MAX31851RATB+T - Maxim Integrated

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

General Description

The MAX31850/MAX31851 cold-junction compensat-

ed, 1-WireM thermocouple-to-digital converters perform

cold-junction compensation and digitize the signal from

a K-, J-, N-, T-, S-, R-, or E-type thermocouple. The con-

verters resolve temperatures to 0.25NC, allow readings

as high as +1768NC and as low as -270NC, and exhibit

thermocouple accuracy of 8 LSBs (2NC) for temperatures

ranging from -200NC to +700NC.

Communication with the master microcontroller is over a

1-Wire bus that by definition requires only one data line

(and ground) for communication. Operating power can

be obtained directly from the data line (“parasite power”),

eliminating the need for an external power supply.

Each device has a unique 64-bit serial code, which

allows multiple units to function on the same 1-Wire bus.

Therefore, it is simple to use one microcontroller (the

master device) to monitor temperature from many ther-

mocouples distributed over a large area.

Four location address inputs simplify mapping of

individual units to specific locations.

Benefits and Features

S Integration Reduces Design Time and Lowers

System Cost

• 14-Bit, 0.25NC Resolution

• Integrated Cold-Junction Compensation

• Available for Multiple Thermocouples Types:

Supports K-, J-, N-, T-, and E-Type (MAX31850);

S- and R-Type (MAX31851)

• Detects Thermocouple Shorts to GND or VDD

• Detects Open Thermocouple

S 1-Wire Multiddrop Capability Simplifies

Multisensor Systems

• 1-Wire Interface (Read-Only); Power Can Be

Obtained from Interface (Parasite-Powered Mode)

Applications

Industrial

Appliances

HVAC

Medical

Block Diagram

19-6725; Rev 2; 1/15

Ordering Information appears at end of data sheet.

1-Wire is a registered trademark of Maxim Integrated Products, Inc.

For related parts and recommended products to use with this part, refer to: www.maximintegrated.com/MAX31850.related

EVALUATION KIT AVAILABLE

MAX31850

MAX31851

64-BIT ROM

AND

1-Wire PORT

ADC

PARASITE-

POWER

CIRCUIT

CPP

VDD

GND

MEMORY

CONTROL LOGIC

COLD-JUNCTION

COMPENSATION

FAULT

DETECTION

REFERENCE

VOLTAGE

POWER-

SUPPLY

SENSE

ADDRESS PIN INPUT

AD0 AD1 AD2 AD3

SCRATCHPAD

T+

T-

For pricing, delivery, and ordering information, please contact Maxim Direct at

1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

2Maxim Integrated

Supply Voltage Range (VDD to GND) .................. -0.3V to +4.0V

All Other Pins ............................................ -0.3V to (VDD + 0.3V)

Continuous Power Dissipation (TA = +70NC)

TDFN (derate 16.70mW/NC above +70NC) ........... 1333.30mW

ESD Protection (All Pins, Human Body Model) ...................±2kV

Operating Temperature Range ........................ -40NC to +125