
–1.0
–0.8
–0.6
–0.4
–0.2
0
–40 0–20 20 80 1006040 120 140
THERMISTOR TEMPERATURE (°C)
ERROR (°C)
29861 G14
1.0
0.8
0.6
0.2
0.4
–1.0
–0.8
–0.6
–0.4
–0.2
0
–40 0–20 20 80 1006040 120 140
THERMISTOR TEMPERATURE (°C)
ERROR (°C)
29861 G15
1.0
0.8
0.6
0.2
0.4
–1.0
–0.8
–0.6
–0.4
–0.2
0
–40 0–20 20 80 1006040 120 140
THERMISTOR TEMPERATURE (°C)
ERROR (°C)
29861 G16
1.0
0.8
0.6
0.2
0.4
–1.0
–0.8
–0.6
–0.4
–0.2
0
–40 0–20 20 80 1006040 120 140
THERMISTOR TEMPERATURE (°C)
ERROR (°C)
29861 G17
1.0
0.8
0.6
0.2
0.4
–1.0
–0.8
–0.6
–0.4
–0.2
0
–40 0–20 20 80 1006040 120 140
THERMISTOR TEMPERATURE (°C)
ERROR (°C)
29861 G18
1.0
0.8
0.6
0.2
0.4
–1.0
–0.8
–0.6
–0.4
–0.2
0
–40 0–20 20 80 1006040 120 140
2.252k Thermistor Error vs
Temperature
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TYPICAL PERFORMANCE CHARACTERISTICS
Adjacent Channel Offset Error vs
Input Fault Voltage (VDD = 5V)
Adjacent Channel Offset Error vs
Input Fault Voltage
CH1 FAULT VOLTAGE (V)
CH2 OFFSET ERROR (µV)
29861 G26
2.5
1.5
2.0
–0.5
0
0.5
1.0
4.95 5.055 5.1 5.2 5.255.15 5.3 5.35
CH1 FAULT VOLTAGE (V)
CH2 OFFSET ERROR (µV)
29861 G27
2.5
1.5
2.0
–0.5
0
0.5
1.0
0 –0.05 –0.1 –0.2 –0.25–0.15 –0.3 –0.35
Offset vs Temperature Noise vs Temperature
ISLEEP vs Temperature
One Shot Conversion Current vs
Temperature VREFOUT vs Temperature
LTC2986 TEMPERATURE (°C)
OFFSET (µV)
29861 G20
2.0
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
–50 –25 50 75250 100 125
VDD = 5.25V
VDD = 4.1V
VDD = 2.85V
LTC2986 TEMPERATURE (°C)
NOISE (µVRMS)
29861 G21
1.2
1.0
0.8
0.6
0.4
0.2
0
–50 500 25–25 10075 125
VDD = 5.25V
VDD = 4.1V
VDD = 2.85V
LTC2986 TEMPERATURE (°C)
ISLEEP (µA)
29861 G22
60
50
40
30
20
10
0
–50 –25 50 75250 100 125
VDD = 5.25V
VDD = 4.1V
VDD = 2.85V
LTC2986 TEMPERATURE (°C)
IIDLE (mA)
29861 G23
16.0
15.8
15.6
15.4
15.2
14.8
14.6
14.4
14.2
15.0
0
–50 50250–25 10075 125
VDD = 5.25V
VDD = 4.1V
VDD = 2.85V
Channel Input Leakage Current vs
Temperature
Diode Error and Repeatability vs
Temperature
DIODE TEMPERATURE (°C)
ERROR (°C)
29861 G19
1.0
0.8
0.6
–0.2
–0.4
–0.6
–0.8
0
0.2
0.4
–1.0
–40 20 80 140
TEMPERATURE (°C)
–50
–30
–10
10
30
50
70
90
110
130
2.4995
2.49975
2.5
2.50025
2.5005
V
REFOUT
(V)
V
REFOUT
vs Temperature
29861 G24
125°C
90°C
25°C
–45°C
INPUT VOLTAGE (V)
–1
0
1
2
3
4
5
6
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
INPUT LEAKAGE (nA)
Temperature
29861 G25
LTC2986/LTC2986-1
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PIN FUNCTIONS
GND (Pins 1, 3, 5, 7, 9, 12, 15, 26-35, 44): Ground.
Connect each of these pins to a common ground plane
through a low impedance connection. All 18 pins must
be grounded for proper operation.
VDD (Pins 2, 4, 6, 8, 45): Analog Power Supply. Tie all
five pins together and bypass as close as possible to the
device, to ground with 0.1µF and 10μF capacitors.
VREF_BYP( Pin 11): Internal Reference Power. This is an
internal supply pin, do not load this pin with external
circuitry. Decouple with a 0.1µF capacitor to GND.
VREFOUT (Pin 13): Reference Output Voltage. Short to
VREFP. A minimum 1µF capacitor to ground is required.
Do not load this pin with external circuitry.
VREFP (Pin 14): Positive Reference Input. Tie to VREFOUT.
CH1 to CH10 (Pin 16 to Pin 25): Analog Inputs. May be
programmed for single-ended, differential, or ratiometric
operation. The voltage on these pins can have any value
between GND – 50mV and VDD – 0.3V. Unused pins can
be grounded or left floating.
COM (Pin 36): Analog Input. The common negative input
for all single-ended configurations. The voltage on this
pin can have any value between GND – 50mV and VDD –
0.3V. This pin is typically tied to ground for temperature
measurements.
INTERRUPT (Pin 37): This pin outputs a LOW when the
device is busy either during start-up or while a conversion
cycle is in progress. This pin goes HIGH at the conclusion
of the start-up state or conversion cycle.
SCK (Pin 38): Serial Clock Pin. Data is shifted out of the
device on the falling edge of SCK and latched by the device
on the rising edge.
SDO (Pin 39): Serial Data Out. During the data output state,
this pin is used as the serial data output. When the chip
select pin is HIGH, the SDO pin is in a high impedance state.
SDI (Pin 40): Serial Data Input. Used to program the device.
Data is latched on the rising edge of SCK.
CS (Pin 41): Active Low Chip Select. A low on this pin
enables the digital input/output. A HIGH on this pin
places SDO in a high impedance state. A falling edge on
CS marks the beginning of a SPI transaction and a rising
edge marks the end.
RESET (Pin 42): Active Low Reset. While this pin is LOW,
the device is forced into the reset state. Once this pin is
returned HIGH, the device initiates its start-up sequence.
LDO (Pin 43): 2.5V LDO Output. Bypass with a 10µF
capacitor to GND. This is an internal supply pin, do not
load this pin with external circuitry.
Q3, Q2, Q1 (Pins 46, 47, 48): External Bypass Pins for
–200mV Integrated Charge Pump. Tie a 10µF X7R capaci-
tor between Q1 and Q2 close to each pin. Tie a 10µF X7R
capacitor from Q3 to Ground. These are internal supply
pins, do not make additional connections.
LTC2986/LTC2986-1
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BLOCK DIAGRAM
29861 BD
11:6 MUX
CH1 TO CH10
COM
ADC1
ROM
RAM
ADC2
INTERRUPT
SDO
SCK
SDI
CS
RESET
ADC3
EXCITATION
CURRENT SOURCES
15ppm/°C REFERENCE
VREFOUT VREFP VREF_BYP
0.1µF
VDD
Q2
Q1
Q3
LDO
PROCESSOR
LDO
CHARGE
PUMP
EEPROM
LTC2986-1
GND
10µF
10µF
10µF
1µF 10µF
0.1µF
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OVERVIEW
The LTC2986 measures the temperature of the most com-
mon sensors (thermocouples, RTDs, thermistors, active
analog temperature sensors, and diodes). It includes all
necessary active circuitry, switches, measurement algo-
rithms, and mathematical conversions to determine the
temperature for each sensor type.
Thermocouples can measure temperatures from as low as
–265°C to over 1800°C. Thermocouples generate a voltage
as a function of the temperature difference between the tip
(thermocouple temperature) and the electrical connection
on the circuit board (cold junction temperature). In order
to determine the thermocouple temperature, an accurate
measurement of the cold junction temperature is required;
this is known as cold junction compensation. The cold
junction temperature is usually determined by placing a
separate (non-thermocouple) temperature sensor at the
cold junction. The LTC2986 allows diodes, active analog
temperature sensors, RTDs, and thermistors to be used
as cold junction sensors. In order to convert the voltage
output from the thermocouple into a temperature result,
a high order polynomial equation (up to 14th order) must
be solved. The LTC2986 has these polynomials built in
for virtually all standard thermocouples (J, K, N, E, R,
S, T, and B). Additionally, inverse polynomials must be
solved for the cold junction temperature. The LTC2986
simultaneously measures the thermocouple output and
the cold junction temperature and performs all required
calculations to report the thermocouple temperature in °C
or °F. It directly digitizes both positive and negative volt-
ages (down to 50mV below ground) from a single ground
referenced supply, includes sensor burn-out detection, and
allows external protection/anti-aliasing circuits without the
need of buffer circuits.
Diodes are convenient low cost sensor elements and
are often used to measure cold junction temperatures in
thermocouple applications. Diodes are typically used to
measure temperatures from –60°C to 130°C, which is
suitable for most cold junction applications. Diodes gen-
erate an output voltage that is a function of temperature
and excitation current. When the difference of two diode
output voltages are taken at two different excitation current
levels, the result (∆VBE) is proportional to temperature.
The LTC2986 accurately generates excitation currents,
measures the diode voltages, and calculates the tempera-
ture in °C or °F.
RTDs and thermistors are resistors that change value as a
function of temperature. RTDs can measure temperatures
over a wide temperature range, from as low as –200°C to
850°C while thermistors typically operate from –40°C to
150°C. In order to measure one of these devices a precision
sense resistor is tied in series with the sensor. An excitation
current is applied to the network and a ratiometric mea-
surement is made. The value, in Ω, of the RTD/thermistor
can be determined from this ratio. This resistance is used
to determine the temperature of the sensor element using
a table lookup (RTDs) or solving Steinhart-Hart equations
(thermistors). The LTC2986 automatically generates the
excitation current, simultaneously measures the sense
resistor and thermistor/RTD voltage, calculates the sensor
resistance and reports the result in °C. The LTC2986 can
digitize most RTD types (PT-10, PT-50, PT-100, PT-200,
PT-500, PT-1000, and NI-120), has built in coefficients
for many curves (American, European, Japanese, and
ITS-90), and accommodates 2-wire, 3-wire, and 4-wire
configurations. It also includes coefficients for calculat-
ing the temperature of standard 2.252k, 3k, 5k, 10k , and
30k thermistors. It can be configured to share one sense
resistor among multiple RTDs/thermistors and to rotate
excitation current sources to remove parasitic thermal
effects. In addition to built-in linearization coefficients,
the LTC2986 provides the means of inserting custom
coefficients for both RTDs and thermistors.
The LTC2986 includes the capability to measure active
analog output temperature sensors. These sensors output
voltage as a function of temperature. The relationship
between voltage and temperature can be stored in the
LTC2986. These sensors can be used as a stand alone
temperature sensor or as the cold junction compensation
for thermocouple measurements.
LTC2986/LTC2986-1
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OVERVIEW
Table 1. LTC2986 Error Contribution and Peak Noise Errors
SENSOR TYPE TEMPERATURE RANGE ERROR CONTRIBUTION PEAK-TO-PEAK NOISE
Type K Thermocouple –200°C to 0°C
0°C to 1372°C
±(Temperature • 0.23% + 0.05)°C
±(Temperature • 0.12% + 0.05)°C ±0.08°C
Type J Thermocouple –210°C to 0°C
0°C to 1200°C
±(Temperature • 0.23% + 0.05)°C
±(Temperature • 0.12% + 0.05)°C ±0.07°C
Type E Thermocouple –200°C to 0°C
0°C to 1000°C
±(Temperature • 0.18% + 0.05)°C
±(Temperature • 0.10% + 0.05)°C ±0.06°C
Type N Thermocouple –200°C to 0°C
0°C to 1300°C
±(Temperature • 0.27% + 0.08)°C
±(Temperature • 0.10% + 0.08)°C ±0.13°C
Type R Thermocouple 0°C to 1768°C ±(Temperature • 0.10% + 0.4)°C ±0.62°C
Type S Thermocouple 0°C to 1768°C ±(Temperature • 0.10% + 0.4)°C ±0.62°C
Type B Thermocouple 400°C to 1820°C ±(Temperature • 0.10%)°C ±0.83°C
Type T Thermocouple –250°C to 0°C
0°C to 400°C
±(Temperature • 0.15% + 0.05)°C
±(Temperature • 0.10% + 0.05)°C ±0.09°C
External Diode (2 Reading) –40°C to 85°C ±0.25°C ±0.05°C
External Diode (3 Reading) –40°C to 85°C ±0.25°C ±0.2°C
Platinum RTD – PT-10, RSENSE = 1kΩ
Platinum RTD – PT-100, RSENSE = 2kΩ
Platinum RTD – PT-500, RSENSE = 2kΩ
Platinum RTD – PT-1000, RSENSE = 2kΩ
–200°C to 800°C
–200°C to 800°C
–200°C to 800°C
–200°C to 800°C
±0.1°C
±0.1°C
±0.1°C
±0.1°C
±0.05°C
±0.05°C
±0.02°C
±0.01°C
Thermistor, RSENSE = 10kΩ –40°C to 85°C ±0.1°C ±0.01°C
Table 1 shows the estimated system accuracy and noise
associated with specific temperature sensing devices.
System accuracy and peak-to-peak noise include the
effects of the ADC, internal amplifiers, excitation current
sources, and integrated reference. Accuracy and noise
are the worst-case errors calculated from the guaranteed
maximum ADC and reference specifications. Peak-to-
peak noise values are calculated at 0°C (except Type B
was calculated at 400°C) and diode measurements use
AVG= ON mode.
Thermocouple errors do not include the errors associated
with the cold junction measurement. Errors associated
with a specific cold junction sensor within the operating
temperature range can be combined with the errors for a
given thermocouple for total temperature measurement
accuracy.
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Table 2A. Memory Map
LTC2986 MEMORY MAP
SEGMENT
START
ADDRESS
END
ADDRESS
SIZE
(BYTES) DESCRIPTION
Command Status Register 0x000 0x0000 1 See Table 6 and 12, Initiate Conversion, Sleep Command,
EEPROM Command
Reserved 0x001 0x000F 15
Temperature Result Memory
10 Words – 40 Bytes
0x010 0x037 40 See Tables 8 to 10, Read Result
Reserved 0x038 0x0AF 120
EEPROM Key 0x0B0 0x0B3 4 See Table 11 (LTC2986-1 Only, Otherwise Reserved)
Reserved 0x0B4 0x0CF 44
EEPROM Read Result Code 0x0D0 0x0D0 1 See Table 11 (LTC2986-1 Only, Otherwise Reserved)
Reserved 0x0D1 0x0EF 15
Global Configuration Register 0x0F0 0x0F0 1 See Table 67 for Global Configuration
Reserved 0x0F1 0x0F3 3
Measure Multiple Channels Bit Mask 0x0F4 0x0F7 4 See Tables 84, 85, Run Multiple Conversions
Reserved 0x0F8 0x0F8 1
EEPROM Status Register 0x0F9 0x0F9 1 See Table 13 (LTC2986-1 Only, Otherwise Reserved)
Reserved 0x0FA 0x0FE 5
MUX Configuration Delay 0x0FF 0x0FF 1 See MUX Configuration Delay Section of Data Sheet
Reserved 0x100 0x1FF 256
Channel Assignment Data 0x200 0x227 40 See Tables 3, 4, Channel Assignment
Reserved 0x228 0x24F 40
Custom Sensor Table Data 0x250 0x3CF 384
Reserved 0x3D0 0x3FF 48
OVERVIEW
Memory Map
The LTC2986 channel assignment, configuration, conver-
sion start, and results are all accessible via the RAM (see
Table 2A). Table 2B details the valid SPI instruction bytes
for accessing memory. The channel conversion results are
mapped into memory locations 0x010 to 0x037 and can be
read using the SPI interface as shown in Figure 1. A read is
initiated by sending the read instruction byte = 0x03
followed by the address and then data. Channel assign-
ment data resides in memory locations 0x200 to 0x227
and can be programmed via the SPI interface as shown in
Figure 2. A write is initiated by sending the write instruc-
tion byte = 0x02 followed by the address and then data.
Conversions are initiated by writing the conversion control
byte (see Table 6) into memory location 0x000 (command
status register).
Table 2B. SPI Instruction Byte
INSTRUCTION SPI INSTRUCTION BYTE DESCRIPTION
Read 0b00000011 See Figure 1
Write 0b00000010 See Figure 2
Invalid 0bxxxxxx0x
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OVERVIEW
Figure 2. Memory Write Operation
Figure 1. Memory Read Operation
SCK
CS
RECEIVER SAMPLES
DATA ON RISING EDGE
TRANSMITTER TRANSITIONS
DATA ON FALLING EDGE
SDI I7 I6 I5 I4 I3 I2 I1 I0
00000011
0 0 0 0 A11 A10 A9 A8
16-BIT ADDRESS FIELD
USER MEMORY READ TRANSACTION
FIRST DATA BYTE
SUBSEQUENT
DATA BYTES
MAY FOLLOW
SPI INSTRUCTION BYTE
READ = 0x03
A7 A6 A5 A4 A3 A2 A1 A0
SDO
29861 F01
D7 D6 D5 D4 D3 D2 D1 D0
• • •
• • •
SCK
CS
RECEIVER SAMPLES
DATA ON RISING EDGE
TRANSMITTER TRANSITIONS
DATA ON FALLING EDGE
SDI I7 I6 I5 I4 I3 I2 I1 I0
00000010
0 0 0 0 A11 A10 A9 A8
16-BIT ADDRESS FIELD
USER MEMORY WRITE TRANSACTION
FIRST DATA BYTE
SUBSEQUENT
DATA BYTES
MAY FOLLOW
SPI INSTRUCTION BYTE
WRITE = 0x02
A7 A6 A5 A4 A3 A2 A1 A0
29861 F02
• • •
D7 D6 D5 D4 D3 D2 D1 D0 • • •
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The LTC2986 combines high accuracy with ease of use.
The basic operation is simple and is composed of five
states (see Figure 3).
APPLICATIONS INFORMATION
Figure 3. Basic Operation
29861 F03
POWER-UP,
SLEEP
OR RESET
(OPTIONAL)
≈ 200ms(MAX)
NO
YES
START-UP
CHANNEL ASSIGNMENT
INITIATE CONVERSION
CONVERSION
READ RESULTS
STATUS CHECK
COMPLETE?
Conversion States Overview
1. Start-Up. After power is applied to the LTC2986
(VDD>2.6V), there is a 200ms wake up period. During
this time, the LDO, charge pump, ADCs, and reference
are powered up and the internal RAM is initialized. Once
start-up is complete, the INTERRUPT pin goes HIGH
and the command status register will return a value of
0x40 (Start bit=0, Done bit=1) when read.
2. Channel Assignment. The device automatically enters
the channel assignment state after start-up is complete.
While in this state, the user writes sensor specific data
for each input channel into RAM. For the LTC2986-1,
the user can also load it from the EEPROM (see the
EEPROM section for more details). The assignment data
contains information about the sensor type, pointers to
cold junction sensors or sense resistors, and sensor
specific parameters.
3. Initiate Conversion. A conversion is initiated by writing
a measurement command into RAM memory location
0x000. This command is a pointer to the channel in
which the conversion will be performed.
4. Conversion. A new conversion begins automatically
following an Initiate Conversion command. In this
state, the ADC is running a conversion on the specified
channel and associated cold junction or RSENSE channel
(if applicable). The user is locked out of RAM access
while in the state (except for reading status location
0x000). The end of conversion is indicated by both
the INTERRUPT pin going HIGH and a status register
START bit going LOW and DONE bit going HIGH.
5. Read Results. In this state, the user has access to
RAM and can read the completed conversion results
and fault status bits. It is also possible for the user to
modify/append the channel assignment data during the
read results state.
Conversion State Details
State 1: Start-Up
The start-up state automatically occurs when power is ap-
plied to the LTC2986. If the power drops below a threshold
of ≈2.6V and then returns to the normal operating voltage
(2.85V to 5.25V), the LTC2986 resets and enters the power-
up state. Note that the LTC2986 also enters the start-up
state at the conclusion of the sleep state. The start-up state
can also be entered at any time during normal operation
by pulsing the RESET pin low.
In the first phase of the start-up state all critical analog
circuits are powered up. This includes the LDO, reference,
charge pump and ADCs. During this first phase, the com-
mand status register will be inaccessible to the user. This
phase takes a maximum of 100ms to complete. Once this
phase completes, the command status register will be
accessible and return a value of 0x80 until the LTC2986
is completely initialized. Once the LTC2986 is initialized
and ready to use, the INTERRUPT pin will go high and the
command status register will return a read value of 0x40
(Start bit=0, Done bit=1). At this point the LTC2986
is fully initialized and is ready to perform a conversion.
State 2: Channel Assignment
The LTC2986 RAM can be programmed with up to 10 sets
of 32-bit (4-byte) channel assignment data. These reside
sequentially in RAM with a one-to-one correspondence
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Table 3. Channel Assignment Memory Map
CHANNEL ASSIGNMENT
NUMBER
CONFIGURATION
DATA START
ADDRESS
CONFIGURATION
DATA
ADDRESS + 1
CONFIGURATION
DATA
ADDRESS + 2
CONFIGURATION
DATA END
ADDRESS + 3 SIZE (BYTES)
CH1 0x200 0x201 0x202 0x203 4
CH2 0x204 0x205 0x206 0x207 4
CH3 0x208 0x209 0x20A 0x20B 4
CH4 0x20C 0x20D 0x20E 0x20F 4
CH5 0x210 0x211 0x212 0x213 4
CH6 0x214 0x215 0x216 0x217 4
CH7 0x218 0x219 0x21A 0x21B 4
CH8 0x21C 0x21D 0x21E 0x21F 4
CH9 0x220 0x221 0x222 0x223 4
CH10 0x224 0x225 0x226 0x227 4
APPLICATIONS INFORMATION
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to each of the 10 analog input channels (see Table 3).
Channels that are not used should have their channel
assignment data set to all zeros (default at START-UP).
The channel assignment data contains all the necessary
information associated with the specific sensor tied to
that channel (see Table 4). The first five bits determine the
sensor type (see Table 5). Associated with each sensor
are sensor specific configurations. These include point-
ers to cold junction or sense resistor channels, pointers
to memory locations of custom linearization data, sense
resistor values and diode ideality factors. Also included
in this data are, if applicable, the excitation current level,
single-ended/differential input mode, as well as sensor
specific controls. Separate detailed operation sections
for thermocouples, RTDs, diodes, thermistors, analog
temperature sensors, and sense resistors describe the
assignment data associated with each sensor type in more
detail. The LTC2986 demonstration software includes
a utility for checking configuration data and generating
annotated C-code for programming the channel assign-
ment data.
Table 4. Channel Assignment Data
SENSOR TYPE SENSOR SPECIFIC CONFIGURATION
Channel
Assignment
Memory Location
Configuration Data
Start Address
Configuration Data
Start Address + 1
Configuration Data
Start Address + 2
Configuration Data
Start Address + 3
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Unassigned
(Default)
Type = 0 Channel Disabled
Thermocouple Type = 1 to 9 Cold Junction Channel
Assignment [4:0]
SGL=1
DIFF=0
OC
Check
OC Current
[1:0]
0 0 0 0 0 0 Custom
Address [5:0]
Custom
Length – 1 [5:0]
RTD Type = 10 to 18 RSENSE Channel Assignment
[4:0]
2, 3, 4 Wire Excitation
Mode
Excitation
Current [3:0]
Curve
[1:0]
Custom
Address [5:0]
Custom
Length – 1 [5:0]
Thermistor Type = 19 to 27 RSENSE Channel Assignment
[4:0]
SGL=1
DIFF=0
Excitation
Mode
Excitation Current
[3:0]
0 0 0 Custom
Address [5:0]
Custom
Length – 1 [5:0]
Diode Type = 28 SGL=1
DIFF=0
2 to 3
Reading
Avg
on
Current
[1:0]
Ideality Factor (2, 20) Value from 0 to 4 with 1/1048576 Resolution
All Zeros Use Factory Set Default in ROM
Sense Resistor Type = 29 Sense Resistor Value (17, 10) Up to 131,072Ω with 1/1024Ω Resolution
Direct ADC Type = 30 SGL=1
DIFF=0
Table
Mode
Not Used Custom
Address [5:0]
Custom
Length – 1 [5:0]
Active Analog
Temperature
Sensor
Type = 31 SGL=1
DIFF=0
Not Used Custom
Address [5:0]
Custom
Length – 1 [5:0]
APPLICATIONS INFORMATION
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Table 5. Sensor Type Selection
31 30 29 28 27 SENSOR TYPE
0 0 0 0 0 Unassigned
0 0 0 0 1 Type J Thermocouple
0 0 0 1 0 Type K Thermocouple
0 0 0 1 1 Type E Thermocouple
0 0 1 0 0 Type N Thermocouple
0 0 1 0 1 Type R Thermocouple
0 0 1 1 0 Type S Thermocouple
0 0 1 1 1 Type T Thermocouple
0 1 0 0 0 Type B Thermocouple
0 1 0 0 1 Custom Thermocouple
0 1 0 1 0 RTD PT-10
0 1 0 1 1 RTD PT-50
0 1 1 0 0 RTD PT-100
0 1 1 0 1 RTD PT-200
0 1 1 1 0 RTD PT-500
0 1 1 1 1 RTD PT-1000
1 0 0 0 0 RTD 1000 (0.00375)
1 0 0 0 1 RTD NI-120
1 0 0 1 0 RTD Custom
1 0 0 1 1 Thermistor 44004/44033 2.252kΩ at 25°C
1 0 1 0 0 Thermistor 44005/44030 3kΩ at 25°C
1 0 1 0 1 Thermistor 44007/44034 5kΩ at 25°C
1 0 1 1 0 Thermistor 44006/44031 10kΩ at 25°C
1 0 1 1 1 Thermistor 44008/44032 30kΩ at 25°C
1 1 0 0 0 Thermistor YSI 400 2.252kΩ at 25°C
1 1 0 0 1 Thermistor Spectrum 1003k 1kΩ
1 1 0 1 0 Thermistor Custom Steinhart-Hart
1 1 0 1 1 Thermistor Custom Table
1 1 1 0 0 Diode
1 1 1 0 1 Sense Resistor
1 1 1 1 0 Direct ADC
1 1 1 1 1 Analog Temperature Sensor
Table 7. Input Channel Mapping
B7 B6 B5 B4 B3 B2 B1 B0 CHANNEL SELECTED
1 0 0 0 0 0 0 0 Multiple Channels
1 0 0 0 0 0 0 1 CH1
1 0 0 0 0 0 1 0 CH2
1 0 0 0 0 0 1 1 CH3
1 0 0 0 0 1 0 0 CH4
1 0 0 0 0 1 0 1 CH5
1 0 0 0 0 1 1 0 CH6
1 0 0 0 0 1 1 1 CH7
1 0 0 0 1 0 0 0 CH8
1 0 0 0 1 0 0 1 CH9
1 0 0 0 1 0 1 0 CH10
1 0 0 1 0 1 1 1 Sleep
All Other Combinations Reserved
Table 6. Command Status Register
B7 B6 B5 B4 B3 B2 B1 B0
Start = 1 Done = 0 0 EEPROM Command and
Channel Selection 1 to 10
Start Conversion
1 0 0 1 0 1 1 1 Initiate Sleep
APPLICATIONS INFORMATION
State 3: Initiate Conversion
Once the channel assignment is complete, the device is
ready to begin a conversion. A conversion is initiated by
writing Start (B7=1) and Done (B6=0) followed by the
desired input channel (B4 – B0) into RAM memory loca-
tion 0x000 (see Tables 6 and 7). It is possible to initiate
a measurement cycle on multiple channels by setting the
channel selection bits (B4 to B0) to 00000; see the Running
Conversions Consecutively on Multiple Channels section
of the data sheet.
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Bits B4 to B0 determine which input channel the conversion
is performed upon and are simply the binary equivalent
of the channel number (see Table 7). These bits are also
used for EEPROM read and write operations (LTC2986-1,
see Table 12).
Bit B5 should be set to 0.
Bits B7 and B6 serve as start/done bits. In order to start
a conversion, these bits must be set to “10” (B7=1 and
B6=0). When the conversion begins, the INTERRUPT pin
goes LOW. Once the conversion is complete, bits B7 and
B6 will toggle to “01” (B7=0 and B6=1) (Address = 0x000)
and the INTERRUPT pin will go HIGH, indicating the con-
version is complete and the result is available.
State 4: Conversion
The measurement cycle starts after the Initiate Conversion
command is written into RAM location 0x000 (Table6).
The LTC2986 simultaneously measures the selected input
sensor, sense resistors (RTDs and thermistors), and cold
junction temperatures if applicable (thermocouples).
Once the conversion is started, the user is locked out of
the RAM, with the exception of reading status data stored
in RAM memory location 0x000.
Once the conversion is started the INTERRUPT pin goes
low. Depending on the sensor configuration, two or three
82ms cycles are required per temperature result. These
correspond to conversion rates of 167ms and 251ms,
respectively (assuming a filter frequency setting of 55Hz).
Details describing these modes are described in the 2- and
3-cycle Conversion Modes section of the data sheet.
The end of conversion can be monitored either through the
INTERRUPT pin (LOW to HIGH transition), or by reading
the command status register in RAM memory location
0x000 (start bit, B7, toggles from 1 to 0 and DONE bit,
B6, toggles from 0 to 1).
APPLICATIONS INFORMATION
State 5: Read Results
Once the conversion is complete, the conversion results
can be read from RAM memory locations corresponding
to the input channel (see Table 8).
The conversion result is 32 bits long and contains both
the sensor temperature (D23 to D0) and sensor fault data
(D31 to D24) (see Tables 9A and 9B).
The result is reported in °C for all temperature sensors
with a range of –273.15°C to 8192°C and 1/1024°C
resolution or in °F with a range of –459.67°F to 8192°F
with 1/1024°F resolution. Included with the conversion
result are seven sensor fault bits and a valid bit. These
sensor fault bits are set to a 1 if there was a problem
associated with the corresponding conversion result
(see Table 10). Two types of errors are reported: hard
errors and soft errors. Hard errors indicate the reading is
invalid and the resulting temperature reported is –999°C
or °F. Soft errors indicate operation beyond the normal
temperature range of the sensor or the input range of the
ADC. In this case, the calculated temperature is reported
but the accuracy may be compromised. Details relating
to each fault type are sensor specific and are described
in detail in the sensor specific sections of this data sheet.
Bit D24 is the valid bit and will be set to a 1 for valid data.
Once the data read is complete, the device is ready for a new
Initiate Conversion command. In cases where new channel
configuration data is required, the user has access to the
RAM in order to modify existing channel assignment data.
Table 8. Conversion Result Memory Map
CONVERSION
CHANNEL
START
ADDRESS END ADDRESS SIZE (BYTES)
CH1 0x010 0x013 4
CH2 0x014 0x017 4
CH3 0x018 0x01B 4
CH4 0x01C 0x01F 4
CH5 0x020 0x023 4
CH6 0x024 0x027 4
CH7 0x028 0x02B 4
CH8 0x02C 0x02F 4
CH9 0x030 0x033 4
CH10 0x034 0x037 4
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Table 9A. Example Data Output Words (°C)
START ADDRESS START ADDRESS + 1 START ADDRESS + 2
START ADDRESS + 3
(END ADDRESS)
D31 D30 D29 D28 D27 D26 D25 D24 D23 D22 D21 D20 D19 D18 D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
Fault Data SIGN MSB LSB
Temperature Sensor
Hard
Fault
ADC
Hard
Fault
CJ
Hard
Fault
CJ
Soft
Fault
Sensor
Over
Range
Fault
Sensor
Under
Range
Fault
ADC
Out
of
Range
Fault
Valid
If 1 4096°C1°C 1/1024°C
8191.999°C 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1024°C 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1°C 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0
1/1024°C 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
0°C 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
–1/1024°C 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
–1°C 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0
–273.15°C 1 1 1 1 1 1 0 1 1 1 0 1 1 1 0 1 1 0 1 1 0 0 1 1 1
Table 9B. Example Data Output Words (°F)
START ADDRESS START ADDRESS + 1 START ADDRESS + 2
START ADDRESS + 3
(END ADDRESS)
D31 D30 D29 D28 D27 D26 D25 D24 D23 D22 D21 D20 D19 D18 D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
Fault Data SIGN MSB LSB
Temperature Sensor
Hard
Fault
ADC
Hard
Fault
CJ
Hard
Fault
CJ
Soft
Fault
Sensor
Over
Range
Fault
Sensor
Under
Range
Fault
ADC
Out
of
Range
Fault
Valid
If 1 4096°F1°F 1/1024°F
8191.999°F 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1024°F 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1°F 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0
1/1024°F 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
0°F 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
–1/1024°F 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
–1°F 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0
–459.67°F 1 1 1 1 1 1 0 0 0 1 1 0 1 0 0 0 1 0 1 0 1 0 0 1 0
APPLICATIONS INFORMATION
Table 10. Sensor Fault Reporting
BIT FAULT ERROR TYPE DESCRIPTION OUTPUT RESULT
D31 Sensor Hard Fault Hard Bad Sensor Reading –999°C or °F
D30 Hard ADC-Out-of-Range Hard Bad ADC Reading (Could Be Large External Noise Event) –999°C or °F
D29 CJ Hard Fault Hard Cold Junction Sensor Has a Hard Fault Error –999°C or °F
D28 CJ Soft Fault Soft Cold Junction Sensor Result Is Beyond Normal Range Suspect Reading
D27 Sensor Over Range Soft Sensor Reading Is Above Normal Range Suspect Reading
D26 Sensor Under Range Soft Sensor Reading Is Below Normal Range Suspect Reading
D25 ADC Out-of-Range Soft ADC Absolute Input Voltage Is Beyond ±1.125 • VREF/2 Suspect Reading
D24 Valid NA Result Valid (Should Be 1) Discard Results if 0 Suspect Reading
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EEPROM OVERVIEW (LTC2986-1)
The LTC2986-1 contains 512 bytes of EEPROM, which
shadows the upper sensor configuration segment of USER
RAM (locations 0x200–0x3CF, see Figure 4). Prior to initial
usage, the user programs the USER RAM with all channel
assignment and custom sensor data. Once the USER RAM
has been programmed, the user can save this segment of
memory into the EEPROM. After subsequent power down
or sleep cycles, the user can reload the USER RAM with this
stored EEPROM data bypassing the channel assignment
and customer sensor programming normally required.
APPLICATIONS INFORMATIONAPPLICATIONS INFORMATION
Figure 4. Shadow EEPROM Memory Map
BYTE
ADDRESS
0000
01FF
0200
03CF
03DO
03FF
LTC2986 SPI
ADDRESS SPACE
USER COMMAND
REGISTERS,
RESULTS DATA,
GLOBAL CONFIGURATION
AND STATUS
SENSOR CONFIGURATION
MEMORY SEGMENT
(CHANNEL ASSIGNMENT
AND
CUSTOM SENSOR DATA)
COMMAND 21
(0x15)
COMMAND 22
(0x16)
EEPROM
SHADOW
RESERVEDRESERVED*
*NOTE: 03D0–03FF IS RESERVED
AND IS NOT SHADOWED BY EEPROM
29861 F04
Figure 5. EEPROM Write Operation
29861 F05
WRITE EEPROM KEY
TO LTC2986-1
CHECK EEPROM
STATUS REGISTER
DONE
USER DEFINED
EEPROM
ERROR HANDLER
SEND EEPROM WRITE COMMAND
(COMMAND 21)
WAIT FOR EEPROM
COMMAND TO COMPLETE
LTC2986-1
READY
WRITE CHANNEL ASSIGNMENT
AND CUSTOM SENSOR DATA
TO LTC2986-1
ELSE
PROGRAM FAILED
STATUS BIT SET
EEPROM WRITE OPERATION
The EEPROM write operation requires 5 states (see Figure5).
1. Sensor Configuration. Write all desired channel assignment
and custom sensor data to the LTC2986-1 USER RAM.
2. Set EEPROM Key. Write the EEPROM Key (0xA53C0F5A)
to the key register space of the LTC2986-1 USER RAM
(Address range 0x0B0–0x0B3, see Table 11). Note the
key is written MSB first.
3. Send EEPROM Write Command. Write the EEPROM write
command (0x15) and start bit (0x80) to the LTC2986-1
command register (Address 0x000). The command plus
start bit is 0x80 + 0x15 = 0x95 (see Table 12).
4. Wait for EEPROM Command to Complete. Completion
of the write operation is indicated by both the INTER-
RUPT pin going HIGH and the status register START
bit going LOW and DONE bit going HIGH.
5. Check EEPROM Status Register. Read EEPROM Status
register (Address 0x0F9) and checks the Program-Failed
status bit (Bit 2) to determine whether the EEPROM write
operation was successful (see Table 13). The Program-Failed
status bit being set indicates that the write operation failed.
Upon successful completion of steps 1–5, the EEPROM
will now contain the image that was present in USER RAM
locations 0x200–0x3CF.
EEPROM READ/WRITE VALIDATION
Access to the EEPROM is key-protected to prevent inad-
vertent access. The EEPROM also has two levels of data
integrity protection. The first level is implemented using
an error correcting code (ECC) on each 32-bit word of
data in the EEPROM. The ECC is capable of correcting
any single bit error per word and detecting 2-bit errors
per word. The second level of protection is implemented
using a 32-bit checksum, which covers the entire contents
of user EEPROM. Status bits are available to the user for
reporting ECC status and checksum error conditions.
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EEPROM READ OPERATION (LTC2986-1)
The LTC2986-1 EEPROM read operation is comprised of
4states (see Figure 6)
APPLICATIONS INFORMATION
Figure 6. Read Operation
29861 F06
WRITE EEPROM KEY
TO LTC2986-1
CHECK EEPROM
READ RESULT
CODE
DONE
USER DEFINED
EEPROM
ERROR HANDLER
SEND EEPROM READ COMMAND
(COMMAND 0x16)
WAIT FOR EEPROM
COMMAND TO COMPLETE
LTC2986-1
READY
ELSE
PASS: READ
RESULT CODE == 0
Table 11. LTC2986-1 EEPROM Related Registers
ADDRESS REGISTER NAME DESCRIPTION
0x0B0 EEPROM Key [3] (MSB) EEPROM Key byte 3 – Set to 0xA5
0x0B1 EEPROM Key [2] EEPROM Key byte 2 – Set to 0x3C
0x0B2 EEPROM Key [1] EEPROM Key byte 1 – Set to 0x0F
0x0B3 EEPROM Key [0] (LSB) EEPROM Key byte 0 – Set to 0x5A
0x0D0 EEPROM Read
Result Code
This register indicates the Pass/Fail
status of the most recent EEPROM
read operation
0x00 = PASS
0xFF = FAIL
0x0F9 EEPROM Status
Register
See LTC2986-1 EPROM Status
Register Tables 12 and 13
Table 12. LTC2986-1 EEPROM Related Commands and Status
B7 B6 B5 B4 B3 B2 B1 B0 DESCRIPTION
10010101EEPROM Write Command –
Transfer the contents of user
memory locations 0x200–0x3CF
to the on-chip shadow EEPROM
10010110EEPROM Read Command –
Transfer the contents of the on-
chip shadow EEPROM to user
memory locations 0x200–0x3CF
Table 13. EEPROM Status Bits
EEPROM STATUS BIT DESCRIPTION
ECC Used Error Correcting Code Used – This bit indicates
that ECC was used to correct data on one or more
locations during the EEPROM read process (Note 20)
ECC Failure Error Correcting Code Failure – This bit indicates
that ECC failed to correct data on one or more
locations during the EEPROM read process. If this
bit is set one or more locations has invalid data
(Note 20)
Program Failure Program Failure – This bit indicates that a write
data error occurred on one or more locations
during the EEPROM programming process
(Note20)
Checksum Error Checksum Error – This bit indicates that a
checksum error occurred during the EEPROM
read process (Note 20)
Note 20: Once bits in the EEPROM status register are set they will remain
set until cleared by the user. The EEPROM status register bits are cleared
by writing 0x00 to address 0x0F9. These bits are also cleared on reset and
after exiting sleep mode.
Table 14. LTC2986-1 EEPROM Status Register (Address 0x0F9)
7 6 5 4 3 2 1 0
– – – – Checksum
Error
Program
Failure
ECC
Failure
ECC
Used
1. Set EEPROM Key. Write the EEPROM Key (0xA53C0F5A)
to the key register space of the LTC2986-1 USER RAM
(Address range 0x0B0–0x0B3, see Table 11). Note the
key is written MSB first.
2. Send EEPROM Read Command. Write the EEPROM read
command (0x16) and start bit (0x80) to the LTC2986-1
command register (Address 0x000). The command plus
start bit would be 0x80 + 0x16 = 0x96 (see Table 12).
3. Wait for EEPROM Command to Complete. Completion
of the read operation is indicated by both the INTER-
RUPT pin going HIGH and the status register START
bit going LOW and DONE bit going HIGH.
4. Check EEPROM Read Result Code. Read the EEPROM
read result code register address (0x0D0) to determine
the pass/fail status of the read operation. A value of zero
indicates that the command completed successfully and
a non-zero value indicates that an error has occurred.
Additional read operation status bits are also available
in the EEPROM Status Register (see Tables 13 and 14).
Upon successful completion of steps 1–4, USER RAM
locations 0x200–0x3CF will now contain the data that was
stored in the LTC2986-1’s shadow EEPROM.
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THERMOCOUPLE MEASUREMENTS
Channel Assignment – Thermocouples
For each thermocouple tied to the LTC2986, a 32-bit channel
assignment word is programmed into a memory location
corresponding to the channel the sensor is tied to (see
Table 15). This word includes (1) thermocouple type, (2)
cold junction channel pointer, (3) sensor configuration,
and (4) custom thermocouple data pointer.
(1) Thermocouple Type
The thermocouple type is determined by the first five input
bits B31 to B27 as shown in Table 16. Standard NIST coef-
ficients for types J, K, E, N, R, S, T and B thermocouples
are stored in the device ROM. If custom thermocouples
are used, the custom thermocouple sensor type can be
selected. In this case, user-specific data can be stored in
the on-chip RAM starting at the address defined in the
custom thermocouple data pointer.
(2) Cold Junction Channel Pointer
The cold junction compensation can be a diode, active
analog temperature sensor, RTD, or thermistor. The cold
junction channel pointer tells the LTC2986 which channel
(1 to 10) the cold junction sensor is assigned to (see Table
17). When a conversion is performed on a channel tied to
a thermocouple, the cold junction sensor is simultaneously
and automatically measured. The final output data uses
the embedded coefficients stored in ROM to automatically
compensate the cold junction temperature and output the
thermocouple sensor temperature.
(3) Sensor Configuration
The sensor configuration field (see Table 18) is used
to select single-ended (B21=1) or differential (B21=0)
input and allows selection of open circuit current
if internal open-circuit detect is enabled (bit B20).
Single-ended readings are measured relative to the
COM pin and differential are measured between the
selected CHTC and adjacent CHTC-1 (see Figure 7).
If open-circuit detection is enabled, B20=1, then the user
can select the pulsed current value applied during open-
circuit detect using bits B18 and B19 . The user determines
the value of the open circuit current based on the size of
the external protection resistor and filter capacitor (typically
10µA). This network needs to settle within 50ms to 1µV
or less. The duration of the current pulse is approximately
8ms and occurs 50ms before the normal conversion cycle.
Thermocouple channel assignments follow the general
convention shown in Figure 7. The thermocouple positive
terminal ties to CHTC (where TC is the selected channel
number) for both the single-ended and differential modes
of operation. For single-ended measurements the thermo-
couple negative terminal and the COM pin are grounded.
The thermocouple negative terminal is tied to CHTC-1
for differential measurements. This node can either be
grounded or tied to a bias voltage.
APPLICATIONS INFORMATION
Table 15. Thermocouple Channel Assignment Word
(1) THERMOCOUPLE
TYPE
(2) COLD JUNCTION
CHANNEL POINTER
(3) SENSOR
CONFIGURATION
(4) CUSTOM THERMOCOUPLE
DATA POINTER
TABLES 4, 16 TABLE 17 TABLE 18 TABLES 86 TO 88
Measurement Type 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Thermocouple Types 1 to 9 Cold Junction
Channel Assignment
[4:0]
SGL=1
DIFF=0
OC
Check
OC
Current
[1:0]
0 0 0 0 0 0 Custom Address
[5:0]
Custom Length –1
[5:0]
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Table 17. Cold Junction Channel Pointer
(2) COLD JUNCTION CHANNEL POINTER
B26 B25 B24 B23 B22 COLD JUNCTION CHANNEL
0 0 0 0 0 No Cold Junction
Compensation, 0°C Used for
Calculations
0 0 0 0 1 CH1
0 0 0 1 0 CH2
0 0 0 1 1 CH3
0 0 1 0 0 CH4
0 0 1 0 1 CH5
0 0 1 1 0 CH6
0 0 1 1 1 CH7
0 1 0 0 0 CH8
0 1 0 0 1 CH9
0 1 0 1 0 CH10
All Other Combinations Invalid
Table 16. Thermocouple Type
(1) THERMOCOUPLE TYPE
B31 B30 B29 B28 B27 THERMOCOUPLE TYPES
0 0 0 0 1 Type J Thermocouple
0 0 0 1 0 Type K Thermocouple
0 0 0 1 1 Type E Thermocouple
0 0 1 0 0 Type N Thermocouple
0 0 1 0 1 Type R Thermocouple
0 0 1 1 0 Type S Thermocouple
0 0 1 1 1 Type T Thermocouple
0 1 0 0 0 Type B Thermocouple
0 1 0 0 1 Custom Thermocouple
Table 18. Sensor Configuration
(3) SENSOR CONFIGURATION
SGL
OC
CHECK OC CURRENT SINGLE-ENDED/
DIFFERENTIAL
OPEN-CIRCUIT
CURRENT
B21 B20 B19 B18
0 0 X X Differential External
0 1 0 0 Differential 10µA
0 1 0 1 Differential 100µA
0 1 1 0 Differential 500µA
0 1 1 1 Differential 1mA
1 0 X X Single-Ended External
1 1 0 0 Single-Ended 10µA
1 1 0 1 Single-Ended 100µA
1 1 1 0 Single-Ended 500µA
1 1 1 1 Single-Ended 1mA
Figure 7. Thermocouple Channel Assignment Convention
SINGLE-ENDED
+
–
+
–
= CHTC (1≤ TC ≤ 10)
COM
CHTC
0.1µF
CHANNEL
ASSIGNMENT
DIFFERENTIAL
29861 F07
= CHTC (2 ≤ TC ≤ 10)
CHTC
CHTC-1
0.1µF
CHANNEL
ASSIGNMENT
APPLICATIONS INFORMATION
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(4) Custom Thermocouple Data Pointer
See Custom Thermocouples section near the end of this
data sheet for more information.
Fault Reporting – Thermocouple
Each sensor type has a unique fault reporting mechanism
indicated in the upper byte of the data output word. Table 19
shows faults reported in the measurement of thermo-
couples.
Bit D31 indicates the thermocouple sensor is open (broken
or not plugged in), the cold junction sensor has a hard
fault, or the ADC is out of range. This is indicated by a
reading well beyond the normal operating range. Bit D30
indicates a bad ADC reading. This can be a result of either
a broken (open) sensor or an excessive noise event (ESD
or static discharge into the sensor path). Either of these
APPLICATIONS INFORMATION
Table 19. Thermocouple Fault Reporting
BIT FAULT ERROR TYPE DESCRIPTION OUTPUT RESULT
D31 Sensor Hard Fault Hard Open Circuit or Hard ADC or Hard CJ –999°C or °F
D30 Hard ADC-Out-of-Range Hard Bad ADC Reading (Could Be Large External Noise Event) –999°C or °F
D29 CJ Hard Fault Hard Cold Junction Sensor Has a Hard Fault Error –999°C or °F
D28 CJ Soft Fault Soft Cold Junction Sensor Result Is Beyond Normal Range Suspect Reading
D27 Sensor Over Range Soft Thermocouple Reading Greater Than High Limit Suspect Reading
D26 Sensor Under Range Soft Thermocouple Reading Less Than Low Limit Suspect Reading
D25 ADC Out-of-Range Soft ADC Absolute Input Voltage Is Beyond ±1.125 • VREF/2 Suspect Reading
D24 Valid NA Result Valid (Should Be 1) Discard Results if 0 Valid Reading
are a hard error and –999°C or °F is reported. In the case
of an excessive noise event, the device should recover and
the following conversions will be valid if the noise event
was a random, infrequent event. Bit D29 indicates a hard
fault occurred at the cold junction sensor and –999°C
or °F is reported. Refer to the specific sensor (diode,
themistor, or RTD) used for cold junction compensation.
Bit D28 indicates a soft fault occurred at the cold junction
sensor. A valid temperature is reported, but the accuracy
may be compromised since the cold junction sensor is
operating outside its normal temperature range. Bits
D27 and D26 indicate over or under temperature limits
have been exceeded for specific thermocouple types, as
defined in Table 20. Bit D25 indicates the absolute voltage
measured by the ADC is beyond its normal operating range.
This fault reflects a reading that is well beyond the normal
range of a thermocouple.
Table 20. Thermocouple Temperature Limits
THERMOCOUPLE TYPE LOW TEMP LIMIT °C HIGH TEMP LIMIT °C
J-Type –210 1200
K-Type –265 1372
E-Type –265 1000
N-Type –265 1300
R-type –50 1768
S-Type –50 1768
T-Type –265 400
B-Type 40 1820
Custom Lowest Table Entry Highest Table Entry
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DIODE MEASUREMENTS
Channel Assignment – Diode
For each diode tied to the LTC2986, a 32-bit channel as-
signment word is programmed into a memory location
corresponding to the channel the sensor is tied to (see
Table 21). This word includes (1) diode sensor selection,
(2) sensor configuration, (3) excitation current, and (4)
diode ideality factor.
1) Sensor Type
The diode is selected by the first five input bits B31 to
B27 (see Table 22).
(2) Sensor Configuration
The sensor configuration field (bits B26 to B24) is used to
define various diode measurement properties. Configura-
tion bit B26 is set high for single-ended (measurement
relative to COM) and low for differential.
Bit B25 sets the measurement algorithm. If B25 is low,
two conversion cycles (one at 1I and one at 8I current
excitation) are used to measure the diode. This is used
in applications where parasitic resistance between the
LTC2986 and the diode is small. Parasitic resistance ef-
fects can be removed by setting bit B25 high, enabling
three conversion cycles (one at 1I, one at 4I and one at 8I).
Bit B24 enables a running average of the diode temperature
reading. This reduces the noise when the diode is used
as a cold junction temperature element on an isothermal
block where temperatures change slowly.
The algorithm used for diode averaging is a simple recursive
running average. The new value is equal to the average of
the current reading plus the previous value.
NEW VALUE =
CURRENT READING
2
+
PREVIOUS VALUE
2
If the current reading is 2°C above or below the previous
value, the new value is reset to the current reading.
(3) Excitation Current
The next field in the channel assignment word (B23
to B22) controls the magnitude of the excitation cur-
rent applied to the diode (see Table 23). In the two
conversion cycle mode, the device performs the
first conversion at a current equal to 8x the excita-
tion current 1I. The second conversion occurs at 1I.
Alternatively, in the three conversion cycle mode the first
conversion excitation current is 8I, the second is 4I and
the 3rd is 1I.
Table 21. Diode Channel Assignment Word
(1) SENSOR
TYPE
(2) SENSOR
CONFIGURATION
(3) EXCITATION
CURRENT (4) DIODE IDEALITY FACTOR VALUE
TABLE 22 TABLE 23 TABLE 24
Measurement Class 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Diode Type = 28 SGL=1
DIFF=0
2 or 3
Readings
Avg
on
Current [1:0] Non-Ideality Factor (2, 20) Value from 0 to 4 with 1/1048576 Resolution
All Zeros Uses a Factory Set Default of 1.003
Table 22. Diode Sensor Selection
(1) SENSOR TYPE
B31 B30 B29 B28 B27 SENSOR TYPE
1 1 1 0 0 Diode
APPLICATIONS INFORMATION
Table 23. Diode Excitation Current Selection
(3) EXCITATION CURRENT
B23 B22 1I 4I 8I
0 0 10µA 40µA 80µA
0 1 20µA 80µA 160µA
1 0 40µA 160µA 320µA
1 1 80µA 320µA 640µA
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(4) Diode Ideality Factor
The last field in the channel assignment word (B21 to B0)
sets the diode ideality factor within the range 0 to 4 with
1/1048576 (2–20) resolution. The top two bits (B21 to B20)
are the integer part and bits B19 to B0 are the fractional
part of the ideality factor (see Table 24).
Diode channel assignments follow the general convention
shown in Figure 8. The anode ties to CHD (where D is
the selected channel number) for both the single-ended
and differential modes of operation, and the cathode is
grounded. For differential diode measurements, the cathode
is also tied to CHD-1.
APPLICATIONS INFORMATION
Table 24. Programming Diode Ideality Factor
(4) DIODE IDEALITY FACTOR VALUE
B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
Example h21202–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10 2–11 2–12 2–13 2–14 2–15 2–16 2–17 2–18 2–19 2–20
1.25 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1.003 (Default) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1.006 0 1 0 0 0 0 0 0 0 1 1 0 0 0 1 0 0 1 0 0 1 1
Table 25. Diode Fault Reporting
BIT FAULT ERROR TYPE DESCRIPTION OUTPUT RESULT
D31 Sensor Hard Fault Hard Open, Short, Reversed, or Hard ADC –999°C or °F
D30 Hard ADC-Out-of-Range Hard Bad ADC Reading (Could Be Large External Noise Event) –999°C or °F
D29 Not Used for Diodes N/A Always 0
D28 Not Used for Diodes N/A Always 0
D27 Sensor Over Range Soft T > 130°C Suspect Reading
D26 Sensor Under Range Soft T < –60°C Suspect Reading
D25 ADC Out-of-Range Soft ADC Absolute Input Voltage Is Beyond ±1.125 • VREF/2 Suspect Reading
D24 Valid NA Result Valid (Should Be 1) Discard Results if 0 Valid Reading
Fault Reporting – Diode
Each sensor type has unique fault reporting mechanism
indicated in the upper byte of the data output word.
Table 25 shows faults reported in the measurement of
diodes.
Bit D31 indicates the diode is open, shorted, not plugged
in, wired backwards, or the ADC reading is bad. Any of
these are hard faults and –999°C or °F is reported. Bit
D30 indicates a bad ADC reading. This can be a result of
either a broken (open) sensor or an excessive noise event
(ESD or static discharge into the sensor path). This is a
hard error and –999°C or °F is reported. In the case of
Figure 8. Diode Channel Assignment Convention
29861 F08
SINGLE-ENDED
= CHD (1 ≤ D ≤ 10)
COM
CHDCHANNEL
ASSIGNMENT
DIFFERENTIAL
= CHD (2 ≤ D ≤ 10)
CHD
CHD-1
CHANNEL
ASSIGNMENT
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an excessive noise event, the device should recover and
the following conversions will be valid if the noise event
was a random, infrequent event. Bits D29 and D28 are not
used for diodes. Bits D27 and D26 indicate over or under
temperature limits (defined as T > 130°C or T < –60°C). The
calculated temperature is reported, but the accuracy may
be compromised. Bit D25 indicates the absolute voltage
measured by the ADC is beyond its normal operating range.
If a diode is used as the cold junction element, any hard
or soft error is flagged in the corresponding thermocouple
result (bits D28 and D29 in Table 19).
Example: Single-Ended Type K and Differential Type T
Thermocouples with Shared Diode Cold Junction
Compensation
Figure 9 shows a typical temperature measurement
system where two thermocouples share a single cold
junction diode. In this example, a Type K thermocouple
is tied to CH1 and a Type T thermocouple is tied to CH3
and CH4. They both share a single cold junction diode
with ideality factor of h=1.003 tied to CH2. Channel as-
signment data for both thermocouples and the diode are
Figure 9. Dual Thermocouple with Diode Cold Junction Example
TYPE K 0.1µF
TYPE T
η = 1.003
29861 F09
TYPE K THERMOCOUPLE ASSIGNED TO CH1 (CHTC=1)
DIODE COLD JUNCTION ASSIGNED TO CH2 (CHD=2)
TYPE T THERMOCOUPLE JUNCTION ASSIGNED TO CH4 (CHTC=4)
CH4
CH3
CH2
CH1
0.1µF COM
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x200 TO 0x203
RESULT MEMORY LOCATIONS 0x010 TO 0x013
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x204 TO 0x207
RESULT MEMORY LOCATIONS 0x014 TO 0x017
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x20C TO 0x20F
RESULT MEMORY LOCATIONS 0x01C TO 0x01F
shown in Tables 26 to 28. Thermocouple #1 (Type K)
sensor type and configuration data are assigned to CH1.
32-bits of binary configuration data are mapped directly
into memory locations 0x200 to 0x203 (see Table 26).
The cold junction diode sensor type and configuration
data are assigned to CH2. 32-bits of binary configuration
data are mapped directly into memory locations 0x204
to 0x207 (see Table 27). Thermocouple #2 (Type T) sen-
sor type and configuration data are assigned to CH4.
32-bits of binary configuration data are mapped directly
into memory locations 0x20C to 0x20F (see Table28). A
conversion is initiated on CH1 by writing 10000001 into
memory location 0x000. Both the Type K thermocouple
and the diode are measured simultaneously. The LTC2986
calculates the cold junction compensation and determines
the temperature of the Type K thermocouple. Once the
conversion is complete, the INTERRUPT pin goes HIGH
and memory location 0x000 becomes 01000001. Similarly,
a conversion can be initiated on CH4 by writing 10000100
into memory location 0x000. The results (in °C) can be
read from memory locations 0x010 to 0x013 for CH1 and
0x01C to 0x01F for CH4.
APPLICATIONS INFORMATION
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Table 26. Thermocouple #1 Channel Assignment (Type K, Cold Junction CH2, Single-Ended, 10µA Open-Circuit Detect)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x200
MEMORY
ADDRESS 0x201
MEMORY
ADDRESS 0x202
MEMORY
ADDRESS 0x203
(1) Thermocouple
Type
Type K 5 00010 0 0 0 1 0
(2) Cold Junction
Channel Pointer
CH25 00010 0 0 0 1 0
(3) Sensor
Configuration
Single-Ended,
10µA Open-Circuit
4 1100 1 1 0 0
Not Used Set These Bits to 0 6 000000 0 0 0 0 0 0
(4) Custom
Thermocouple
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Table 27. Diode Channel Assignment (Single-Ended 3-Reading, Averaging On, 20µA/80µA Excitation, Ideality Factor = 1.003))
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x204
MEMORY
ADDRESS 0x205
MEMORY
ADDRESS 0x206
MEMORY
ADDRESS 0x207
(1) Sensor Type Diode 5 11100 1 1 1 0 0
(2) Sensor
Configuration
Single-Ended,
3-Reading,
Average On
3 111 1 1 1
(3) Excitation
Current
20µA, 80µA,
160µA
2 01 0 1
(4) Ideality Factor 1.003 22 0100000000110001001001 0 1 0 0 0 0 0 0 0 0 1 1 0 0 0 1 0 0 1 0 0 1
Table 28. Thermocouple #2 Channel Assignment (Type T, Cold Junction CH2, Differential, 100µA Open-Circuit Detect)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x20C
MEMORY
ADDRESS 0x20D
MEMORY
ADDRESS 0x20E
MEMORY
ADDRESS 0x20F
(1) Thermocouple
Type
Type T 5 00111 0 0 1 1 1
(2) Cold Junction
Channel Pointer
CH2500010 0 0 0 1 0
(3) Sensor
Configuration
Differential,
100µA Open-
Circuit Current
40101 0 1 0 1
Not Used Set These Bits
to 0
6000000 0 0 0 0 0 0
(4) Custom
Thermocouple
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
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RTD MEASUREMENTS
Channel Assignment – RTD
For each RTD tied to the LTC2986, a 32-bit channel as-
signment word is programmed into a memory location
corresponding to the channel the sensor is tied to (see
Table29). This word includes (1) RTD type, (2) sense resis-
tor channel pointer, (3) sensor configuration, (4) excitation
current, (5) RTD curve, and (6) custom RTD data pointer.
(1) RTD Type
The RTD type is determined by the first five input bits B31
to B27 as shown in Table 30. Linearization coefficients
for RTD types PT-10, PT-50, PT-100, PT-200, PT-500,
PT-1000, and NI-120 with selectable common curves
(α = 0.003850, α = 0.003911, α = 0.003916, and
α = 0.003926) are built into the device. If custom RTDs
are used, RTD Custom can be selected. In this case, user
specific data can be stored in the on-chip RAM starting
at the address defined by the custom RTD data pointers.
(2) Sense Resistor Channel Pointer
RTD measurements are performed ratiometrically relative
to a known RSENSE resistor. The sense resistor channel
pointer field indicates the differential channel the sense
resistor is tied to for the RTD (see Table 31). Sense resis-
tors are always measured differentially.
APPLICATIONS INFORMATION
(3) Sensor Configuration
The sensor configuration field is used to define various
RTD properties. Configuration bits B20 and B21 determine
if the RTD is a 2-, 3-, or 4-wire type (see Table 32).
The simplest configuration is the 2-wire configuration.
While this setup is simple, parasitic errors due to IR drops
in the leads result in systematic temperature errors. The
3-wire configuration cancels RTD lead resistance errors
(if the lines are equal resistance) by applying two matched
current sources to the RTD, one per lead. Mismatches in
the two current sources are removed through transparent
background calibration. 4-wire RTDs remove unbalanced
RTD lead resistance by measuring directly across the
sensor using a high impedance Kelvin sensing. 4-wire
measurements with Kelvin RSENSE are useful in applica-
tions where sense resistor wiring parasitics can lead to
errors; this is especially useful for low resistance PT-10
type RTDs. In this case, both the RTD and sense resistor
have Kelvin sensing connections.
The next sensor configuration bits (B18 and B19) deter-
mine the excitation current mode. These bits are used to
enable RSENSE sharing, where one sense resistor is used
for multiple 2-, 3-, and/or 4-wire RTDs. In this case, the
RTD ground connection is internal and each RTD points
to the same RSENSE channel.
Bits B18 and B19 are also used to enable excitation current
rotation to automatically remove parasitic thermocouple
effects. Parasitic thermocouple effects may arise from the
physical connected between the RTD and the measure-
ment instrument. This mode is available for all 4-wire
configurations using internal current source excitation.
Table 29. RTD Channel Assignment Word
(1) RTD TYPE
(2) SENSE RESISTOR
CHANNEL POINTER
(3) SENSOR
CONFIGURATION
(4) EXCITATION
CURRENT
(5) RTD
CURVE (6) CUSTOM RTD DATA POINTER
TABLE 30 TABLE 31 TABLE 32 TABLE 33 TABLE 34 TABLES 92 TO 94
Measurement Class 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
RTD Type = 10 to 18 RSENSE Channel
Assignment [4:0]
2, 3, 4
Wire
Excitation
Mode
Excitation
Current [3:0]
Curve
[1:0]
Custom Address
[5:0]
Custom Length-1
[5:0]
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APPLICATIONS INFORMATION
Table 30. RTD Type
(1) RTD TYPE
B31 B30 B29 B28 B27 RTD TYPE
0 1 0 1 0 RTD PT-10
0 1 0 1 1 RTD PT-50
0 1 1 0 0 RTD PT-100
0 1 1 0 1 RTD PT-200
0 1 1 1 0 RTD PT-500
0 1 1 1 1 RTD PT-1000
1 0 0 0 0 RTD 1000 (α = 0.00375)
1 0 0 0 1 RTD NI-120
1 0 0 1 0 RTD Custom
Table 31. Sense Resistor Channel Pointer
(2) SENSE RESISTOR CHANNEL POINTER
B26 B25 B24 B23 B22 SENSE RESISTOR CHANNEL
0 0 0 0 0 Invalid
0 0 0 0 1 Invalid
0 0 0 1 0 CH2-CH1
0 0 0 1 1 CH3-CH2
0 0 1 0 0 CH4-CH3
0 0 1 0 1 CH5-CH4
0 0 1 1 0 CH6-CH5
0 0 1 1 1 CH7-CH6
0 1 0 0 0 CH8-CH7
0 1 0 0 1 CH9-CH8
0 1 0 1 0 CH10-CH9
All Other Combinations Invalid
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Table 32. RTD Sensor Configuration Selection
(3) SENSE
CONFIGURATION MEASUREMENT MODE BENEFITS
NUMBER
OF WIRES
EXCITATION
MODE
NUMBER
OF WIRES
GROUND
CONNECTION
CURRENT
SOURCE
ROTATION
SENSE
RESISTOR
SHARING
RTDs
POSSIBLE
PER
DEVICE
CANCELS RTD
MATCHED
LEAD
RESISTANCE
CANCELS RTD
MISMATCH
LEAD
RESISTANCE
CANCELS
PARASITIC
THERMOCOUPLE
EFFECTS
CANCELS
RSENSE
LEAD
RESISTANCE
B21 B20 B19 B18
0 0 0 0 2-Wire External No No 2
0 0 0 1 2-Wire Internal No Yes 4
0 1 0 0 3-Wire External No No 2 •
0 1 0 1 3-Wire Internal No Yes 4 •
0 1 1 X Reserved
1 0 0 0 4-Wire External No No 2 • •
1 0 0 1 4-Wire Internal No Yes 2 • •
1 0 1 0 4-Wire Internal Yes Yes 2 • • •
1 0 1 1 Reserved
1 1 0 0 4-Wire,
Kelvin
RSENSE
External No No 2 • • •
1 1 0 1 4-Wire,
Kelvin
RSENSE
Internal No Yes 2 • • •
1 1 1 0 4-Wire,
Kelvin
RSENSE
Internal Yes Yes 2 • • • •
1 1 1 1 Reserved
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APPLICATIONS INFORMATION
(4) Excitation Current
The next field in the channel assignment word (B17 to
B14) controls the magnitude of the excitation current
applied to the RTD (see Table 33). The current selected
is the total current flowing through the RTD independent
of the wiring configuration. The RSENSE current is 2x the
sensor excitation current for 3-wire RTDs.
Table 33. Total Excitation Current for All RTD Wire Types
(4) EXCITATION CURRENT
B17 B16 B15 B14 CURRENT
0 0 0 0 External
0 0 0 1 5µA
0 0 1 0 10µA
0 0 1 1 25µA
0 1 0 0 50µA
0 1 0 1 100µA
0 1 1 0 250µA
0 1 1 1 500µA
1 0 0 0 1mA
Table 34. RTD Curves: RT = R0 • (1 + a • T + b • T2 + (T – 100°C) • c • T3) for T < 0°C, RT = R0 • (1 + a • T + b • T2) for T > 0°C
(5) CURVE
B13 B12 CURVE ALPHA a b c
0 0 European Curve 0.00385 3.908300E-03 –5.775000E-07 –4.183000E-12
0 1 American 0.003911 3.969200E-03 –5.849500E-07 –4.232500E-12
1 0 Japanese 0.003916 3.973900E-03 –5.870000E-07 –4.400000E-12
1 1 ITS-90 0.003926 3.984800E-03 –5.870000E-07 –4.000000E-12
X X RTD1000-375 0.00375 3.810200E-03 –6.018880E-07 –6.000000E-12
X X *NI-120 N/A N/A N/A N/A
*NI-120 uses table based data.
In order to prevent soft or hard faults, select a current
such that the maximum voltage drop across the sensor or
sense resistor is nominally 1.0V. For example, if RSENSE
is 10kΩ and the RTD is a PT-100, select an excitation
current of 100µA for 2-wire and 4-wire RTDs and select
50µA for a 3-wire RTD. Alternatively, using a 1kΩ sense
resistor with a PT-100 RTD allows 500µA excitation for
any wiring configuration.
(5) RTD Curve
Bits B13 and B12 set the RTD curve used and the cor-
responding Callendar-Van Dusen constants (shown in
Table 34).
(6) Custom RTD Data Pointer
In the case where an RTD not listed in Table 34 is used,
a custom RTD table may be entered into the LTC2986.
See Custom RTD section near the end of this data sheet
for more information.
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Fault Reporting – RTD
Each sensor type has unique fault reporting mechanism
indicated in the most significant byte of the data output
word. Table 35 shows faults reported in the measurement
of RTDs.
Bit D31 indicates the RTD or RSENSE is open, shorted, or not
plugged in. This is a hard fault and –999°C or °F is reported.
Bit D30 indicates a bad ADC reading. This can be a result
of either a broken (open) sensor or an excessive noise
event (ESD or static discharge into the sensor path). This
is a hard error and –999°C or °F is reported. In the case of
an excessive noise event, the device should recover and
the following conversions will be valid if the noise was a
random infrequent event. Bits D29 and D28 are not used
for RTDs. Bits D27 and D26 indicate over or under tem-
perature limits (see Table 36). The calculated temperature
is reported, but the accuracy may be compromised. Bit
D25 indicates the absolute voltage measured by the ADC
is beyond its normal operating range. If an RTD is used
as the cold junction element, any hard or soft error is also
flagged in the thermocouple result.
Sense Resistor
Channel Assignment
For each sense resistor tied to the LTC2986, a 32-bit
channel assignment word is programmed into a memory
location corresponding to the channel the sensor is tied
to (see Table 37). This word includes (1) sense resistor
selection and (2) sense resistor value.
Table 36. Voltage and Resistance Ranges
RTD TYPE MIN Ω MAX Ω LOW TEMP LIMIT °C HIGH TEMP LIMIT °C
PT-10 1.95 34.5 –200 850
PT-50 9.75 172.5 –200 850
PT-100 19.5 345 –200 850
PT-200 39 690 –200 850
PT-500 97.5 1725 –200 850
PT-1000 195 3450 –200 850
NI-120 66.6 380.3 –80 260
Custom Table Lowest Table Entry Highest Table Entry Lowest Table Entry Highest Table Entry
Table 35. RTD Fault Reporting
BIT FAULT ERROR TYPE DESCRIPTION OUTPUT RESULT
D31 Sensor Hard Fault Hard Open or Short RTD or RSENSE –999°C or °F
D30 Hard ADC-Out-of-Range Hard Bad ADC Reading (Could Be Large External Noise Event) –999°C or °F
D29 Not Used for RTDs N/A Always 0 Valid Reading
D28 Not Used for RTDs N/A Always 0 Valid Reading
D27 Sensor Over Range Soft T > High Temp Limit (See Table 36) Suspect Reading
D26 Sensor Under Range Soft T < Low Temp Limit (See Table 36) Suspect Reading
D25 ADC Out-of-Range Soft ADC Absolute Input Voltage Is Beyond ±1.125 • VREF/2 Suspect Reading
D24 Valid N/A Result Valid (Should Be 1) Discard Results if 0 Valid Reading
APPLICATIONS INFORMATION
Table 37. Sense Resistor Channel Assignment Word
(1) SENSOR TYPE (2) SENSE RESISTOR VALUE (Ω)
Table 38 Table 39
Measurement Class 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Sense Resistor Type = 29 Sense Resistor Value (17, 10) Up to ≈ 131,072Ω with 1/1024Ω Resolution
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APPLICATIONS INFORMATION
(1) Sensor Type
The sense resistor is selected by setting the first 5 input
bits, B31 to B27, to 11101 (see Table 38).
Sense resistor channel assignments follow the general
convention shown in Figure 11. The sense resistor is tied
between CHRSENSE and CHRSENSE-1, where CHRSENSE is
tied to the 2nd terminal of the RTD. Channel assignment
data (see Table 37) is mapped into a memory location
corresponding to CHRSENSE.
Table 38. Sense Resistor Selection
(1) SENSOR TYPE
B31 B30 B29 B28 B27 SENSOR TYPE
1 1 1 0 1 Sense Resistor
(2) Sense Resistor Value
The last field in the channel assignment word (B26 to B0)
sets the value of the sense resistor within the range 0 to
131,072Ω with 1/1024Ω precision (see Table 39). The top
17 bits (B26 to B10) create the integer and bits B9 to B0
create the fraction of the sense resistor value.
Example: 2-Wire RTD
The simplest RTD configuration is the 2-wire configura-
tion, 2-wire RTDs follow the general convention shown in
Figure 10. They require only two connections per RTD and
can be tied directly to 2-lead RTD elements. This topology,
however, causes errors due to parasitic lead resistance. If
sharing is not selected (1 RSENSE per RTD), then CHRTD
should be grounded. The ground connection should be
removed if sharing is enabled (1 RSENSE for multiple RTDs).
Figure 10. 2-Wire RTD Channel Assignment Convention
29861 F10
OPTIONAL GND, REMOVE FOR RSENSE SHARING
2ND TERMINAL TIES TO SENSE RESISTOR (CHRSENSE)
2
1
CHRTD-1
CH
RTD
EXCITATION
CURRENT
FLOW = CHRTD (2 ≤ RTD ≤ 10)
CHANNEL
ASSIGNMENT
Figure 11. Sense Resistor Channel Assignment Convention
for 2-Wire RTDs
29861 F11
CHRSENSE-1
CHRSENSE
RSENSE
EXCITATION
CURRENT
FLOW
= CHRSENSE (2 ≤ RSENSE ≤ 10)
CHANNEL
ASSIGNMENT
Example: 2-Wire RTDs with Shared RSENSE
Figure 12 shows a typical temperature measurement system
using multiple 2-wire RTDs. In this example, a PT-1000
RTD ties to CH7 and CH8 and an NI-120 RTD ties to CH9
and CH10. Using this configuration, the LTC2986 can
digitize up to four 2-wire RTDs with a single sense resistor.
RTD #1 sensor type and configuration data are assigned
to CH8. 32 bits of binary configuration data are mapped
directly into memory locations 0x21C to 0x21F (see
Table40). RTD #2 sensor type and configuration data are
assigned to CH10. 32-bits of binary configuration data are
mapped directly into memory locations 0x224 to 0x227
(see Table 41). The sense resistor is assigned to CH6.
The user-programmable value of this resistor is 5001.5Ω.
32bits of binary configuration data are mapped directly
into memory locations 0x214 to 0x217 (see Table 42).
A conversion is initiated on CH8 by writing 10001000 into
memory location 0x000. Once the conversion is complete,
the INTERRUPT pin goes HIGH and memory location
0x000 becomes 01001000. The resulting temperature in
°C can be read from memory locations 0x02C to 0x02F
(corresponding to CH8). A conversion can be initiated and
read from CH10 in a similar fashion.
Table 39. Example Sense Resistor Values
(2) SENSE RESISTOR VALUE (Ω)
B26 B25 B24 B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
Example R 216 215 214 213 212 211 210 292827262524232221202–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10
10,000.2Ω 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 1 1 0 0 1 1 0 1
99.99521kΩ 1 1 0 0 0 0 1 1 0 1 0 0 1 1 0 1 1 0 0 1 1 0 1 0 1 1 1
1.0023kΩ 0 0 0 0 0 0 0 1 1 1 1 1 0 1 0 1 0 0 1 0 0 1 1 0 0 1 1
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Table 40. Channel Assignment Data for 2-Wire RTD #1 (PT-1000, RSENSE on CH6, 2-Wire, Shared RSENSE, 10µA Excitation Current,
α = 0.003916 Curve)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x21C
MEMORY
ADDRESS 0x21D
MEMORY
ADDRESS 0x21E
MEMORY
ADDRESS 0x21F
(1) RTD TYPE PT-1000 5 01111 0 1 1 1 1
(2) Sense Resistor
Channel Pointer
CH65 00110 0 0 1 1 0
(3) Sensor
Configuration
2-Wire with
Shared RSENSE
4 0001 0 0 0 1
(4) Excitation
Current
10µA 4 0010 0 0 1 0
(5) Curve Japanese,
α = 0.003916
2 10 1 0
(6) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Figure 12. Shared 2-Wire RTD Example
RSENSE
5001.5Ω
0.01µF
29861 F12
0.01µF
SENSE RESISTOR ASSIGNED TO CH6 (CHRSENSE=6)
RTD #1 ASSIGNED TO CH8 (CHRTD=8)
RTD #2 ASSIGNED TO CH10 (CHRTD=10)
CH10
CH9
CH6
CH5
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x214 TO 0x217
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x21C TO 0x21F
RESULT MEMORY LOCATIONS 0x02C TO 0x02F
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x224 TO 0x227
RESULT MEMORY LOCATIONS 0x034 TO 0x037
0.01µF
0.01µF
CH7
CH8
0.01µF
0.01µF
2-WIRE PT-1000
2-WIRE NI-120
2
1
2
1
APPLICATIONS INFORMATION
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Table 41. Channel Assignment Data for 2-Wire RTD #2 (NI-120, RSENSE on CH6, 2-Wire, Shared RSENSE, 100µA Excitation Current)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x224
MEMORY
ADDRESS 0x225
MEMORY
ADDRESS 0x226
MEMORY
ADDRESS 0x227
(1) RTD TYPE NI-120 5 10001 1 0 0 0 1
(2) Sense Resistor
Channel Pointer
CH65 00110 0 0 1 1 0
(3) Sensor
Configuration
2-Wire with
Shared RSENSE
4 0001 0 0 0 1
(4) Excitation
Current
100µA 4 0101 0 1 0 1
(5) Curve European
α = 0.00385
2 00 0 0
(6) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Table 42. Channel Assignment Data for Sense Resistor (Value = 5001.5Ω)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x214
MEMORY
ADDRESS 0x215
MEMORY
ADDRESS 0x216
MEMORY
ADDRESS 0x217
(1) Sensor Type Sense Resistor 5 11101 1 1 1 0 1
(2) Sense
Resistor Value
5001.5Ω 27 000010011100010011000000000 000010011100010011000000000
APPLICATIONS INFORMATION
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Example: 3-Wire RTD
3-wire RTD channel assignments follow the general con-
vention shown in Figure 13. Terminals 1 and 2 tie to the
input/excitation current sources and terminal 3 connects
to the sense resistor
. Channel assignment data is mapped
to memory locations corresponding to CHRTD.
Sense resistor channel assignments follow the general
convention shown in Figure 14. The sense resistor is tied
between CHRSENSE and CHRSENSE-1, where CHRSENSE is
tied to the 3rd terminal of the RTD and CHRSENSE-1 is tied
to ground (or left floating for RSENSE sharing). Channel
assignment data (see Table 37) is mapped into the memory
location corresponding to CHRSENSE.
Figure 15 shows a typical temperature measurement sys-
tem using a 3-wire RTD. In this example, a 3-wire RTD’s
terminals tie to CH9, CH8, and CH7. The sense resistor
ties to CH7 and CH6. The sense resistor and RTD connect
together at CH7.
The 3-wire RTD reduces the errors associated with para-
sitic lead resistance by applying excitation current to each
RTD input. This first order cancellation removes matched
lead resistance errors. This cancellation does not remove
errors due to thermocouple effects or mismatched lead
resistances. The RTD sensor type and configuration data
are assigned to CH9. 32 bits of binary configuration data
are mapped directly into memory locations 0x220 to 0x223
(see Table 43). The sense resistor is assigned to CH7. The
user-programmable value of this resistor is 12150.39Ω.
32 bits of binary configuration data are mapped directly
into memory locations 0x218 to 0x21B (see Table 44).
A conversion is initiated on CH9 by writing 10001001 into
memory location 0x000 . Once the conversion is complete,
the INTERRUPT pin goes HIGH and memory location
0x000 becomes 01001001. The resulting temperature in
°C can be read from memory locations 0x030 to 0x033
(corresponding to CH9).
APPLICATIONS INFORMATION
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Figure 13. 3-Wire RTD Channel Assignment Convention
Figure 14. 3-Wire Sense Resistor Channel Assignment
Convention for 3-Wire RTDs
29861 F13
3RD TERMINAL TIES TO SENSE RESISTOR
2
3
1
CHRTD-1
CHRSENSE
CH
RTD
EXCITATION
CURRENT
FLOW = CHRTD (2 ≤ RTD ≤ 10)
CHANNEL
ASSIGNMENT
29861 F14
CHRSENSE-1
CHRSENSE
RSENSE
2x EXCITATION
CURRENT
FLOW = CHRSENSE (2 ≤ RSENSE ≤ 10)
(OPTIONAL GND, REMOVE FOR RSENSE SHARING)
CHANNEL
ASSIGNMENT
APPLICATIONS INFORMATION
Figure 15. 3-Wire RTD Example
RSENSE
12,150.39Ω
29861 F15
0.01µF
RSENSE ASSIGNED TO CH7 (CHRSENSE=7)
3-WIRE RTD ASSIGNED TO CH9 (CHRTD=9)
CH7
CH6
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x218 TO 0x21B
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x220 TO 0x223
RESULT MEMORY LOCATIONS 0x030 TO 0x033
0.01µF
0.01µF
CH8
CH9
3-WIRE PT-200
2
3
1
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Table 43. Channel Assignment Data for 3-Wire RTD (PT-200, RSENSE on CH7, 3-Wire, 50µA Excitation Current, α = 0.003911 Curve)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x220
MEMORY
ADDRESS 0x221
MEMORY
ADDRESS 0x222
MEMORY
ADDRESS 0x223
(1) RTD TYPE PT-200 5 01101 0 1 1 0 1
(2) Sense
Resistor Channel
Pointer
CH75 00111 0 0 1 1 1
(3) Sensor
Configuration
3-Wire 4 0100 0 1 0 0
(4) Excitation
Current
50µA 4 0100 0 1 0 0
(5) Curve American,
α = 0.003911
2 01 0 1
(6) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Table 44. Channel Assignment Data for Sense Resistor (Value = 12150.39Ω)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x218
MEMORY
ADDRESS 0x219
MEMORY
ADDRESS 0x21A
MEMORY
ADDRESS 0x21B
(1) Sensor Type Sense Resistor 5 11101 1 1 1 0 1
(2) Sense Resistor
Value
12150.39Ω 27 000101111011101100110001111 0 0 0 1 0 1 1 1 1 0 1 1 1 0 1 1 0 0 1 1 0 0 0 1 1 1 0
APPLICATIONS INFORMATION
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Example: Standard 4-Wire RTD (No Rotation or RSENSE
Sharing)
Standard 4-wire RTD channel assignments follow the
general convention shown in Figure 16. Terminal 1 is
tied to ground, terminals 2 and 3 (Kelvin sensed signal)
tie to CHRTD and CHRTD-1, and the 4th terminal ties to the
sense resistor. Channel assignment data (see Table 29)
is mapped to memory locations corresponding to CHRTD.
Sense resistor channel assignments follow the general
convention shown in Figure 17. The sense resistor is tied
between CHRSENSE and CHRSENSE-1, where CHRSENSE is
tied to the 4th terminal of the RTD. Channel assignment
data (see Table 37) is mapped into a memory location
corresponding to CHRSENSE.
Figure 18 shows a typical temperature measurement sys-
tem using a 4-wire RTD. In this example, a 4-wire RTD’s
APPLICATIONS INFORMATION
Figure 16. 4-Wire RTD Channel Assignment Convention
Figure 17. Sense Resistor Channel Assignment Convention for
4-Wire RTDs
29861 F16
3
4
1
2
CHRTD-1
CH
RTD
CH
RSENSE
4TH TERMINAL TIES TO SENSE RESISTOR (CH
RSENSE
)
EXCITATION
CURRENT
FLOW = CHRTD (2 ≤ RTD ≤ 10)
CHANNEL
ASSIGNMENT
29861 F17
CHRSENSE-1
CHRSENSE
RSENSE
EXCITATION
CURRENT
FLOW = CHRSENSE (2 ≤ RSENSE ≤ 10)
CHANNEL
ASSIGNMENT
terminals tie to GND, CH4, CH3, and CH2. The sense resistor
ties to CH2 and CH1. The sense resistor and RTD share
a common connection at CH2. The RTD sensor type and
configuration data are assigned to CH4. 32 bits of binary
configuration data are mapped directly into memory loca-
tions 0x20C to 0x20F (see Table 45). The sense resistor
is assigned to CH2. The user-programmable value of this
resistor is 5000.2Ω. 32 bits of binary configuration data
are mapped directly into memory locations 0x204 to 0x207
(see Table 46).
A conversion is initiated on CH4 by writing 10000100
into the data byte at memory location 0x000. Once the
conversion is complete, the INTERRUPT pin goes HIGH
and memory location 0x000 becomes 01000100. The
resulting temperature in °C can be read from memory
locations 0x01C to 0x01F (corresponding to CH4).
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APPLICATIONS INFORMATION
Table 45. Channel Assignment Data for 4-Wire RTD (PT-1000, RSENSE on CH2, Standard 4-Wire, 25µA Excitation Current,
α = 0.00385 Curve)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x20C
MEMORY
ADDRESS 0x20D
MEMORY
ADDRESS 0x20E
MEMORY
ADDRESS 0x20F
(1) RTD TYPE PT-1000 5 01111 0 1 1 1 1
(2) Sense
Resistor Channel
Pointer
CH25 00010 0 0 0 1 0
(3) Sensor
Configuration
4-Wire,
No Rotate,
No Share
4 1000 1 0 0 0
(4) Excitation
Current
25µA 4 0011 0 0 1 1
(5) Curve European,
α =0.00385
2 00 0 0
(6) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Table 46. Channel Assignment Data for Sense Resistor (Value = 5000.2Ω)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x204
MEMORY
ADDRESS 0x205
MEMORY
ADDRESS 0x206
MEMORY
ADDRESS 0x207
(1) Sensor Type Sense Resistor 5 11101 11101
(2) Sense
Resistor Value
5000.2Ω 27 000010011100010000011001100 0 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 1 1 0 0 1 1 0 0
Figure 18. Standard 4-Wire RTD Example
RSENSE
5000.2Ω
0.01µF
29861 F18
0.01µF
SENSE RESISTOR ASSIGNED TO CH2 (CHRSENSE=2)
RTD ASSIGNED TO CH4 (CHRTD=4)
CH2
CH1
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x204 TO 0x207
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x20C TO 0x20F
RESULT MEMORY LOCATIONS 0x01C TO 0x01F
0.01µF
0.01µF
CH3
CH4
4-WIRE PT-1000
3
4
2
1
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Example: 4-Wire RTD with Rotation
One method to improve the accuracy of an RTD over the
standard 4-wire implementation is by rotating the excita-
tion current source. Parasitic thermocouple effects are
automatically removed through autorotation. In order to
perform autorotation, the 1st terminal of the RTD ties to
CHRTD+1 instead of GND, as in the standard case. This
allows the LTC2986 to automatically change the direc-
tion of the current source without the need for additional
external components.
4-wire RTD with rotation channel assignments follow
the general convention shown in Figure 19. Terminal 1 is
tied to CHRTD+1, terminals 2 and 3 (Kelvin sensed signal)
tie to CHRTD and CHRTD-1, and the 4th terminal ties to the
sense resistor. Channel assignment data (see Table29) is
mapped to memory locations corresponding to CHRTD.
Sense resistor channel assignments follow the general
convention shown in Figure 20. The sense resistor is tied
between CHRSENSE and CHRSENSE-1, where CHRSENSE is
tied to the 4th terminal of the RTD. Channel assignment
APPLICATIONS INFORMATION
data is mapped into a memory location corresponding to
CHRSENSE.
Figure 21 shows a typical temperature measurement sys-
tem using a rotating 4-wire RTD. In this example a 4-wire
RTD’s terminals tie to CH10, CH9, CH8, and CH6. The sense
resistor is tied to CH6 and CH5. The sense resistor and
RTD connect together at CH6. The RTD sensor type and
configuration data are assigned to CH9. 32 bits of binary
configuration data are mapped directly into memory loca-
tions 0x220 to 0x223 (see Table 47). The sense resistor
is assigned to CH6. The user programmable value of this
resistor is 10.0102kΩ. 32 bits of binary configuration
data are mapped directly into memory locations 0x214
to 0x217 (see Table 48).
A conversion is initiated on CH9 by writing 10001001 into
memory location 0x000. Once the conversion is complete,
the INTERRUPT pin goes HIGH and memory location
0x000 becomes 01001001. The resulting temperature in
°C can be read from memory locations 0x030 to 0x033
(corresponding to CH9).
Figure 19. 4-Wire RTD Channel Assignment Convention
29861 F19
3
4
1
2
CHRTD–1
CHRTD
CHRTD+1
EXCITATION
CURRENT
FLOW = CHRTD (2 ≤ RTD ≤ 9)
CHRSENSE 4TH TERMINAL TIES TO SENSE RESISTOR
CHANNEL
ASSIGNMENT
Figure 20. Sense Resistor Channel Assignment Convention for
4-Wire RTDs with Rotation
29861 F20
CHRSENSE-1
CHRSENSE
RSENSE
EXCITATION
CURRENT
FLOW = CHRSENSE (2 ≤ RSENSE ≤ 10)
CHANNEL
ASSIGNMENT
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APPLICATIONS INFORMATION
Table 47. Channel Assignment Data for Rotating 4-Wire RTD (PT-100, RSENSE on CH6, Rotating 4-Wire, 100µA Excitation Current,
α = 0.003911 Curve)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x220
MEMORY
ADDRESS 0x221
MEMORY
ADDRESS 0x222
MEMORY
ADDRESS 0x223
(1) RTD TYPE PT-100 5 01100 0 1 1 0 0
(2) Sense
Resistor Channel
Pointer
CH65 00110 0 0 1 1 0
(3) Sensor
Configuration
4-Wire with
Rotation
4 1010 1 0 1 0
(4) Excitation
Current
100µA 4 0101 0 1 0 1
(5) Curve American,
α =0.003911
2 01 0 1
(6) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Table 48. Channel Assignment Data for Sense Resistor (Value = 10.0102kΩ)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x214
MEMORY
ADDRESS 0x215
MEMORY
ADDRESS 0x216
MEMORY
ADDRESS 0x217
(1) Sensor Type Sense Resistor 5 11101 1 1 1 0 1
(2) Sense Resistor
Value
10.0102kΩ 27 000100111000110100011001100 0 0 0 1 0 0 1 1 1 0 0 0 1 1 0 1 0 0 0 1 1 0 0 1 1 0 0
Figure 21. Rotating 4-Wire RTD Example
RSENSE
10.0102k
0.01µF
29861 F21
0.01µF
SENSE RESISTOR ASSIGNED TO CH6 (CHRSENSE=6)
RTD ASSIGNED TO CH9 (CHRTD=9)
CH6
CH5
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x214 TO 0x217
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x220 TO 0x223
RESULT MEMORY LOCATIONS 0x030 TO 0x033
0.01µF
CH8
CH9
CH10
PT-100 0.01µF
0.01µF
3
4
2
1
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APPLICATIONS INFORMATION
Example: Multiple 4-Wire RTDs with Shared RSENSE
Figure 22 shows a typical temperature measurement
system using two 4-wire RTDs with a shared RSENSE.
The LTC2986 can support up to two 4-wire RTDs with
a single sense resistor. In this example, the first 4-wire
RTD’s terminals tie to CH5, CH4, CH3, and CH2 and the
2nd ties to CH8, CH7, CH6, and CH2. The sense resistor
ties to CH1 and CH2. The sense resistor and both RTDs
connect together at CH2. This channel assignment conven-
tion is identical to that of the rotating RTD. This topology
supports both rotated and non-rotated RTD excitations.
Channel assignment data for each sensor is shown in
Tables 49 to 51.
A conversion is initiated on CH4 by writing 10000100 into
memory location 0x000. Once the conversion is complete,
the INTERRUPT pin goes HIGH and memory location
0x000 becomes 01000100. The resulting temperature in
°C can be read from memory locations 0x01C to 0x01F
(corresponding to CH4). A conversion can be initiated and
read from CH7 in a similar fashion.
Table 49. Channel Assignment Data for 4-Wire RTD #1 (PT-100, RSENSE on CH2, 4-Wire, Shared RSENSE, Rotated 100µA Excitation
Current, α = 0.003926 Curve)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x20C
MEMORY
ADDRESS 0x20D
MEMORY
ADDRESS 0x20E
MEMORY
ADDRESS 0x20F
(1) RTD TYPE PT-100 5 01100 0 1 1 0 0
(2) Sense
Resistor Channel
Pointer
CH25 00010 0 0 0 1 0
(3) Sensor
Configuration
4-Wire
Rotated
4 1010 1 0 1 0
(4) Excitation
Current
100µA 4 0101 0 1 0 1
(5) Curve ITS-90,
α =0.003926
2 11 1 1
(6) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Figure 22. Shared RSENSE 4-Wire RTD Example
RSENSE
10k
0.01µF
29861 F22
0.01µF
SENSE RESISTOR ASSIGNED TO CH2 (CHRSENSE=2)
RTD #1 ASSIGNED TO CH4 (CHRTD=4)
CH2
CH1
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x204 TO 0x207
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x20C TO 0x20F
RESULT MEMORY LOCATIONS 0x01C TO 0x01F
0.01µF
CH3
CH4
CH5
CH6
CH7
CH8
RTD #2 ASSIGNED TO CH7 (CHRTD=7)
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x218 TO 0x21B
RESULT MEMORY LOCATIONS 0x028 TO 0x02B
4-WIRE PT-100 0.01µF
0.01µF
3
4
2
1
0.01µF
4-WIRE PT-500 0.01µF
0.01µF
3
4
2
1
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APPLICATIONS INFORMATION
Table 50. Channel Assignment Data for 4-Wire RTD #2 (PT-500, RSENSE on CH2, 4-Wire, Rotated 50µA Excitation Current,
α = 0.003911 Curve)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x218
MEMORY
ADDRESS 0x219
MEMORY
ADDRESS 0x21A
MEMORY
ADDRESS 0x21B
(1) RTD TYPE PT-500 5 01110 0 1 1 1 0
(2) Sense
Resistor Channel
Pointer
CH25 00010 0 0 0 1 0
(3) Sensor
Configuration
4-Wire
Shared,
No Rotation
4 1001 1 0 0 1
(4) Excitation
Current
50µA 4 0100 0 1 0 0
(5) Curve American,
α =0.003911
2 01 0 1
(6) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Table 51. Channel Assignment Data for Sense Resistor (Value = 10.000kΩ)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x204
MEMORY
ADDRESS 0x205
MEMORY
ADDRESS 0x206
MEMORY
ADDRESS 0x207
(1) Sensor Type Sense Resistor 5 11101 11101
(2) Sense
Resistor Value
10.000kΩ 27 000100111000100000000000000 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
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Figure 23. Sense Resistor with Kelvin Connections Channel Assignment Convention
29861 F23
3
4
1
2
CHRSENSE–1
CHRSENSE–2
CHRSENSE
RSENSE
TIES TO RTD TERMINAL 4
EXCITATION
CURRENT
FLOW = CHRSENSE (3 ≤ RSENSE ≤ 10)
CHANNEL
ASSIGNMENT
Figure 24. Sense Resistor with Kelvin Connections Example
29861 F24
SENSE RESISTOR ASSIGNED TO CH6 (CHRSENSE=6)
CH4
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x214 TO 0x217
0.01µF
CH5
CH6
CH8
CH9
CH10
RTD ASSIGNED TO CH9 (CHRTD=9)
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x220 TO 0x223
RESULTS MEMORY LOCATIONS 0x030 TO 0x033
RSENSE
1k
0.01µF
0.01µF
3
4
2
1
0.01µF
4-WIRE PT-10 0.01µF
0.01µF
3
4
2
1
Example: 4-Wire RTD with Kelvin RSENSE
It is possible to cancel the parasitic lead resistance in
the sense resistors by configuring the 4-wire RTD with
a 4-wire (Kelvin connected) sense resistor. This is useful
when using a PT-10 or PT-50 with a small valued RSENSE
or when the sense resistor is remotely located or in ap-
plications requiring extreme precision.
The 4-wire RTD channel assignments follow the general
conventions previously defined (Figure 19) for a standard
4-wire RTD. The sense resistor follows the convention
shown in Figure 23.
Figure 24 shows a typical temperature measurement sys-
tem using a 4-wire RTD with a Kelvin connected RSENSE.
In this example, the 4-wire RTD’s terminals tie to CH10,
CH9, CH8, and CH6. The sense resistor ties to CH6, CH5,
and CH4 and excitation current is applied to CH4 and
CH10. The sense resistor’s nominal value is 1kΩ in order
to accommodate a 1mA excitation current. The sense
resistor and RTD connect together at CH6. This topology
supports rotated, shared and standard 4-wire RTD topolo-
gies. If rotated or shared configuration are not used then
terminal 1 of the RTD is tied to ground instead of CH10,
freeing up one input channel. Channel assignment data
is shown in Tables 52 and 53.
A conversion is initiated on CH9 by writing 10001001 into
memory location 0x000. Once the conversion is complete,
the INTERRUPT pin goes HIGH and memory location
0x000 becomes 01001001 (see Table 6). The resulting
temperature in °C can be read from memory locations
0x030 to 0x033 (corresponding to CH9).
APPLICATIONS INFORMATION
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Table 52. Channel Assignment Data for 4-Wire RTD with Kelvin Connected RSENSE (PT-10, RSENSE on CH6, 4-Wire, Kelvin RSENSE with
Rotated 1mA Excitation Current, α = 0.003916 Curve)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x220
MEMORY
ADDRESS 0x221
MEMORY
ADDRESS 0x222
MEMORY
ADDRESS 0x223
(1) RTD TYPE PT-10 5 01010 0 1 0 1 0
(2) Sense Resistor
Channel Pointer
CH65 00110 0 0 1 1 0
(3) Sensor
Configuration
4-Wire Kelvin RSENSE
and Rotation
4 1110 1 1 1 0
(4) Excitation Current 1mA 4 1000 1 0 0 0
(5) Curve Japanese,
α =0.003916
2 10 1 0
(6) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Table 53. Channel Assignment Data for Sense Resistor (Value = 1000Ω)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x214
MEMORY
ADDRESS 0x215
MEMORY
ADDRESS 0x216
MEMORY
ADDRESS 0x217
(1) Sensor Type Sense Resistor 5 11101 11101
(2) Sense
Resistor Value
1000Ω 27 000000011111010000000000000 0 0 0 0 0 0 0 1 1 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0
APPLICATIONS INFORMATION
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THERMISTOR MEASUREMENTS
Channel Assignment – Thermistor
For each thermistor tied to the LTC2986, a 32-bit channel
assignment word is programmed into a memory location
corresponding to the channel the sensor is tied to (see
Table 54). This data includes (1) thermistor type, (2)
sense resistor channel pointer, (3) sensor configuration,
(4) excitation current, (5) Steinhart-Hart address pointer
or custom table address pointer.
(1) Thermistor Type
The thermistor type is determined by the first five input
bits (B31 to B27) as shown in Table 55. Linearization coef-
ficients based on Steinhart-Hart equation for commonly
APPLICATIONS INFORMATION
Table 54. Thermistor Channel Assignment Word
(1) THERMISTOR
TYPE
(2) SENSE RESISTOR
CHANNEL POINTER
(3) SENSOR
CONFIGURATION
(4) EXCITATION
CURRENT
(5) CUSTOM THERMISTOR
DATA POINTER
TABLE 55 TABLE 31 TABLE 56 TABLE 57 TABLES 96, 97, 98, 100, 101
Measurement Class 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Thermistor Type = 19 to 27 RSENSE Channel
Pointer [4:0]
SGL = 1
DIFF = 0
Excitation
Mode
Excitation Current
[3:0]
Not Used
0 0 0
Custom Address
[5:0]
Custom Length –1
[5:0]
Table 55. Thermistor Type: 1/T=A+B•ln(R)+C•ln(R)2 + D•ln(R)3 + E•ln(R)4 + F•ln(R)5
B31 B30 B29 B28 B27 THERMISTOR TYPE A B C D E F
1 0 0 1 1 Thermistor 44004/44033
2.252kΩ at 25°C
1.46800E-03 2.38300E-04 0 1.00700E-07 0 0
1 0 1 0 0 Thermistor 44005/44030
3kΩ at 25°C
1.40300E-03 2.37300E-04 0 9.82700E-08 0 0
1 0 1 0 1 Thermistor 44007/44034
5kΩ at 25°C
1.28500E-03 2.36200E-04 0 9.28500E-08 0 0
1 0 1 1 0 Thermistor 44006/44031
10kΩ at 25°C
1.03200E-03 2.38700E-04 0 1.58000E-07 0 0
1 0 1 1 1 Thermistor 44008/44032
30kΩ at 25°C
9.37600E-04 2.20800E-04 0 1.27600E-07 0 0
1 1 0 0 0 Thermistor YSI-400
2.252kΩ at 25°C
1.47134E-03 2.37624E-04 0 1.05034E-07 0 0
1 1 0 0 1 Spectrum 1003k 1kΩ
at 25°C
1.445904E-3 2.68399E-04 0 1.64066E-07 0 0
1 1 0 1 0 Thermistor Custom
Steinhart-Hart
user input user input user input user input user input user input
1 1 0 1 1 Thermistor Custom Table not used not used not used not used not used not used
used Thermistor types 44004/44033, 44005/44030,
44006/44031, 44007/44034, 44008/44032 and YSI-400
are built into the device. If other custom thermistors are
used, Thermistor Custom Steinhart-Hart or Thermis-
tor Custom Table (temperature vs resistance) can be
selected. In this case, user specific data can be stored
in the on-chip RAM starting at the address defined in
Thermistor Custom Steinhart-Hart or Thermistor Custom
Table address pointers.
(2) Sense Resistor Channel Pointer
Thermistor measurements are performed ratiometrically
relative to a known RSENSE resistor. The sense resistor
channel pointer field indicates the differential channel
the sense resistor is tied to for the current thermistor
(see Table 31).
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APPLICATIONS INFORMATION
(3) Sensor Configuration
The sensor configuration field is used to define various
thermistor properties. Configuration bit B21 is set high
for single-ended (measurement relative to COM) and low
for differential (see Table 56).
(4) Excitation Current
The next field in the channel assignment word (B18 to B15)
controls the magnitude of the excitation current applied to
the thermistor (see Table 57). In order to prevent hard or
soft faults, select a current such that the maximum volt-
age drop across the sensor or sense resistor is nominally
1.0V. The LTC2986 has no special requirements related
to the ratio between the voltage drop across the sense
resistor and the sensor. Consequently, it is possible to
have a sense resistor several orders of magnitude smaller
than the maximum sensor value. For optimal performance
over the full thermistor temperature range, auto ranged
current can be selected. In this case, the LTC2986 conver-
sion is performed in three cycles (instead of the standard
two cycles) (see Table 83). The first cycle determines the
optimal excitation current for the sensor resistance value
and RSENSE value. The following two cycles use that cur-
rent to measure the thermistor temperature.
(5) Steinhart-Hart Address/Custom Table Address
See Custom Thermistors section near the end of this data
sheet for more information.
Table 56. Sensor Configuration Data
(3) SENSOR
CONFIGURATION
SGL
EXCITATION
MODE
SINGLE-ENDED/
DIFFERENTIAL
SHARE
RSENSE ROTATE
B21 B20 B19
0 0 0 Differential No No
0 0 1 Differential Yes Yes
0 1 0 Differential Yes No
0 1 1 Reserved
1 0 0 Single-Ended No No
1 0 1 Reserved
1 1 0 Reserved
1 1 1 Reserved
Table 57. Excitation Current for Thermistors
(4) EXCITATION CURRENT
B18 B17 B16 B15 CURRENT
0 0 0 0 Reserved
0 0 0 1 250nA
0 0 1 0 500nA
0 0 1 1 1µA
0 1 0 0 5µA
0 1 0 1 10µA
0 1 1 0 25µA
0 1 1 1 50µA
1 0 0 0 100µA
1 0 0 1 250µA
1 0 1 0 500µA
1 0 1 1 1mA
1 1 0 0 Auto Range*
1 1 0 1 Invalid
1 1 1 0 Invalid
1 1 1 1 External
*Auto Range not allowed for custom sensors.
The next sensor configuration bits (B19 and B20) deter-
mine the excitation current mode. These bits are used to
enable RSENSE sharing, where one sense resistor is used
for multiple thermistors. In this case, the thermistor ground
connection is internal and each thermistor points to the
same RSENSE channel.
Bits B19 and B20 are also used to enable excitation current
rotation to automatically remove parasitic thermocouple
effects. Parasitic thermocouple effects may arise from
the physical connection between the thermistor and the
measurement instrument. This mode is available for dif-
ferential thermistor configurations using internal current
source excitation.
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Fault Reporting – Thermistor
Each sensor type has unique fault reporting mechanism
indicated in the upper byte of the data output word.
Table58 shows faults reported during the measurement
of thermistors.
Bit D31 indicates the thermistor or RSENSE is open, shorted,
or not plugged in. This is a hard fault and –999°C is re-
ported. Bit D30 indicates a bad ADC reading. This could be
a result of either a broken (open) sensor or an excessive
noise event (ESD or static discharge into the sensor path).
Table 58. Thermistor Fault Reporting
BIT FAULT ERROR TYPE DESCRIPTION OUTPUT RESULT
D31 Sensor Hard Fault Hard Open or Short Thermistor or RSENSE –999°C
D30 Hard ADC-Out-of-Range Hard Bad ADC Reading (Could Be Large External Noise Event) –999°C
D29 Not Used for Thermistors N/A Always 0 Valid Reading
D28 Not Used for Thermistors N/A Always 0 Valid Reading
D27 Sensor Over Range* Soft T > High Temp Limit Suspect Reading
D26 Sensor Under Range* Soft T < Low Temp Limit Suspect Reading
D25 ADC Out-of-Range Soft ADC Absolute Input Voltage Is Beyond ±1.125 • VREF/2 Suspect Reading
D24 Valid N/A Result Valid (Should Be 1) Discard Results if 0 Valid Reading
*Do not apply to custom Steinhart-Hart sensor type. Custom table thermistor over/under range is determined by the resistor table values, see custom
thermistor table example for details.
APPLICATIONS INFORMATION
This is a hard error and –999°C is output. In the case of
an excessive noise event, the device should recover and
the following conversions will be valid if the noise event
was a random infrequent event. Bits D29 and D28 are not
used for thermistors. Bits D27 and D26 indicate the read-
ing is over or under temperature limits (see Table 59). The
calculated temperature is reported, but the accuracy may
be compromised. Bit D25 indicates the absolute voltage
measured by the ADC is beyond its normal operating range.
If a thermistor is used as the cold junction element, any
hard or soft error is flagged in the thermocouple result.
Table 59. Thermistor Temperature/Resistance Range
THERMISTOR TYPE MIN (Ω) MAX (Ω) LOW Temp Limit (°C) HIGH Temp Limit (°C)
Thermistor 44004/44033 2.252kΩ at 25°C 41.9 75.79k –40 150
Thermistor 44005/44030 3kΩ at 25°C 55.6 101.0k –40 150
Thermistor 44007/44034 5kΩ at 25°C 92.7 168.3k –40 150
Thermistor 44006/44031 10kΩ at 25°C 237.0 239.8k –40 150
Thermistor 44008/44032 30kΩ at 25°C 550.2 884.6k –40 150
Thermistor YSI 400 2.252kΩ at 25°C 6.4 1.66M –80 250
Spectrum 1003K 1kΩ at 25°C 51.1 39.51k –50 125
Thermistor Custom Steinhart-Hart N/A N/A N/A N/A
Thermistor Custom Table Second Table Entry Last Table Entry
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Figure 25. Single-Ended Thermistor Channel Assignment
Convention
29861 F25
2
1
CHTHERM
COM
EXCITATION
CURRENT
FLOW
= CHTHERM (1 ≤ THERM ≤ 10)
2ND TERMINAL TIES TO SENSE RESISTOR (CHRSENSE)
CHANNEL
ASSIGNMENT
Figure 26. Sense Resistor Channel Assignment Convention
29861 F26
CHRSENSE-1
CHRSENSE
RSENSE
EXCITATION
CURRENT
FLOW = CHRSENSE (2 ≤ RSENSE ≤ 10)
CHANNEL
ASSIGNMENT
Figure 27. Single-Ended Thermistor Example
29861 F27
SENSE RESISTOR ASSIGNED TO CH4 (CHRSENSE=4)
CH3
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x20C TO 0x20F
CH4
CH5
COM
THERMISTOR ASSIGNED TO CH5 (CHTHERM=5)
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x210 TO 0x213
RESULT MEMORY LOCATIONS 0x020 TO 0x023
RSENSE
10.1k
100pF
100pF
TYPE 44031
100pF
2
1
Example: Single-Ended Thermistor
The simplest thermistor configuration is the single-ended
configuration. Thermistors using this configuration share
a common ground (COM) between all sensors and are
each tied to a unique sense resistor (RSENSE sharing is
not allowed for single-ended thermistors). Single-ended
thermistors follow the convention shown in Figure 25.
Terminal 1 ties to ground (COM) and terminal 2 ties to
CHTHERM and the sense resistor. Channel assignment
data (see Table 54) is mapped to memory locations cor-
responding to CHTHERM.
Sense resistor channel assignments follow the general
convention shown in Figure 26. The sense resistor is tied
between CHRSENSE and CHRSENSE-1, where CHRSENSE is tied
Figure 27 shows a typical temperature measurement
system using a single-ended thermistor. In this example
a 10kΩ (44031 type) thermistor is tied to a 10.1kΩ sense
resistor. The thermistor is assigned channel CH5 (memory
locations 0x210 to 0x213) and the sense resistor to CH4
(memory locations 0x20C to 0x20F). Channel assignment
data are shown in Tables 60 and 61.
A conversion is initiated on CH5 by writing 10000101 into
memory location 0x000. Once the conversion is complete,
the INTERRUPT pin goes HIGH and memory location
0x000 becomes 01000101. The resulting temperature in
°C can be read from memory locations 0x020 to 0x023
(corresponding to CH5).
to the 2nd terminal of the thermistor. Channel assignment
data (see Table 37) is mapped into the memory location
corresponding to CHRSENSE.
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Table 60. Channel Assignment Data for Single-Ended Thermistor (44006/44031 10kΩ at 25°C Type Thermistor, Single-Ended
Configuration, RSENSE on CH4, 1µA Excitation Current)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x210
MEMORY
ADDRESS 0x211
MEMORY
ADDRESS 0x212
MEMORY
ADDRESS 0x213
(1) Thermistor
Type
44006/44031
10kΩ at 25°C
5 10110 10110
(2) Sense
Resistor Channel
Pointer
CH45 00100 0 0 1 0 0
(3) Sensor
Configuration
Single-Ended 3 100 1 0 0
(4) Excitation
Current
1µA 4 0011 0 0 1 1
Not Used Set These Bits
to 0
3 000 0 0 0
(5) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Table 61. Channel Assignment Data for Sense Resistor (Value = 10.1kΩ)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x20C
MEMORY
ADDRESS 0x20D
MEMORY
ADDRESS 0x20E
MEMORY
ADDRESS 0x20F
(1) Sensor Type Sense Resistor 5 11101 11101
(2) Sense
Resistor Value
10.1kΩ 27 000100111011101000000000000 0 0 0 1 0 0 1 1 1 0 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0
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Example: Differential Thermistor
The differential thermistor configuration allows separate
ground sensing for each sensor. In this standard differ-
ential configuration, one sense resistor is used for each
thermistor. Differential thermistors follow the convention
shown in Figure 28. Terminal 1 ties to CHTHERM and is
shorted to ground and terminal 2 ties CHTHERM-1 to and
the sense resistor. Channel assignment data (see Table 54)
is mapped to memory locations corresponding to CHTHERM.
APPLICATIONS INFORMATION
Sense resistor channel assignments follow the general
convention shown in Figure 29. The sense resistor is tied
between CHRSENSE and CHRSENSE-1, where CHRSENSE is tied
to the 2nd terminal of the thermistor. Channel assignment
data (see Table 37) is mapped into a memory location
corresponding to CHRSENSE.
Figure 28. Differential Thermistor Channel Assignment
Convention
29861 F28
2
1CHTHERM
CHTHERM–1
EXCITATION
CURRENT
FLOW = CHTHERM (2 ≤ THERM ≤ 10)
2ND TERMINAL TIES TO SENSE RESISTOR
1ST TERMINAL TIES TO GND
CHANNEL
ASSIGNMENT
Figure 30 shows a typical temperature measurement
system using a differential thermistor. In this example a
30kΩ(44032 type) thermistor is tied to a 9.99kΩ sense
resistor. The thermistor is assigned channel CH9 (memory
locations 0x220 to 0x223) and the sense resistor to CH7
(memory locations 0x218 to 0x21B). Channel assignment
data is shown in Table 62 and Table 63).
A conversion is initiated on CH9 by writing 10001001 into
memory location 0x000. Once the conversion is complete,
the INTERRUPT pin goes HIGH and memory location
0x000 becomes 01001001. The resulting temperature in
°C can be read from memory locations 0x030 to 0x033
(Corresponding to CH9).
Figure 29. Sense Resistor Channel Assignment Convention
29861 F29
CHRSENSE-1
CHRSENSE
RSENSE
EXCITATION
CURRENT
FLOW = CHRSENSE (2 ≤ RSENSE ≤ 10)
CHANNEL
ASSIGNMENT
Figure 30. Differential Thermistor Example
29861 F30
SENSE RESISTOR ASSIGNED TO CH7 (CHRSENSE=7)
CH6
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x218 TO 0x21B
CH7
CH8
CH9
THERMISTOR ASSIGNED TO CH9 (CHTHERM=9)
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x220 TO 0x223
RESULT MEMORY LOCATIONS 0x030 TO 0x033
RSENSE
9.99k
100pF
100pF
TYPE 44032 100pF
2
1
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Table 62. Channel Assignment Data for Differential Thermistor (44008/44032 30kΩ at 25°C Type Thermistor, Differential
Configuration, RSENSE on CH7, Auto Range Excitation)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x220
MEMORY
ADDRESS 0x221
MEMORY
ADDRESS 0x222
MEMORY
ADDRESS 0x223
(1) Thermistor
Type
44008/44032
30kΩ at 25°C
5 10111 1 0 1 1 1
(2) Sense
Resistor Channel
Pointer
CH75 00111 0 0 1 1 1
(3) Sensor
Configuration
Differential,
No Share,
No Rotate
3 000 0 0 0
(4) Excitation
Current
Auto Range 4 1100 1 1 0 0
Not Used Set These Bits
to 0
2 000 0 0 0
(5) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Table 63. Channel Assignment Data for Sense Resistor (Value = 9.99kΩ)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x218
MEMORY
ADDRESS 0x219
MEMORY
ADDRESS 0x21A
MEMORY
ADDRESS 0x21B
(1) Sensor Type Sense
Resistor
5 11101 1 1 1 0 1
(2) Sense
Resistor Value
9.99kΩ 27 000100111000001100000000000 0 0 0 1 0 0 1 1 1 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0
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Example: Shared/Rotated Differential Thermistor
The differential thermistor configuration allows separate
internal ground sensing for each sensor. In this configura-
tion, one sense resistor can be used for multiple thermis-
tors. Differential thermistors follow the convention shown
in Figure 31. Terminal 1 ties to CHTHERM and terminal 2
ties to CHTHERM-1 and the sense resistor. Channel assign-
ment data (see Table 54) is mapped to memory locations
corresponding to CHTHERM.
Figure 31. Thermistor with Shared RSENSE Channel
Assignment Convention
29861 F31
2
1
CHTHERM–1
CHTHERM
EXCITATION
CURRENT
FLOW = CHTHERM (2 ≤ THERM ≤ 10)
2ND TERMINAL TIES TO SENSE RESISTOR
CHANNEL
ASSIGNMENT
Sense resistor channel assignments follow the general
convention shown in Figure 32. The sense resistor is tied
between CHRSENSE and CHRSENSE-1, where CHRSENSE is
tied to the 2nd terminal of the thermistor. Channel as-
signment data (see Table 37) is mapped into a memory
location corresponding to CHTHERM.
Figure 32. Sense Resistor Channel Assignment
Convention for Thermistors
29861 F32
CHRSENSE-1
CHRSENSE
RSENSE
EXCITATION
CURRENT
FLOW = CHRSENSE (2 ≤ RSENSE ≤ 10)
CHANNEL
ASSIGNMENT
Figure 33 shows a typical temperature measurement
system using a shared sense resistor and one rotated/
non-rotated differential thermistors. In this example a
30kΩ(44032 Type) thermistor is tied to a 10.0kΩ sense
resistor and configured as rotated/shared. The second
thermistor a 2.25kΩ (44033 type) is configured as a
non-rotated/shared. Channel assignment data are shown
in Tables 64 to 66.
A conversion is initiated on CH8 by writing 10001000 into
memory location 0x000. Once the conversion is complete,
the INTERRUPT pin goes HIGH and memory location
0x000 becomes 01001000. The resulting temperature in
°C can be read from memory locations 0x02C to 0x02F
(corresponding to CH8). A conversion can be initiated and
read from CH10 in a similar fashion.
Figure 33. Rotated and Shared Thermistor Example
29861 F33
THERMISTOR #1 ASSIGNED TO CH8 (CHTHERM=8)
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x21C TO 0x21F
RESULT MEMORY LOCATIONS 0x02C TO 0x02F
100pF
CH7
CH6
CH5
CH8
CH9
CH10
THERMISTOR #2 ASSIGNED TO CH10 (CHTHERM=10)
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x224 TO 0x227
RESULT MEMORY LOCATIONS 0x034 TO 0x037
SENSE RESISTOR ASSIGNED TO CH6 (CHRSENSE=6)
CHANNEL ASSIGNMENT
MEMORY LOCATIONS 0x214 TO 0x217
TYPE 44032
RSENSE
10k
100pF
2
1
100pF
TYPE 44033 100pF
2
1
100pF
100pF
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Table 64. Channel Assignment Data Differential Thermistor (44008/44032 30kΩ at 25°C Type Thermistor, Differential Configuration
with Sharing and Rotation, RSENSE on CH6, 250nA Excitation Current)
CONFIGURATION
FIELD DESCRIPTION # BITS BINARY DATA
MEMORY
ADDRESS 0x21C
MEMORY
ADDRESS 0x21D
MEMORY
ADDRESS 0x21E
MEMORY
ADDRESS 0x21F
(1) Thermistor
Type
44008/44032
30kΩ at 25°C
5 10111 1 0 1 1 1
(2) Sense
Resistor Channel
Pointer
CH65 00110 0 0 1 1 0
(3) Sensor
Configuration
Differential,
Rotate and
Shared
3 001 0 0 1
(4) Excitation
Current
250nA
Excitation
Current
4 0001 0 0 0 1
Not Used Set These Bits
to 0
3 000 0 0 0
(5) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
Table 66. Channel Assignment Data for Sense Resistor (Value = 10.0kΩ)
Configuration
Field Description # Bits Binary Data
MEMORY
ADDRESS 0x214
MEMORY
ADDRESS 0x215
MEMORY
ADDRESS 0x216
MEMORY
ADDRESS 0x217
(1) Sensor Type Sense Resistor 5 11101 11101
(2) Sense
Resistor Value
10.0kΩ 27 000100111000100000000000000 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Table 65. Channel Assignment Data Differential Thermistor (44004/44033 2.252kΩ at 25°C Type Thermistor, Differential
Configuration with Sharing and No Rotation, RSENSE on CH6, 10µA Excitation Current)
Configuration
Field Description # Bits Binary Data
MEMORY
ADDRESS 0x224
MEMORY
ADDRESS 0x225
MEMORY
ADDRESS 0x226
MEMORY
ADDRESS 0x227
(1) Thermistor
Type
44004/44033
2.252kΩ at
25°C
5 10011 1 0 0 1 1
(2) Sense
Resistor Channel
Pointer
CH65 00110 0 0 1 1 0
(3) Sensor
Configuration
Differential,
No Rotate
and Shared
3 010 0 1 0
(4) Excitation
Current
10µA
Excitation
Current
4 0101 0 1 0 1
Not Used Set These
Bits to 0
3 000 0 0 0
(5) Custom RTD
Data Pointer
Not Custom 12 000000000000 0 0 0 0 0 0 0 0 0 0 0 0
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GLOBAL CONFIGURATION REGISTER
Table 67 shows a summary of the global configuration
register. The global configuration register is 1 byte long
and is at memory location 0x0F0. Bits G0 – G1 set the
notch frequency of the on chip digital filter, Bit G2 sets the
temperature results unit (°C or °F), Bits G3 and G7 are
reserved and should be set low, Bits G4 – G6 determine
the Kelvin current excitation mode.
Table 67. Global Configuration Register
Bit # Field Name Description
G0
G1
Filter Frequency Select [1:0] 00 = 55Hz
01 = 60Hz
10 = 50Hz
G2 Temperature Result Format 0 = Celsius
1 = Fahrenheit
G3 Reserved Set to 0
G4 3-Wire RTD Kelvin Current
Excitation Mode
Excitation Current on
Adjacent Channels
G5 2-Wire RTD Kelvin Current
Excitation Mode
Excitation Current on
Adjacent Channels
G6 Thermistor Kelvin Current
Excitation Mode
Excitation Current on
Adjacent Channels
G7 Reserved Set to 0
INPUT OVERVOLTAGE PROTECTION – OVERVIEW
Temperature sensors are often used in harsh environ-
ments. The sensors or leads can short to high voltages
or each other
. Resistive circuits can protect the LTC2986
from these fault conditions. These external resistors can
potentially introduce measurement errors; however, the
LTC2986 includes special modes and features that reduce
these effects
The tip of a thermocouple is often unshielded, creating a
low impedance conductive path to the input of the mea-
surement device. In order to protect the LTC2986 from
damage due to overvoltage conditions, current-limiting
resistors can be placed between the input channels and the
thermocouple sensor
. The value of this resistor is chosen
such that at the maximum overvoltage, the current enter-
ing the LTC2986 is less than ±15mA. Errors due to these
protection resistors are minimal during normal operation
due to the very low input leakage (1nA) specifications of
the LTC2986.
Most RTD sensor elements are electrically isolated from
the sensor leads either through a non-conductive encap-
sulation or a separate grounding shield. While these types
of sensors may not require input overvoltage protection,
certain applications may require current limiting resistors
between the RTD and the LTC2986. One such application
is a universal input device where an input terminal can
see either an RTD or a thermocouple. Other applications
may require protection against erroneous connections, for
example, connecting a voltage source accidentally to the
RTD input terminals. The protection circuits implemented
for RTDs should accommodate 2-wire, 3-wire, and 4-wire
configurations.
Thermistors are 2-wire resistance to temperature sensors
with a non-conductive encapsulation enclosing the sensor
element. Similar to the RTD, overvoltage protection for
thermistors may be required for both universal sensor
input applications and inadvertent user applied overvoltage.
The LTC2986 offer several current excitation modes for
eliminating errors due to resistive overvoltage protection
circuits. The following sections describe overvoltage
protection circuits for thermocouples, 2-, 3-, and 4-wire
RTDs and thermistors with an emphasis on universality
(sharing the same protection scheme for all sensor types).
Input Overvoltage Protection – Resistor Value Selection
The maximum continuous current the LTC2986 can sus-
tain without damage is ±15mA. In order to determine the
value of the overvoltage protection resistor and its power
rating, the maximum voltage is required. This voltage is
application specific and depends on the maximum antici-
pated overvoltage. For example, a system with possible
overvoltage of 40V would require a resistance R > 2.7kΩ
and a power rating (see Figure 34 and Figure 35):
P > 600mW, where R >
V
MAX
– V
DD
15mA
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Figure 36. Thermocouple with Protection Resistors
29861 F36
CH1
COM
2k
2k ERROR = 4μV
+
–
1nA (MAX)
= EXTERNAL TERMINAL CAN BE
EXPOSED TO FAULT VOLTAGE.
Input Overvoltage Protection – Thermocouples
Thermocouples are low impedance devices that generate
voltage as a function of temperature differences. Since
the LTC2986 input impedance is very high (input leak-
age<1nA) external overvoltage protection resistors have
minimal effect on the temperature measurement accuracy.
For example, a 2kΩ protection resistor results in a worst-
case error of4µV (see Figure 36). This corresponds to a
0.1°C error for a Type K thermocouple at 25°C.
In addition to the protection resistors, 100pF capacitors
should be added to each input for anti-alias filtering; these
are not shown in the following schematics for simplicity.
Figure 34. Maximum Fault Voltage vs Minimum
Protection Resistance
Figure 35. Maximum Fault Voltage vs Minimum
Protection Resistor Power Rating
MAXIMUM FAULT VOLTAGE (V)
0
40
80
120
160
200
0
1.3
2.7
4.0
5.3
6.7
8.0
9.3
10.7
12.0
13.3
MINIMUM PROTECTION RESISTANCE (kΩ)
vs Minimum Protection Resistance
29861 F34
MAXIMUM FAULT VOLTAGE (V)
0
40
80
120
160
200
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
RESISTOR WATTAGE (W)
MINIMUM PROTECTION
Power Rating
29861 F35
Input Overvoltage Protection – RTDs
RTDs are resistive devices that require excitation current
in order to determine their temperature. The excitation is
applied to the series network consisting of the RTD and
a sense resistor in order to make a ratiometric measure-
ment. Overvoltage protection is implemented by placing
a resistor between each RTD terminal and the LTC2986
input channels.
4-Wire RTDs
The simplest RTD configuration to protect is the 4-wire RTD.
A protection resistor is tied to each of the 4 RTD terminals
(see Figure 37). Excitation current flows through the sense
resistor (RSENSE), the RTD, and protection resistors RP1
and RP4. The LTC2986 measures the voltage drop across
the RTD using CH3 and CH4 through protection resistors
RP2 and RP3. Since the excitation current does not flow
through RP2 and RP3, errors due to the protection resis-
tance are negligible. Measurement errors are dominated
by input leakage current (I < 1nA). For example, errors
due to leakage current for a PT-100 RTD with 1kΩ sense
resistor and 1kΩ protection resistors are below 0.025°C.
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Figure 37. 4-Wire RTD with Protection Resistors
29861 F37
CH2
CH1
CH3
CH5
CH4
RP1
RP2
ILEAK
= EXTERNAL TERMINAL CAN BE
EXPOSED TO FAULT VOLTAGE.
RP4
RP3
ILEAK
3
4
2
1
RSENSE
EXCITATION
CURRENT
FLOW
29861 F38
CH2
CH1
CH3
CH4
RP1
RP2
= EXTERNAL TERMINAL CAN BE
EXPOSED TO FAULT VOLTAGE.
RP3
2
3
1
RSENSE
EXCITATION
CURRENT
FLOW
I2
I1
Figure 38. 3-Wire RTD with Protection Resistors
3-Wire RTDs
3-wire RTDs are more difficult to protect than 4-wire RTDs.
Normally, protection resistors are tied to each of the 3 RTD
terminals (see Figure 38). The LTC2986 provides two matched
excitation currents, I1 and I2. These currents flow from CH3
and CH4 through RP2 and RP3 into the RTD. The resulting
voltage is measured between CH3 and CH4. Assuming RP2
= RP3 and I1 = I2, the errors resulting from the protection
resistors are cancelled. While the LTC2986 provides matched
current source excitation, external protection resistors may
be difficult to match. Every 1Ω mismatch in RP2 and RP3
translates to a 1Ω error in the RTD measurement.
The LTC2986 offers a 3-wire RTD Kelvin current source
mode in order to remove errors due to mismatched protec-
tion resistors. This feature is enabled by setting the global
3-wire RTD Kelvin current excitation mode bit (G4 = 1, see
Table 67) prior to initiating a conversion start. This mode
uses the adjacent channels (in this example CH5 and CH6)
for the current source excitation and performs the mea-
surement on CH3 and CH4 (see Figure 39). Two additional
resistors are placed between the RTD and the channels
tied to the excitation current sources, but the protection
resistor matching constraint is removed for all resistors.
The excitation current no longer flows through RP2 or
RP3, removing the voltage drop across them. Figure 40
shows the channel assignment convention for this mode.
Figure 39. 3-Wire RTD Kelvin Current Mode (G4 = 1)
Figure 40. 3-Wire RTD Kelvin Current Mode Channel
Assignment Convention (G4 = 1)
29861 F39
CH2
CH1
CH3
CH4
RP1
RP2
= EXTERNAL TERMINAL CAN BE
EXPOSED TO FAULT VOLTAGE.
RP3
CH5
RP4
CH6
RP5
2
3
1
RSENSE
EXCITATION
CURRENT
FLOW
I2
I1
ILEAK
ILEAK
29861 F40
23
1
EXCITATION
CURRENT
FLOW
CHRSENSE
CHRTD–1
CHRTD+1
CHRTD
CHRTD+2
= CHRTD (2 ≤ RTD ≤ 8)
GLOBAL REGISTER G4 = 1
CHANNEL
ASSIGNMENT
APPLICATIONS INFORMATION
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Figure 41. 2-wire RTD with Protection Resistors
29861 F41
CH2
CH1
CH3
CH4
CH5
RP1
RP2
= EXTERNAL TERMINAL CAN BE
EXPOSED TO FAULT VOLTAGE.
RP3
2
1
RSENSE
EXCITATION
CURRENT
FLOW
I1
29861 F42
CH2
CH1
CH3
CH4
CH5
RP1
RP2
= EXTERNAL TERMINAL CAN BE
EXPOSED TO FAULT VOLTAGE.
RP3
RP4
2
1
RSENSE
EXCITATION
CURRENT
FLOW
I1
ILEAK
ILEAK
Figure 42. 2-Wire RTD Kelvin Current Mode (G5 = 1)
2-Wire RTDs
2-wire RTDs are difficult to protect because the protection
resistor (RP3) is in series with the RTD (see Figure 41).
Every 1Ω of protection resistance adds 1Ω measurement
error to the RTD.
The LTC2986 offers a 2-wire Kelvin current source mode
in order to remove the errors associated with protection
resistors. This feature is enabled by setting the global
2-wire RTD Kelvin current excitation mode bit (G5 = 1, see
Table 67) prior to initiating a conversion start. This current
excitation mode uses the adjacent channel (CH5 for this
example) for the internal ground connection (see Figure
42). One additional protection resistor is added between
the RTD and CH5. The excitation current no longer flows
through RP3, removing the voltage drop across it. Figure 43
shows the channel assignment convention for this mode.
Thermistors
Similar to the 2-wire RTD, thermistors are difficult to
protect because the protection resistor RP3 is in series
with the sensor (see Figure 44). Every 1Ω of protection
resistance adds 1Ω measurement error to the thermistor.
Figure 43. 2-Wire Kelvin Current Mode Channel
Assignment Convention (G5 = 1)
29861 F43
2
1
EXCITATION
CURRENT
FLOW
CHRTD–1
CHRTD
CHRTD+1
= CHRTD (2 ≤ RTD ≤ 9)
GLOBAL REGISTER G5 = 1
CHANNEL
ASSIGNMENT
2ND TERMINAL TIES TO SENSE RESISTOR (CHRSENSE)
Figure 44. Thermistor with Protection Resistors
29861 F44
CH2
CH1
CH3
CH4
RP1
RP2
= EXTERNAL TERMINAL CAN BE
EXPOSED TO FAULT VOLTAGE.
RP3
RSENSE
EXCITATION
CURRENT
FLOW
11
The LTC2986 offers a thermistor Kelvin current source
mode in order to remove the errors associated with protec-
tion resistors. This feature is enabled by setting the global
thermistor Kelvin current excitation mode bit (G6= 1,
see Table 67) prior to initiating a conversion start. This
current excitation mode uses the adjacent channel (CH5
for this example) for the internal ground connection (see
Figure45). One additional protection resistor is added
between the thermistor and CH5. The excitation current
no longer flows through RP3, removing the voltage drop
across it. Figure 46 shows the channel assignment con-
vention for this mode.
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APPLICATIONS INFORMATION
Universal Example
The LTC2986 is capable of sharing one protected interface
with multiple sensor types (see Figure 47). This includes
all RTD configurations (2-, 3- and 4-wire), thermistors,
and thermocouples. Switching between sensors only
requires a new channel assignment word via software
control. The multi-sensor circuit provides 4 input terminals,
each protected against external overvoltage conditions.
In order to enable Kelvin current excitation mode for all
sensors set all 3 global configuration bits G4, G5, and G6
to 1 (see Table 67).
29861 F45
CH2
CH1
CH3
CH4
RP1
RP2
= EXTERNAL TERMINAL CAN BE
EXPOSED TO FAULT VOLTAGE.
RP3
RSENSE
EXCITATION
CURRENT
FLOW
I1
CH5
RP4
ILEAK
ILEAK
29861 F46
EXCITATION
CURRENT
FLOW
CHTHERM–1
CHTHERM
CHTHERM+1
= CHTHERM (2 ≤ THERM ≤ 9)
GLOBAL REGISTER G6 = 1
CHANNEL
ASSIGNMENT
2ND TERMINAL TIES TO SENSE RESISTOR (CHRSENSE)
Figure 46. Thermistor Kelvin Current Mode Channel
Assignment Convention (G6 = 1)
Figure 45. Thermistor Kelvin Current Source Mode (G6 = 1)
In Figure 48, a 4-wire RTD is tied directly to the 4 input
terminals. In this case, the 4-wire RTD is assigned to CH6
and the sense resistor is tied to CH2. The excitation current
flows through protection resisters RP1 and RP6. Since
RP6 is grounded, RSENSE sharing and excitation current
rotation are off.
Figure 47. Universal Multi-Sensor Schematic
RP6
29861 F47
CH3
CH2
CH1
CH4
CH5
CH8
CH9
COM
CH7
CH6
RP2
RP3
RP8
RP5
RSENSE
MULTI SENSOR:
2-, 3,- 4-WIRE RTD
THERMISTOR
THERMOCOUPLE
RP4
RP1
T2
T3
T4
T1
RP7
Figure 48. Protected Multi-Sensor 4-Wire RTD Connection
RP6
29861 F48
CH3
CH2
CH1
CH4
CH5
CH8
CH9
COM
CH7
CH6
RP2
RP3
RP8
RP5
RSENSE
RP4
RP1
RP7
3
4
2
1
T2
T3
T4
T1
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APPLICATIONS INFORMATION
Figure 49 shows the interface to a 3-wire RTD using the
multi-sensor circuit. In this case, the RTD ties directly to
terminals T1 – T3 and terminal T4 is left floating. Matched
excitation currents flow from CH7 and CH8 through protec-
tion resistors RP7 and RP8 while the RTD measurement
occurs at CH5 and CH6. The 3-wire RTD is assigned to
CH6 and RSENSE sharing is turned on.
Figure 50 shows the interface to a 2-wire RTD using the
LTC2986 multi-sensor circuit. In this case, the RTD ties
directly to terminals T1 and T2 while terminals T3 and T4
are left floating. The excitation currents flows from CH1
through RSENSE and protection resistors RP1 and RP4
(CH5 is internally grounded) while the RTD measurement
occurs at CH3 and CH4. The 2-wire RTD is assigned to
CH4 and RSENSE sharing is turned on.
Figure 51 shows the interface to a thermistor using the
LTC2986 multi-sensor circuit. In this case, the thermistor
ties directly to terminals T1 and T2 while terminals T3 and
T4 are left floating. The excitation current flows from CH1
through RSENSE and protection resistors RP1 and RP4 (CH5
is internally grounded) while the thermistor differential
measurement occurs at CH3 and CH4. The thermistor is
assigned to CH4 and RSENSE sharing is enabled in order
to provide an internal ground connection.
Figure 49. Protected Multi-Sensor 3-Wire RTD Connection
RP6
29861 F49
CH3
CH2
CH1
CH4
CH5
CH8
CH9
COM
CH7
CH6
RP2
RP3
RP8
RP5
RSENSE
RP4
RP1
RP7
2
3
1
T2
T3
T4
T1
Figure 51. Protected Multi-Sensor Thermistor Connection
RP6
29861 F51
CH3
CH2
CH1
CH4
CH5
CH8
CH9
COM
CH7
CH6
RP2
RP3
RP8
RP5
RSENSE
RP4
RP1
RP7
T2
T3
T4
T1
Figure 50. Protected Multi-Sensor 2-Wire RTD Connection
RP6
29861 F50
CH3
CH2
CH1
CH4
CH5
CH8
CH9
COM
CH7
CH6
RP2
RP3
RP8
RP5
RSENSE
RP4
RP1
RP7
2
1T2
T3
T4
T1
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APPLICATIONS INFORMATION
Figure 52 shows the interface to a thermocouple us-
ing the LTC2986 multi-sensor circuit. In this case, the
thermocouple ties directly to terminals T3 and T4 while
terminals T1 and T2 can be left floating or tied to an RTD
(Global Register G5 = 1) or thermistor (Global Register
G6 = 1) for cold junction compensation. Alternatively, a
diode (tied to CH9) can be used for cold junction com-
pensation. The thermocouple is assigned to CH6 with
single ended measurement mode.
Figure 52. Protected Multi-Sensor Thermocouple Connection
RP6
29861 F52
CH3
CH2
CH1
CH4
CH5
CH8
CH9
COM
CH7
CH6
RP2
RP3
RP8
RP5
RSENSE
RP4
RP1
RP7
2
1
OPTIONAL
RTD FOR CJ
G5 = 1
OPTIONAL
THERMISTOR
FOR CJ
G6 = 1
THERMOCOUPLE
OPTIONAL
DIODE
FOR CJ
T2
T3
T4
T1
ACTIVE ANALOG TEMPERATURE SENSORS
In addition to passive type temperature sensors, the
LTC2986 also supports active analog temperature sensors
(i.e. LTC2997). In this mode, the LTC2986 measures the
voltage output from the analog temperature sensor and
does a table lookup to convert the measured voltage to
temperature. This sensor option is fully customizable and
can be used for direct temperature measurement or cold
junction compensation.
Analog Sensor Channel Assignment and Result Formats
For the active analog temperature sensor type = 31 (see
Table 4 and Figure 53), the channel assignment word is
0xF800 0000 for differential measurements and 0xFC00
0000 for single-ended. When the LTC2986 is configured
for the active analog temperature sensor type, it will take
the measured voltage value from the ADC and perform a
table lookup to produce a temperature result. The result
format of the table lookup will be a 24-bit signed fixed-
point temperature result along with the error status byte.
The fixed-point format of the temperature is identical to all
the other LTC2986 temperature sensor types. This format
can be seen in Table 9.
Figure 53. Active Analog Temperature Sensor Channel Assignment Conventions
29861 F53
CHADC
SINGLE-ENDED
CHANNEL
ASSIGNMENT = CHADC (1 ≤ CHADC ≤ 10)
= CHADC (2 ≤ CHADC ≤ 10)
24-BIT
∆∑ ADC
24-BIT
∆∑ ADC
CHANNEL
ASSIGNMENT
DIFFERENTIAL
COM
CHADC
CHADC-1
–
+
–
+
LOOKUP
TABLE
WITH
INTERPOLATION
TEMPERATURE
OUTPUT
LOOKUP
TABLE
WITH
INTERPOLATION
TEMPERATURE
OUTPUT
ACTIVE ANALOG
TEMPERATURE
SENSOR
VDD
ACTIVE ANALOG
TEMPERATURE
SENSOR
VDD
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APPLICATIONS INFORMATION
The error status byte is located in the upper byte of the
data output word. Table 68 shows format of the error
status byte for the analog sensor type. Bit D31 and D30
indicate a bad ADC reading. This can be a result of either
a broken (open) sensor or an excessive noise event (ESD
or static discharge into the sensor path). Either of these
are a hard error and –999°C or °F is reported. In the case
of an excessive noise event, the device should recover and
the following conversions will be valid if the noise event
was a random, infrequent event. Bits D27 and D26 indicate
over or under temperature limits have been exceeded the
table limits as described in Table 68. Bit D25 indicates
the absolute voltage measured by the ADC is beyond its
normal operating range.
Example: Differential Active Analog
TemperatureSensor
In this example, a simplified temperature curve is imple-
mented (see Figure 54). Points P1 to P9 represent the
normal operating range of the custom device. Voltage read-
ings above point P9 result in a soft fault and the reported
result is a linear extrapolation using a slope determined
by points P8 and P9 (the final two table entries). Voltage
readings below point P1 are also reported as soft faults.
The reported result is the extrapolation between point P1
and P0, where P0 is typically the lowest possible sensor
output voltage. Sensor output voltages below P0 (in mV)
will report P0 output.
In order to program the LTC2986 with an active analog
temperature sensor table, both the mV data and the Kelvin
data are converted to 24-bit binary values (represented as
two, 3-byte table entries, see Table 69). Since some analog
sensors generate negative output voltages, the table mV
values are 2’s compliment. The sensor output voltage (units
= mV) follows the convention shown in Table 71, where
the first bit is the sign, the next 11 bits are the integer part
and the remaining 12 bits are the fractional part.
Table 68. Active Analog Temperature Sensor Fault Data Byte
BIT FAULT ERROR TYPE DESCRIPTION OUTPUT RESULT
D31 Hard Fault Hard VADC < –1.75 • VREF/2 or VADC > 1.75 • VREF/2 –999
D30 Range Hard Fault Hard VADC < –1.75 • VREF/2 or VADC > 1.75 • VREF/2 –999
D29 Not Used N/A NA NA
D28 Not Used N/A NA NA
D27 Soft Above Soft VADC > Last Table Point Voltage Suspect Reading
D26 Soft Below Soft VADC < Second Table Point (P1) Voltage Suspect Reading
D25 Soft Range Soft VADC < –1.125 • VREF/2 or VADC > 1.125 • VREF/2 Suspect Reading
D24 Result Valid (Always 1) NA NA NA
Active Analog Temperature Sensor Table Format
Table 69. Active Analog Temperature Sensor Table Format
ADDRESS BYTE 0 BYTE 1 BYTE 2 BYTE 3 BYTE 4 BYTE 5
0x250 + 6 • Start Address Table Entry #1 (mV) Table Entry #1 (Kelvin)
0x250 + 6 • Start Address + 6 Table Entry #2 (mV) Table Entry #2 (Kelvin)
0x250 + 6 • Start Address + 12 Table Entry #3 (mV) Table Entry #3 (Kelvin)
• • •
• • •
• • •
Max Address = 0x3CA Table Entry #64 (mV) Table Entry #64 (Kelvin)
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APPLICATIONS INFORMATION
The temperature values are input in Kelvin as unsigned
fixed-point values, but the final temperatures reported by
the LTC2986 are reported in °C or °F. The sensor tempera-
ture (Kelvin) follows the convention shown in Table72,
where the first 14 bits are the integer part and the remaining
10 bits are the fractional part. In this example, a custom
analog sensor is tied to CH2, and is programmed with
the channel assignment data shown in Table 73. In this
case the custom data begins at memory location 0x250
(starting address is 0). The starting address (offset from
0x250) is entered in the analog sensor data pointer field
of the channel assignment data. The table data length –1
(9 in this example) is entered into the data length field of
the analog temperature sensor channel assignment word.
Refer to Table 70 for the location and format of the 10
six-byte table entries.
Figure 54. Active Analog Temperature Sensor Table Example
Table 70. Active Analog Temperature Sensor Example Table Data Memory Map
POINT SENSOR OUTPUT
VOLTAGE (mV)
TEMPERATURE
KELVIN
START
ADDRESS
STOP
ADDRESS BYTE 0 BYTE 1 BYTE 2 BYTE 3 BYTE 4 BYTE 5
P0 –50.22 0 0x250 0x255
P1 –30.2 99.1 0x256 0x25B
P2 –5.3 135.4 0x25C 0x261
P3 20.33 220.3 0x262 0x267
P4 40.2 361.2 0x268 0x26D mV Data Temperature Data
P5 55.3 522.1 0x26E 0x273
P6 88.3 720.3 0x274 0x279
P7 132.2 811.2 0x27A 0x27F
P8 188.7 922.5 0x280 0x285
P9 460.4 1000 0x286 0x28B
29861 F54
TEMPERATURE (K)
p9
p8
p7
p6
p5
p4
p3
(0mV, 0K)
NOTE:
P0 SHOULD BE THE
EXTRAPOLATION
POINT TO 0K
VOLTAGE (mV)
p2
p1
p0
VOLTAGE < p1
SOFT FAULT
CONDITION
VOLTAGE > p9
SOFT FAULT
CONDITION
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Table 73. Example Active Analog Temperature Sensor Channel Assignment Data
CONFIGURATION
FIELD DESCRIPTION # BITS
BINARY
DATA
MEMORY
ADDRESS 214
MEMORY
ADDRESS 215
MEMORY
ADDRESS 216
MEMORY
ADDRESS 217
(1) Analog Temp
Sensor
Sensor Type 5 11110 1 1 1 1 0
(2) SE/Diff Single-Ended or
Differential
1 0 0
(3) Not Used Set to 0 14 00000000000000 00000000000000
(4) Direct ADC Table
Data Pointer
Start Address = 0
(Start at 0x250)
6 000000 0 0 0 0 0 0
(5) Direct ADC Table
Data Length-1
Data Length-1 = 9 6 001001 0 0 1 0 0 1
Table 72. Example Active Analog Temperature Sensor Temperature Values
BYTE 3 BYTE 4 BYTE 5
B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
Temperature 213 212 211 210 292827262524232221202–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
99.1 0 0 0 0 0 0 0 1 1 0 0 0 1 1 0 0 0 1 1 0 0 1 1 0
135.4 0 0 0 0 0 0 1 0 0 0 0 1 1 1 0 1 1 0 0 1 1 0 0 1
220.3 0 0 0 0 0 0 1 1 0 1 1 1 0 0 0 1 0 0 1 1 0 0 1 1
361.2 0 0 0 0 0 1 0 1 1 0 1 0 0 1 0 0 1 1 0 0 1 1 0 0
522.1 0 0 0 0 1 0 0 0 0 0 1 0 1 0 0 0 0 1 1 0 0 1 1 0
720.3 0 0 0 0 1 0 1 1 0 1 0 0 0 0 0 1 0 0 1 1 0 0 1 1
811.2 0 0 0 0 1 1 0 0 1 0 1 0 1 1 0 0 1 1 0 0 1 1 0 0
922.5 0 0 0 0 1 1 1 0 0 1 1 0 1 0 1 0 0 0 0 0 0 0 0 0
1000 0 0 0 0 1 1 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0
Table 71. Example Active Analog Temperature Sensor Voltage Values
BYTE 0 BYTE 1 BYTE 2
B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
mV Sign 210 292827262524232221202–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10 2–11 2–12
–50.22 1 1 1 1 1 1 0 0 1 1 0 1 1 1 0 0 0 1 1 1 1 0 1 1
–30.2 1 1 1 1 1 1 1 0 0 0 0 1 1 1 0 0 1 1 0 0 1 1 0 1
–5.3 1 1 1 1 1 1 1 1 1 0 1 0 1 0 1 1 0 0 1 1 0 0 1 1
20.33 0 0 0 0 0 0 0 1 0 1 0 0 0 1 0 1 0 1 0 0 1 0 0 0
40.2 0 0 0 0 0 0 1 0 1 0 0 0 0 0 1 1 0 0 1 1 0 0 1 1
55.3 0 0 0 0 0 0 1 1 0 1 1 1 0 1 0 0 1 1 0 0 1 1 0 1
88.3 0 0 0 0 0 1 0 1 1 0 0 0 0 1 0 0 1 1 0 0 1 1 0 1
132.2 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1 1 0 0 1 1 0 0 1 1
188.7 0 0 0 0 1 0 1 1 1 1 0 0 1 0 1 1 0 0 1 1 0 0 1 1
460.4 0 0 0 1 1 1 0 0 1 1 0 0 0 1 1 0 0 1 1 0 0 1 1 0
APPLICATIONS INFORMATION
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APPLICATIONS INFORMATION
DIRECT ADC MEASUREMENTS
In addition to measuring temperature sensors, the LTC2986
can perform direct ADC (i.e. voltage) measurements. Any
channel may be configured to perform direct single-ended
or differential measurements. Direct ADC channel assign-
ments follow the general convention shown in Figure 55.
The 32-bit channel assignment word is programmed into
a memory location corresponding to the input channel.
The Direct ADC mode is configurable to single-ended as
well as differential inputs. The positive input channel ties
to CHADC for both single-ended and differential modes.
For single-ended measurements the ADC negative input
is COM while for differential measurements it is CHADC-1.
For single-ended measurements, COM can be driven with
any voltage above GND – 50mV and below VDD – 0.3V. The
direct ADC results are available in memory at a location
corresponding to the conversion channel.
There are two result mode options for direct ADC operation.
The first mode is direct voltage output and the second is
table-driven output. When configured for direct voltage
output, the LTC2986 will return a 24-bit fixed-point volt-
age result along with the 8-bit fault status byte. When
configured for table-lookup, the LTC2986 will perform a
table lookup on the raw ADC voltage and return a 24-bit
signed-integer table-lookup result along with the 8-bit
fault status byte.
Figures 56 to Figures 58 show typical integral nonlinearity
variation as a function of supply voltage and temperature
for a differential input voltage (±VREF/2) and VREF/2 com-
mon mode input voltage.
Figure 55. Direct ADC Channel Assignment Conventions
29861 F55
CHADC
SINGLE-ENDED CHANNEL
ASSIGNMENT = CHADC (1 ≤ CHADC ≤ 10)
= CHADC (2 ≤ CHADC ≤ 10)
24-BIT
∆∑ ADC
24-BIT
∆∑ ADC CHANNEL
ASSIGNMENT
DIFFERENTIAL
COM
CHADC
CHADC-1
–
+
–
+
Figure 56. Integral Nonlinearity as a Function of
Temperature at VDD = 5.25V
Figure 57. Integral Nonlinearity as a Function of
Temperature at VDD = 3.3V
DIFFERENTIAL INPUT VOLTAGE (V)
INL ERROR (ppm)
29861 F37
20
15
10
5
0
–5
–10
–15
–20
–1.5 –0.5 0 0.5 1 1.5–1
90°C
25°C
–45°C
DIFFERENTIAL INPUT VOLTAGE (V)
INL ERROR (ppm)
29861 F38
20
15
10
5
0
–5
–10
–15
–20
–1.5 –0.5 0 0.5 1 1.5–1
90°C
25°C
–45°C
Figure 58. Integral Nonlinearity as a Function of
Temperature at VDD = 2.85V
DIFFERENTIAL INPUT VOLTAGE (V)
INL ERROR (ppm)
29861 F39
20
15
10
5
0
–5
–10
–15
–20
–1.5 –0.5 0 0.5 1 1.5–1
90°C
25°C
–45°C
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Voltage Output Mode
For standard (non table mode) voltage output mode, the
channel assignment word is 0xF000 0000 for differential
readings and 0xF400 0000 for single-ended (see Table 75).
The data is represented as a 32-bit word (see Table 74)
where the eight most significant bits are fault bits and
the bottom 24 are the ADC reading in volts. For direct
APPLICATIONS INFORMATION
ADC readings hard fault errors do not clamp the digital
output. Readings beyond ±1.125 • VREF/2 exceed the
normal accuracy range of the LTC2986 and flag a soft
error; these results should be discarded. Readings
beyond ±1.75 •VREF/2 exceed the usable range of the
LTC2986; these result in a hard fault and should be
discarded.
Table 74. Direct ADC Voltage Output Result Format
START ADDRESS START ADDRESS + 1 START ADDRESS + 2
START ADDRESS + 3
(END ADDRESS)
D31 D30 D29 D28 D27 D26 D25 D24 D23 D22 D21 D20 D19 D18 D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
Fault Data SIGN MSB LSB
Volts Sensor
Hard
Fault
Range
Hard
Fault
NA NA Soft
Above
Soft
Below
Soft
Range
Valid
Always
1 ± 2V 1V 0.5V 0.25V ...
Integer Fraction
>VREF 1 1 0 0 1 0 1 1 CLAMPED to Factory Programmed Value
of VREF
1.75 • VREF/2 1 1 0 0 1 0 1 1 0 1 0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1.125 • VREF/2 0 0 0 0 1 0 1 1 0 0 1 0 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
VREF/2 0 0 0 0 0 0 0 1 0 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
2–21V 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
–2–21V 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
–VREF/2 0 0 0 0 0 0 0 1 1 1 0 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
–1.125 • VREF 0 0 0 0 0 1 1 1 1 1 0 1 0 0 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
–1.75 • VREF 1 1 0 0 0 1 1 1 1 0 1 1 1 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
< –VREF 1 1 0 0 0 1 1 1 CLAMPED to Factory Programmed Value
of –VREF
Table 75. Direct ADC Channel Assignment Data
CONFIGURATION
FIELD DESCRIPTION # BITS
BINARY
DATA
MEMORY
ADDRESS 0x200
MEMORY
ADDRESS 0x201
MEMORY
ADDRESS 0x202
MEMORY
ADDRESS 0x203
(1) Direct ADC Directly Measure
ADC
5 11110 1 1 1 1 0
(2) SE/Diff Single-Ended or
Differential
1 0 0
(3) TBL Table Lookup 1 0 0
(4) Not Used Set to 0 13 0000000000000 0000000000000
(4) Direct ADC Table
Data Pointer
Start Address = 0 6 000000 0 0 0 0 0 0
(5) Direct ADC Table
Data Length-1
Data Length-1 = 0 6 000000 0 0 0 0 0 0
Example: Direct ADC with Differential Input
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Table Lookup Mode
For table-driven output mode, the channel assignment
word's 1st two bytes are 0xF200 for differential readings
and 0xF600 for single-ended. The 12 least significant bits
APPLICATIONS INFORMATION
contain length and pointer information for the custom
table data. When the LTC2986 is configured for table-
driven output data, it will take the voltage value from the
ADC and perform a table lookup. The result format of the
table lookup will be a 24-bit signed integer along with the
error status byte.
Table 78. Direct ADC Table Format
ADDRESS BYTE 0 BYTE 1 BYTE 2 BYTE 3 BYTE 4 BYTE 5
0x250 + 6 • Start Address Table Entry #1 (mV) Table Entry #1 (Integer Value)
0x250 + 6 • Start Address + 6 Table Entry #2 (mV) Table Entry #2 (Integer Value)
0x250 + 6 • Start Address + 12 Table Entry #3 (mV) Table Entry #3 (Integer Value)
• • •
• • •
• • •
Max Address = 0x3CA Table Entry #64 (mV) Table Entry #64 (Integer Value)
Table 77. Direct ADC Table Lookup Fault Data Byte
BIT FAULT DESCRIPTION
D31 Sensor Hard Fault VADC < –1.75 • VREF/2 or VADC > 1.75 • VREF/2
D30 Range Hard Fault VADC < –1.75 • VREF/2 or VADC > 1.75 • VREF/2
D29 Not Used NA
D28 Not Used NA
D27 Soft Above VADC > Last Table Point Voltage
D26 Soft Below VADC < Second Table Point (P1) Voltage
D25 Soft Range VADC < –1.125 • VREF/2 or VADC > 1.125 • VREF/2
D24 Result Valid (Always 1) NA
Table 76. Direct ADC Table Lookup Result Format
START ADDRESS START ADDRESS + 1 START ADDRESS + 2
START ADDRESS + 3
(END ADDRESS)
D31 D30 D29 D28 D27 D26 D25 D24 D23 D22 D21 D20 D19 D18 D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
Fault Data SIGN MSB LSB
Sensor
Hard
Fault
Range
Hard
Fault
NA NA Soft
Above
Soft
Below
Soft
Range
Valid
Always
1
Table Lookup Result – Signed Integer
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Example: Direct ADC with Differential Input and
TableLookup
In this example, a simplified custom curve is implemented
(see Figure 59). Points P1 to P9 represent the normal
operating range of the custom device. Voltage readings
above point P9 result in a soft fault and the reported
result is a linear extrapolation using a slope determined
by points P8 and P9 (the final two table entries). Voltage
readings below point P1 are also reported as soft faults.
The reported result is the extrapolation between point P1
and P0, where P0 is typically the lowest possible sensor
output voltage. Sensor output voltages below P0 (in mV)
will report P0 output.
APPLICATIONS INFORMATION
In order to program the LTC2986 with the custom ADC
table, both the mV data and the result data are converted
to 24-bit binary values (represented as two 3-byte table
entries). To accommodate sensors with bipolar output
voltages, the mV values input to the LTC2986 are 2’s
compliment. The sensor output voltage (units = mV) fol-
lows the convention shown in Table 80, where the first bit
is the sign, the next 11 bits are the integer part and the
remaining 12 bits are the fractional part.
The result-side of table entries are input as signed 24-bit
integers, the final result reported by the LTC2986 is also
a 24-bit integer. The result format follows the convention
shown in Table 81, where the first bit is the sign bit and
the remaining 23 bits are the integer magnitude. In this
example, a custom differential sensor is tied to CH2 with
the channel assignment data shown in Table 82. In this
case the custom data begins at memory location 0x250
(starting address is 0). The starting address (offset from
0x250) is entered in the data pointer field of the channel
assignment data. The table data length –1 (9 in this example)
is entered into the data length field of the sensor channel
assignment word. Refer to Table 79 for the location and
format of the 10 six-byte table entries.
Figure 59. Direct ADC Table Example
29861 F59
TABLE OUTPUT VALUE
p9
p8
p7
p6
p5
p4
p3
(0mV, 0 INTEGER)
NOTE:
P0 SHOULD BE THE
EXTRAPOLATION
POINT TO LOWEST
EXPECTED VOLTAGE VOLTAGE (mV)
p2
p1
p0
VOLTAGE < p1
SOFT FAULT
CONDITION
VOLTAGE > p9
SOFT FAULT
CONDITION
Table 79. Direct ADC Table Example Data Memory Map
POINT SENSOR OUTPUT
VOLTAGE (mV)
INTEGER
OUTPUT DATA
START
ADDRESS
STOP
ADDRESS BYTE 0 BYTE 1 BYTE 2 BYTE 3 BYTE 4 BYTE 5
P0 –50.22 –100 0x250 0x255
P1 –30.2 –50 0x256 0x25B
P2 –5.3 0 0x25C 0x261
P3 20.33 2203 0x262 0x267
P4 40.2 3612 0x268 0x26D mV Data Integer Output Data
P5 55.3 5221 0x26E 0x273
P6 88.3 7203 0x274 0x279
P7 132.2 8112 0x27A 0x27F
P8 188.7 9225 0x280 0x285
P9 460.4 10000 0x286 0x28B
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APPLICATIONS INFORMATION
Table 82. Example Table Lookup Mode Channel Assignment Data
CONFIGURATION
FIELD DESCRIPTION # BITS
BINARY
DATA
MEMORY
ADDRESS 214
MEMORY
ADDRESS 215
MEMORY
ADDRESS 216
MEMORY
ADDRESS 217
(1) Direct ADC Directly Measure
ADC
5 11110 1 1 1 1 0
(2) SE/Diff Single-Ended or
Differential
1 0 0
(3) TBL Table Lookup 1 1 1
(4) Not Used Set to 0 13 00000000000000 0000000000000
(5) Direct ADC Table
Data Pointer
Start Address = 0
(Start at 0x250)
6 000000 0 0 0 0 0 0
(5) Direct ADC Table
Data Length-1
Data Length-1 = 9 6 001001 0 0 1 0 0 1
Table 81. Example Table Output Values
BYTE 3 BYTE 4 BYTE 5
B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
Temperature Sign 222 221 220 219 218 217 216 215 214 213 212 211 210 29282726252423222120
–100 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 1 1 0 0
–50 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 1 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
2203 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 1 1 0 1 1
3612 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 1 1 1 0 0
5221 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 1 1 0 0 1 0 1
7203 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 1 0 0 0 1 1
8112 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0 1 1 0 0 0 0
9225 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0 0 0 1 0 0 1
10000 0 0 0 0 0 0 0 0 0 0 1 0 0 1 1 1 0 0 0 1 0 0 0 0
Table 80. Example ADC Voltage Values
BYTE 0 BYTE 1 BYTE 2
B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
mV Sign 210 292827262524232221202–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10 2–11 2–12
–50.22 1 1 1 1 1 1 0 0 1 1 0 1 1 1 0 0 0 1 1 1 1 0 1 1
–30.2 1 1 1 1 1 1 1 0 0 0 0 1 1 1 0 0 1 1 0 0 1 1 0 1
–5.3 1 1 1 1 1 1 1 1 1 0 1 0 1 0 1 1 0 0 1 1 0 0 1 1
20.33 0 0 0 0 0 0 0 1 0 1 0 0 0 1 0 1 0 1 0 0 1 0 0 0
40.2 0 0 0 0 0 0 1 0 1 0 0 0 0 0 1 1 0 0 1 1 0 0 1 1
55.3 0 0 0 0 0 0 1 1 0 1 1 1 0 1 0 0 1 1 0 0 1 1 0 1
88.3 0 0 0 0 0 1 0 1 1 0 0 0 0 1 0 0 1 1 0 0 1 1 0 1
132.2 0 0 0 0 1 0 0 0 0 1 0 0 0 0 1 1 0 0 1 1 0 0 1 1
188.7 0 0 0 0 1 0 1 1 1 1 0 0 1 0 1 1 0 0 1 1 0 0 1 1
460.4 0 1 0 1 1 1 0 0 1 1 0 0 0 1 1 0 0 1 1 0 0 1 1 0
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2- AND 3-CYCLE CONVERSION MODES
The LTC2986 performs multiple internal conversions in
order to determine the sensor temperature. Normally, two
internal conversion cycles are required for each tempera-
ture result providing a maximum output time of 167.2ms.
The LTC2986 uses these two cycles to automatically
remove offset/offset drift errors, reduce 1/f noise, auto-
calibrate matched internal current sources, and provide
simultaneous 50/60Hz noise rejection.
In addition to performing two conversion cycles per result,
the LTC2986 also offers several unique features by utilizing
a 3rd conversion cycle. In this case, the maximum output
time is 251ms and all the benefits of the 2-cycle modes
are present (see Table 83).
One feature utilizing the three conversion cycle mode is the
internal open circuit detect mode. Typically, thermocouple
open circuit detection is performed by adding a high re-
sistance pull-up between the thermocouple and VDD. This
method can be used with the LTC2986 while operating
in the two conversion cycle mode (OC=0). This external
pull-up can interact with the input protection circuitry and
lead to temperature measurement errors and increased
noise. These problems are eliminated by selecting the
internal open circuit detection mode (OC=1). In this case,
a current is pulsed for 8ms and allowed to settle during
one conversion cycle. This is followed by the normal two
conversion cycle measurement of the thermocouple. If
the thermocouple is broken, the current pulse will result
in an open circuit fault.
A second feature taking advantage of the 3rd conversion
cycle is thermistor excitation current auto ranging. Since
a thermistor’s resistance varies many orders of magni-
tude, the performance in the low resistance regions are
compromised by the small currents required by the high
resistance regions of operation. The auto ranging mode
applies a test current during the first conversion cycle in
order to determine the optimum current for the resistance
state of the thermistor. It then uses that current to perform
the thermistor measurement using the normal 2-cycle
measurement. If a 3-cycle thermistor measurement is used
as the cold junction sensor for a 2-cycle thermocouple
SUPPLEMENTAL INFORMATION
measurement, the thermocouple conversion result is
ready after three cycles.
A third feature requiring a 3rd conversion cycle is the
three current diode measurement. In this mode, three
ratioed currents are applied to the external diode in order
to cancel parasitic lead resistance effects. This is useful
in applications where the diode is remotely located and
significant, unknown parasitic lead resistance requires
cancellation. If a 3-cycle diode or thermistor measure-
ment is used as the cold junction sensor for a 2-cycle
thermocouple measurement, the thermocouple conversion
result is ready after three cycles.
Table 83. 2- and 3-Cycles Conversion Modes
TYPE OF SENSOR CONFIGURATION
NUMBER OF
CONVERSION
CYCLES
MAXIMUM
OUTPUT TIME
Thermocouple OC = 0 2 167.2ms
RTD All 2 167.2ms
Thermistor Non-Autorange
Current
2 167.2ms
Diode Two Readings 2 167.2ms
Thermocouple OC = 1 3 251ms
Thermocouple OC = 0, 3-Cycle
Cold Junction
3 251ms
Thermistor Autorange
Current
3 251ms
Diode Three Readings 3 251ms
RUNNING CONVERSIONS CONSECUTIVELY ON
MULTIPLE CHANNELS
Generally, during the Initiate Conversion state, a conver-
sion measurement is started on a single input chan-
nel determined by the channel number (bits B[4:0] =
00001 to 01010) written into memory location 0x000.
Multiple consecutive conversions can be initiated by writing
bits B[4:0]=00000 into memory location 0. Conversions
will be initiated on each channel selected in the mask
register (see Table 84).
For example, using the mask data shown in Table 85, after
1000000 is written into memory location 0, conversions
are initiated consecutively on CH10, CH8, CH6, and CH1.
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Once the conversions begin, the INTERRUPT pin goes LOW
and remains LOW until all conversions are complete. If
the mask register is set for a channel that has no assign-
ment data, that conversion step is skipped. All the results
are stored in the conversion result memory locations and
can be read at the conclusion of the measurement cycle.
ENTERING/EXITING SLEEP MODE
The LTC2986 can be placed into sleep mode by writing
0x97 to memory location 0x000. On the rising edge of
CS following the memory write (see Figure 2) the device
enters the low power sleep state. It remains in this state
until CS is brought low or RESET is asserted. Once one
of these two signals is asserted, the LTC2986 begins its
start-up cycle as described in State 1: Start-Up section
of this data sheet.
MUX CONFIGURATION DELAY
The LTC2986 performs 2 or 3 internal conversion cycles
per temperature result. Each conversion cycle is performed
with different excitation and input multiplexer configura-
tions. Prior to each conversion, these excitation circuits
Table 84. Multiple Conversion Mask Register
MEMORY LOCATION B7 B6 B5 B4 B3 B2 B1 B0
0x0F4 Reserved
0x0F5
0x0F6 CH10 CH9
0x0F7 CH8 CH7 CH6 CH5 CH4 CH3 CH2 CH1
Table 85. Example Mask Register Select CH10, CH8, CH6, and CH1
MEMORY LOCATION B7 B6 B5 B4 B3 B2 B1 B0
0x0F4 Reserved
0x0F5
0x0F6 1 0
0x0F7 1 0 1 0 0 0 0 1
SUPPLEMENTAL INFORMATION
and input switch configurations are changed and an
internal 1ms (typical) delay ensures settling prior to the
conversion cycle in most cases.
If excessive RC time constants are present in external
sensor circuits (large bypass capacitors used for thermis-
tors or RTDs) it is possible to increase the settling time
between current source excitation and MUX switching.
The extra delay is determined by the value written into
the MUX configuration delay register (memory location
0x0FF). The value written into this memory location is
multiplied by 100µs; therefore, the maximum extra MUX
delay is 25.5ms (i.e. 0x0FF = 255 • 100µs).
REFERENCE CONSIDERATIONS
The mechanical stress of soldering the LTC2986 to a PC
board can cause the output voltage reference to shift and
temperature coefficient to change. These two changes are
not correlated. For example, the voltage may shift but the
temperature coefficient may not. To reduce the effects of
stress-related shifts, mount the reference near the short
edge of the PC board or in a corner.
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Figure 60. Custom Thermocouple Example (mV vs Kelvin)
29861 F60
TEMPERATURE (K)
p9
p8
p7
p6
p5
p4
p3
(0mV, 0K)
NOTE:
P0 SHOULD BE THE
EXTRAPOLATION
POINT TO 0K
VOLTAGE (mV)
p2
p1
p0
VOLTAGE < p1
SOFT FAULT
CONDITION
VOLTAGE > p9
SOFT FAULT
CONDITION
(0mV, 273.15K)
CUSTOM THERMOCOUPLES
In addition to digitizing standard thermocouples, the
LTC2986 can also digitize user-programmable, custom ther-
mocouples (thermocouple type=0b01001, see Table16).
Custom sensor data (minimum of three, maximum of 64
pairs) reside sequentially in memory and are arranged in
blocks of six bytes of monotonically increasing tabular
data as mV vs temperature (see Table 86).
Table 86. Custom Thermocouple Tabular Data Format
ADDRESS BYTE 0 BYTE 1 BYTE 2 BYTE 3 BYTE 4 BYTE 5
0x250 + 6 • Start Address Table Entry #1 (mV) Table Entry #1 (Kelvin)
0x250 + 6 • Start Address + 6 Table Entry #2 (mV) Table Entry #2 (Kelvin)
0x250 + 6 • Start Address + 12 Table Entry #3 (mV) Table Entry #3 (Kelvin)
• • •
• • •
• • •
Max Address = 0x3CA Table Entry #64 (mV) Table Entry #64 (Kelvin)
Custom Thermocouple Example
In this example, a simplified thermocouple curve is
implemented (see Figure 60). Points P1 to P9 represent
the normal operating range of the custom thermocouple.
Voltage readings above point P9 result in a soft fault and
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the reported temperature is a linear extrapolation using a
slope determined by points P8 and P9 (the final two table
entries in Table 87). Voltage readings below point P1 are
also reported as soft faults. The temperature reported is
the extrapolation between point P1 and P0, where P0 is
typically the sensor output voltage at 0 Kelvin. If P0 is
above 0 Kelvin, then all sensor output voltages below P0
(in mV) will report 0 Kelvin. Sensor readings below P1
are reported as soft faults
CUSTOM THERMOCOUPLES
Table 87. Thermocouple Example mV vs Kelvin (K) Data Memory Map
POINT
SENSOR OUTPUT
VOLTAGE (mV)
TEMPERATURE
KELVIN
START
ADDRESS
STOP
ADDRESS BYTE 0 BYTE 1 BYTE 2 BYTE 3 BYTE 4 BYTE 5
P0 –50.22 0 0x250 0x255
P1 –30.2 99.1 0x256 0x25B
P2 –5.3 135.4 0x25C 0x261
P3 0 273.15 0x262 0x267
P4 40.2 361.2 0x268 0x26D mV Data Temperature Data
P5 55.3 522.1 0x26E 0x273 (see Table 88) (see Table 89)
P6 88.3 720.3 0x274 0x279
P7 132.2 811.2 0x27A 0x27F
P8 188.7 922.5 0x280 0x285
P9 460.4 1000 0x286 0x28B
Table 88. Example Thermocouple Output Voltage Values (mV)
BYTE 0 BYTE 1 BYTE 2
B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
mV Sign 2827262524232221202–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10 2–11 2–12 2–13 2–14
–50.22 1 1 1 1 0 0 1 1 0 1 1 1 0 0 0 1 1 1 1 0 1 1 0 0
–30.2 1 1 1 1 1 0 0 0 0 1 1 1 0 0 1 1 0 0 1 1 0 1 0 0
–5.3 1 1 1 1 1 1 1 0 1 0 1 0 1 1 0 0 1 1 0 0 1 1 0 1
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
40.2 0 0 0 0 1 0 1 0 0 0 0 0 1 1 0 0 1 1 0 0 1 1 0 0
55.3 0 0 0 0 1 1 0 1 1 1 0 1 0 0 1 1 0 0 1 1 0 0 1 1
88.3 0 0 0 1 0 1 1 0 0 0 0 1 0 0 1 1 0 0 1 1 0 0 1 1
132.2 0 0 1 0 0 0 0 1 0 0 0 0 1 1 0 0 1 1 0 0 1 1 0 0
188.7 0 0 1 0 1 1 1 1 0 0 1 0 1 1 0 0 1 1 0 0 1 1 0 0
460.4 0 1 1 1 0 0 1 1 0 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1
In order to program the LTC2986 with the custom ther-
mocouple table, both the mV data and the Kelvin data are
converted to 24-bit binary values (represented as two 3-byte
table entries). Since most thermocouples generate negative
output voltages, the mV values input to the LTC2986 are
2’s compliment. The sensor output voltage (units=mV),
follows the convention shown in Table 88, where the first
bit is the sign, the next nine are the integer part and the
remaining 14 bits are the fractional part.
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In order to simplify the temperature field, temperature
values are input in Kelvin as an unsigned value, but the
final temperatures reported by the LTC2986 are reported
in °C or °F (see Table 9). The sensor temperature (Kelvin),
follows the convention shown in Table 89, where the first
14 bits are the integer part and the remaining 10 bits are
the fractional part.
In this example, a custom thermocouple tied to CH1, with a
cold junction sensor on CH2, is programmed with the chan-
CUSTOM THERMOCOUPLES
Table 89. Example Thermocouple Temperature Values
BYTE 3 BYTE 4 BYTE 5
B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
Temperature 213 212 211 210 292827262524232221202–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
99.1 0 0 0 0 0 0 0 1 1 0 0 0 1 1 0 0 0 1 1 0 0 1 1 0
135.4 0 0 0 0 0 0 1 0 0 0 0 1 1 1 0 1 1 0 0 1 1 0 0 1
273.15 0 0 0 0 0 1 0 0 0 1 0 0 0 1 0 0 1 0 0 1 1 0 0 1
361.2 0 0 0 0 0 1 0 1 1 0 1 0 0 1 0 0 1 1 0 0 1 1 0 0
522.1 0 0 0 0 1 0 0 0 0 0 1 0 1 0 0 0 0 1 1 0 0 1 1 0
720.3 0 0 0 0 1 0 1 1 0 1 0 0 0 0 0 1 0 0 1 1 0 0 1 1
811.2 0 0 0 0 1 1 0 0 1 0 1 0 1 1 0 0 1 1 0 0 1 1 0 0
922.5 0 0 0 0 1 1 1 0 0 1 1 0 1 0 1 0 0 0 0 0 0 0 0 0
1000 0 0 0 0 1 1 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0
Table 90. Custom Thermocouple Channel Assignment Data
CONFIGURATION
FIELD DESCRIPTION # BITS
BINARY
DATA
MEMORY
ADDRESS 0x200
MEMORY
ADDRESS 0x201
MEMORY
ADDRESS 0x202
MEMORY
ADDRESS 0x203
(1) Thermocouple
Type
Type Custom 5 01001 0 1 0 0 1
(2) Cold Junction
Channel Pointer
CH25 00010 0 0 0 1 0
(3) Sensor
Configuration
Single-Ended,
10µA Open Circuit
4 1100 1 1 0 0
Not Used Set These Bits to 0 6 000000 0 0 0 0 0 0
(4) Custom
Thermocouple Data
Pointer
Start Address = 0
(Start at 0x250)
6 000000 0 0 0 0 0 0
Custom
Thermocouple Data
Length-1
Data Length –1
= 9
(10 Paired Entries)
6 001001 0 0 1 0 0 1
nel assignment data shown in Table 90 (refer to Figure9
for similar format). In this case the custom data begins at
memory location 0x250 (starting address is 0). The start-
ing address (offset from 0x250) is entered in the custom
thermocouple data pointer field of the channel assignment
data. The table data length –1 (9 in this example) is entered
into the custom thermocouple data length field of the
thermocouple channel assignment word. Refer to Table 87
where the number of six byte entries is 10.
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In addition to digitizing standard RTDs, the LTC2986
can also digitize custom RTDs (RTD type=0b10010, see
Table30). Custom sensor data (minimum of three, maxi-
mum of 64 pairs) reside sequentially in memory and are
arranged in blocks of six bytes of monotonically increasing
tabular data Ω vs temperature (see Table 91).
Table 91. Custom RTD/Thermistor Tabular Data Format
ADDRESS BYTE 0 BYTE 1 BYTE 2 BYTE 3 BYTE 4 BYTE 5
0x250 + 6 • Start Address Table Entry #1 (Ω) Table Entry #1 (Kelvin)
0x250 + 6 • Start Address + 6 Table Entry #2 (Ω) Table Entry #2 (Kelvin)
0x250 + 6 • Start Address + 12 Table Entry #3 (Ω) Table Entry #3 (Kelvin)
• • •
• • •
• • •
Max Address = 0x3CA Table Entry #64 (Ω) Table Entry #64 (Kelvin)
CUSTOM RTDS
Figure 61. Custom RTD Example (Ω vs Kelvin )
29861 F61
p9
p8
p7
p6
p5
p4
p3
NOTE:
P0 SHOULD BE THE
EXTRAPOLATION
POINT TO 0Ω
RESISTANCE (Ω)
TEMPERATURE (K)
p2
p1
0
0p0
RESISTANCE < p1
SOFT FAULT
CONDITION
RESISTANCE > p9
SOFT FAULT
CONDITION
Custom RTD Example
In this example, a simplified RTD curve is implemented (see
Figure 61). Points P1 to P9 represent the normal operating
range of the custom RTD. Resistance readings above point
P9 result in a soft fault and the reported temperature is
a linear extrapolation using a slope determined by points
P8 and P9 (the final two table entries). Resistance read-
ings below point P1 are also reported as soft faults. The
temperature reported is the extrapolation between point
P1 and P0, where P0 is the sensor output temperature
at 0Ω (This point should be 0Ω for proper interpolation
below point P1).
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Custom RTD table data is formatted in Ω (sensor output
resistance) vs Kelvin (see Table 92). Each table entry pair
spans six bytes. The first set of data can begin at any
memory location greater than or equal to 0x250 and end
at or below 0x3CF.
In order to program the LTC2986 with the custom RTD
table, both the resistance data and the Kelvin data are
converted to 24-bit binary values. The sensor output
CUSTOM RTDS
resistance (units=Ω) follows the convention shown in
Table 93, where the first 13 bits are the integer part and
the remaining 11 bits are the fractional part.
In order to simplify the temperature field, temperature
values are input in Kelvin as an unsigned value, but the
final temperatures reported by the LTC2986 are reported
in °C or °F. The sensor temperature (Kelvin) follows the
Table 92. RTD Example Resistance vs Kelvin Data Memory Map
POINT
SENSOR OUTPUT
RESISTANCE (Ω)
TEMPERATURE
(K)
START
ADDRESS
STOP
ADDRESS BYTE 1 BYTE 2 BYTE 3 BYTE 1 BYTE 2 BYTE 3
P0 0 112.3 0x28C 0x291
P1 80 200.56 0x292 0x297
P2 150 273.16 0x298 0x29D
P3 257.36 377.25 0x29E 0x2A3
P4 339.22 489.66 0x2A4 0x2A9 Resistance Data Temperature Data
P5 388.26 595.22 0x2AA 0x2AF
P6 512.99 697.87 0x2B0 0x2B5
P7 662.3 765.14 0x2B6 0x2BB
P8 743.5 801.22 0x2BC 0x2C1
P9 2001.89 900.5 0x2C2 0x2C7
Table 93. Example RTD Resistance Values
BYTE 1 BYTE 2 BYTE 3
B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
Resistance 212 211 210 292827262524232221202–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10 2–11
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
80 0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
150 0 0 0 0 0 1 0 0 1 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0
257.36 0 0 0 0 1 0 0 0 0 0 0 0 1 0 1 0 1 1 1 0 0 0 0 1
339.22 0 0 0 0 1 0 1 0 1 0 0 1 1 0 0 1 1 1 0 0 0 0 1 0
388.26 0 0 0 0 1 1 0 0 0 0 1 0 0 0 1 0 0 0 0 1 0 1 0 0
512.99 0 0 0 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0 1 0 1 1
662.3 0 0 0 1 0 1 0 0 1 0 1 1 0 0 1 0 0 1 1 0 0 1 1 0
743.5 0 0 0 1 0 1 1 1 0 0 1 1 1 1 0 0 0 0 0 0 0 0 0 0
2001.89 0 0 1 1 1 1 1 0 1 0 0 0 1 1 1 1 0 0 0 1 1 1 1 0
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CUSTOM RTDS
convention shown in Table 94, where the first 14 bits
are the integer part and the remaining 10 bits are the
fractional part.
In this example, a custom RTD tied to CH3/CH4, with a
sense resistor on CH1/CH2, is programmed with the chan-
nel assignment data shown in Table 95 (refer to Figure 18
for a similar format). In this case, the custom data begins
at memory location 0x28C (starting address is 10). The
starting address (offset from 0x250) is entered in the
custom RTD data pointer field of the channel assignment
data. The table data length –1 (9 in this case) is entered
into the custom RTD data length field of the channel as-
signment word. Refer to Table 91 where the total number
of paired entries is 10.
Table 94. Example RTD Temperature Values
BYTE 1 BYTE 2 BYTE 3
B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
Temperature 213 212 211 210 292827262524232221202–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10
112.3 0 0 0 0 0 0 0 1 1 1 0 0 0 0 0 1 0 0 1 1 0 0 1 1
200.56 0 0 0 0 0 0 1 1 0 0 1 0 0 0 1 0 0 0 1 1 1 1 0 1
273.16 0 0 0 0 0 0 1 0 0 1 0 0 0 1 0 0 1 0 1 0 0 0 1 1
377.25 0 0 0 0 0 0 1 1 1 1 1 0 0 1 0 1 0 0 0 0 0 0 0 0
489.66 0 0 0 0 0 0 0 1 1 0 1 0 0 1 1 0 1 0 1 0 0 0 1 1
595.22 0 0 0 0 1 0 0 1 0 1 0 0 1 1 0 0 1 1 1 0 0 0 0 1
697.87 0 0 0 0 1 0 1 0 1 1 1 0 0 1 1 1 0 1 1 1 1 0 1 0
765.14 0 0 0 0 1 1 0 1 1 1 1 1 0 1 0 0 1 0 0 0 1 1 1 1
801.22 0 0 0 0 1 0 1 0 1 0 0 0 0 1 0 0 1 1 1 0 0 0 0 1
900.5 0 0 0 0 1 1 1 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 0
Table 95. Custom RTD Channel Assignment Data
CONFIGURATION
FIELD DESCRIPTION # BITS
BINARY
DATA
MEMORY
ADDRESS 0x20C
MEMORY
ADDRESS 0x20D
MEMORY
ADDRESS 0x20E
MEMORY
ADDRESS 0x20F
(1) RTD Type Custom 5 10010 1 0 0 1 0
(2) Sense Resistor
Channel Pointer
CH25 00010 0 0 0 1 0
(3) Sensor
Configuration
4-Wire, No
Rotate, No Share
4 1000 1 0 0 0
(4) Excitation Current 25µA 4 0011 0 0 1 1
(5) Curve Not Used for
Custom
2 00 0 0
(6) Custom RTD Data
Pointer
Start Address
= 10
6 001010 0 0 1 0 1 0
(6) Custom RTD Data
Length-1
Data Length –1
= 9
10 Paired Entries
6 001001 0 0 1 0 0 1
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CUSTOM THERMISTORS
Figure 62. Custom NTC Thermistor Example (Ω vs Kelvin) Figure 63. Custom PTC Thermistor Example (Ω vs Kelvin)
29861 F62
p9
p8
p7
p6
p5
p4
p3
NOTE:
P0 SHOULD BE THE
EXTRAPOLATION
POINT TO 0Ω
RESISTANCE (Ω)
TEMPERATURE (K)
p2
p1
0
0
p0
RESISTANCE < p1
SENSOR UNDER-RANGE
SOFT FAULT CONDITION
RESISTANCE > p9
SENSOR OVER-RANGE
SOFT FAULT CONDITION
29861 F63
p9
p8
p7
p6
p5
p4
p3
NOTE:
P0 SHOULD BE THE
EXTRAPOLATION
POINT TO 0Ω
RESISTANCE (Ω)
TEMPERATURE (K)
p2
p1
0
0p0
RESISTANCE < p1
SENSOR UNDER-RANGE
SOFT FAULT CONDITION
RESISTANCE > p9
SENSOR OVER-RANGE
SOFT FAULT CONDITION
In addition to digitizing standard thermistors, the
LTC2986 can also digitize custom thermistors (thermistor
type=0b11011, see Table 55). Custom sensor data (mini-
mum of three, maximum of 64 pairs) reside sequentially
in memory and are arranged in blocks of six bytes of
monotonically increasing tabular data Ω vs temperature
(see Table 91).
Custom Thermistor Table Example
In this example, a simplified thermistor NTC (negative tem-
perature coefficient) curve is implemented (see Figure 62).
Points P1 to P9 represent the normal operating range of
the custom thermistor. Resistance readings above point
P9 result in a soft fault and the reported temperature is
a linear extrapolation using a slope determined by points
P8 and P9 (the final two table entries). Resistance read-
ings below point P1 are also reported as soft faults. The
temperature reported is the extrapolation between point
P1 and P0, where P0 is the sensor output temperature
at 0Ω (This point must be 0Ω for proper interpolation
below point P1).
In addition to NTC type thermistors, it is also possible to
implement PTC (positive temperature coefficient) type
thermistors (see Figure 63). In both cases, table entries
start at the minimum resistance and end at the maximum
resistance value.
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Custom thermistor table data is formatted in Ω (sensor
output resistance) vs Kelvin (see Table 96). Each table
entry pair spans six bytes. The first set of data can begin
at any memory location greater than or equal to 0x250
and end below 0x3CF.
In order to program the LTC2986 with the custom therm-
istor table, both the resistance data and the Kelvin data
are converted to 24-bit binary values. The sensor output
resistance (units = Ω) follows the convention shown in
Table 97, where the first 20 bits are the integer part and
the remaining four bits are the fractional part.
In order to simplify the temperature field, temperature
values are input in Kelvin as an unsigned value, but the
final temperatures reported by the LTC2986 are reported
in °C or °F. The sensor temperature (Kelvin) follows the
convention shown in Table 98, where the first 14 bits
are the integer part and the remaining 10 bits are the
fractional part.
Table 96. NTC Thermistor Example Resistance vs Kelvin Data Memory Map
POINT SENSOR OUTPUT
RESISTANCE(Ω)
TEMPERATURE
(K)
START
ADDRESS
STOP
ADDRESS BYTE 1 BYTE 2 BYTE 3 BYTE 1 BYTE 2 BYTE 3
P0 0 457.5 0x2C8 0x2CD
P1 80 400.2 0x2CE 0x2D3
P2 184 372.3 0x2D4 0x2D9
P3 423.2 320.1 0x2DA 0x2DF
P4 973.36 290.55 0x2E0 0x2E5 Resistance Data Temperature Data
P5 2238.728 249.32 0x2E6 0x2EB
P6 5149.0744 240.3 0x2EC 0x2F1
P7 26775.18688 230 0x2F2 0x2F7
P8 139230.9718 215.3 0x2F8 0x2FD
P9 724001.0532 200 0x2FE 0x303
Table 97. Example Thermistor Resistance Values
BYTE 1 BYTE 2 BYTE 3
B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
Resistance 219 218 217 216 215 214 213 212 211 210 292827262524232221202–1 2–2 2–3 2–4
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
80 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0
184 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 1 1 0 0 0 0 0 0 0
423.2 0 0 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 1 1 0 0 1 1
973.36 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 0 1 1 0 1 0 1 0 1
2238.728 0 0 0 0 0 0 0 0 1 0 0 0 1 0 1 1 1 1 1 0 1 0 1 1
5149.074 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 1 1 1 0 1 0 0 0 1
26775.19 0 0 0 0 0 1 1 0 1 0 0 0 1 0 0 1 0 1 1 1 0 0 1 1
139231 0 0 1 0 0 0 0 1 1 1 1 1 1 1 0 1 1 1 1 1 0 0 0 0
724001.1 1 0 1 1 0 0 0 0 1 1 0 0 0 0 1 0 0 0 0 1 0 0 0 1
CUSTOM THERMISTORS
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Table 98. Example Thermistor Temperature Values
BYTE 1 BYTE 2 BYTE 3
B23 B22 B21 B20 B19 B18 B17 B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
Temperature 213 212 211 210 292827262524232221202–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8 2–9 2–10
457.5 0 0 0 0 0 1 1 1 0 0 1 0 0 1 1 0 0 0 0 0 0 0 0 0
400.2 0 0 0 0 0 1 1 0 0 1 0 0 0 0 0 0 1 1 0 0 1 1 0 0
372.3 0 0 0 0 0 1 0 1 1 1 0 1 0 0 0 1 0 0 1 1 0 0 1 1
320.1 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 0 1 1 0 0 1 1 0
290.55 0 0 0 0 0 1 0 0 1 0 0 0 1 0 1 0 0 0 1 1 0 0 1 1
249.32 0 0 0 0 0 0 1 1 1 1 1 0 0 1 0 1 0 1 0 0 0 1 1 1
240.3 0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 1 0 0 1 1 0 0 1 1
230 0 0 0 0 0 0 1 1 1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0
215.3 0 0 0 0 0 0 1 1 0 1 0 1 1 1 0 1 0 0 1 1 0 0 1 1
200 0 0 0 0 0 0 1 1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0
Table 99. Custom Thermistor Channel Assignment Data
CONFIGURATION
FIELD DESCRIPTION # BITS
BINARY
DATA
MEMORY
ADDRESS 0x210
MEMORY
ADDRESS 0x211
MEMORY
ADDRESS 0x212
MEMORY
ADDRESS 0x213
(1) Thermistor Type Custom Table 5 11011 1 1 0 1 1
(2) Sense Resistor
Channel Pointer
CH45 00100 0 0 1 0 0
(3) Sensor
Configuration
Single-Ended 3 100 1 0 0
(4) Excitation Current 1µA 4 0011 0 0 1 1
Not Used Set These Bits
to 0
3 00 0 0 0
(5) Custom Thermistor
Data Pointer
Start Address
= 20
6 010100 0 1 0 1 0 0
(5) Custom Thermistor
Length-1
Length –1 = 9 6 001001 0 0 1 0 0 1
In this example, a custom thermistor tied to CH5, with a
sense resistor on CH3/4, is programmed with the channel
assignment data shown in Table 99 (refer to Figure 27
for similar format). In this case the custom data begins
at memory location 0x2C8 (starting address is 20). The
starting address (offset from 0x250) is entered in the
custom thermistor data pointer field of the channel as-
signment data. The table data length –1 (9 in this case)
is entered into the custom thermistor data length field of
the thermistor channel assignment word.
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CUSTOM THERMISTORS
In addition to custom table driven thermistors, it is also
possible to directly input Steinhart-Hart coefficients into
the LTC2986 (thermistor Type 11010, see Table 55).
Steinhart-Hart coefficients are commonly specified
parameters provided by thermistor manufacturers. The
Steinhart-Hart equation is:
1
T= A+B•ln(R)+C•ln(R)2+D•ln(R)3+E•ln(R)4
+F •ln(R)5
Steinhart-Hart data is stored sequentially in any memory
location greater than or equal to 0x250 and below 0x3CF.
Each coefficient is represented by a standard, single-
precision, IEEE754 32-bit value (see Table 100).
Example Custom Steinhart-Hart Thermistor
In this example a Steinhart-Hart equation is entered into
memory starting at location 0x2C8 (see Table 101).
Table 100. Steinhart-Hart Custom Thermistor Data Format
ADDRESS COEFFICIENT VALUE
0x250 + 4 • Start Address A 32-Bit Single-Precision Floating Point Format
0x250 + 4 • Start Address + 4 B 32-Bit Single-Precision Floating Point Format
0x250 + 4 • Start Address + 8 C 32-Bit Single-Precision Floating Point Format
0x250 + 4 • Start Address + 12 D 32-Bit Single-Precision Floating Point Format
0x250 + 4 • Start Address + 16 E 32-Bit Single-Precision Floating Point Format
0x250 + 4 • Start Address + 20 F 32-Bit Single-Precision Floating Point Format
Table 101. Custom Steinhart-Hart Data Example
COEFFICIENT VALUE
START
ADDRESS SIGN
EXPONENT MANTISSA
MSB LSB MSB LSB
A 1.45E-03 0x2C8 0 0 1 1 1 0 1 0 1 0 1 1 1 1 1 0 0 0 0 0 1 1 0 1 1 1 1 0 1 1 0 1
B 2.68E-04 0x2CC 0 0 1 1 1 0 0 1 1 0 0 0 1 1 0 0 1 0 0 0 0 0 1 0 0 1 0 1 1 0 1 0
C 0 0x2D0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
D 1.64E-07 0x2D4 0 0 1 1 0 1 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 1 1 1 1 1 1 1 1 0 1 0
E 0 0x2D8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
F 0 0x2DC 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
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A custom thermistor tied to CH5, with a sense resistor on
CH3/4, is programmed with the channel assignment data
shown in Table 102 (refer to Figure 27 for a similar format).
In this case the custom data begins at memory location
0x2C8 (starting address is 30). The starting address
(offset from 0x250) is entered in the custom thermistor
data pointer field of the channel assignment data. The data
length (set to 0) is always six 32-bit floating point words.
Table 102. Custom Steinhart-Hart Channel Assignment Data
CONFIGURATION
FIELD DESCRIPTION # BITS
BINARY
DATA
MEMORY
ADDRESS 0x210
MEMORY
ADDRESS 0x211
MEMORY
ADDRESS 0x212
MEMORY
ADDRESS 0x213
(1) Thermistor Type Custom
Steinhart-Hart
5 11010 1 1 0 1 0
(2) Sense Resistor
Channel Pointer
CH45 00100 0 0 1 0 0
(3) Sensor
Configuration
Single-Ended 3 100 1 0 0
(4) Excitation Current 1µA 4 0011 0 0 1 1
Not Used Set These Bits
to 0
3 00 0 0 0
(5) Custom Thermistor
Data Pointer
Start Address
= 30
6 011110 0 1 1 1 1 0
(5) Custom Steinhart-
Hart Length Always
Set to 0
Fixed at Six
32-Bit Words
6 000000 0 0 0 0 0 0
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PACKAGE DESCRIPTION
Please refer to http://www.linear.com/product/LTC2986#packaging for the most recent package drawings.
LX48 LQFP 0113 REV A
0° – 7°
11° – 13°
0.45 – 0.75
1.00 REF
11° – 13°
9.00 BSC
A A
7.00 BSC
1
2
7.00 BSC
9.00 BSC
48
1.60
MAX
1.35 – 1.45
0.05 – 0.150.09 – 0.20 0.50
BSC 0.17 – 0.27
GAUGE PLANE
0.25
NOTE:
1. PACKAGE DIMENSIONS CONFORM TO JEDEC #MS-026 PACKAGE OUTLINE
2. DIMENSIONS ARE IN MILLIMETERS
3. DIMENSIONS OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.25mm ON ANY SIDE, IF PRESENT
4. PIN-1 INDENTIFIER IS A MOLDED INDENTATION, 0.50mm DIAMETER
5. DRAWING IS NOT TO SCALE
SEE NOTE: 4
C0.30 – 0.50
R0.08 – 0.20
7.15 – 7.25
5.50 REF
1
2
5.50 REF
7.15 – 7.25
48
PACKAGE OUTLINE
RECOMMENDED SOLDER PAD LAYOUT
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
SECTION A – A
0.50 BSC
0.20 – 0.30
1.30 MIN
LX Package
48-Lead Plastic LQFP (7mm × 7mm)
(Reference LTC DWG # 05-08-1760 Rev A)
e3
LTCXXXX
LX-ES
Q_ _ _ _ _ _
XXYY
TRAY PIN 1
BEVEL PACKAGE IN TRAY LOADING ORIENTATION
COMPONENT
PIN “A1”
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 09/16 Added H-grade. 3 - 5
LTC2986/LTC2986-1
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LINEAR TECHNOLOGY CORPORATION 2016
LT 0916 REV A • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC2986
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Universal Inputs Allow Common Hardware Sharing for Thermocouples, Diodes,
Thermistors, 3-Wire RTDs, and 4-Wire RTDs
29861 TA02
0.1µF
10µF
10µF
2.85V TO 5.25V
LTC2986
48
47
46
13
14
11
43
37
2, 4, 6, 8, 45
17
18
19
16
21
22
23
24
25
20
1, 3, 5, 7, 9, 12, 15, 26–35, 44
COM
VDD
VREFOUT
VREFP
Q1
Q2
Q3
1µF
VREF_BYP 1µF
LDO 10µF
INTERRUPT
GND
RP6
29861 TA02
CH3
CH2
CH1
CH4
CH5
CH8
CH9
CH10
CH7
CH6
RP2
RP3
RP8
RP5
RSENSE
RP4
RP1
RP7
CS
SDI
SDO
SCK
42
41
40
39
38
(OPTIONAL, DRIVE
LOW TO RESET)
SPI INTERFACE
RESET
COLD JUNCTION
RTD OR
THERMISTOR OR
DIODE OR
ANALOG TEMP
2
1
2
3
1
3
4
2
1
THERMOCOUPLE THERMISTOR 2-WIRE RTD 3-WIRE RTD 4-WIRE RTD
UNIVERSAL PROTECTED MULTI-SENSOR INPUT
36
T2
T3
T4
T1
10µF
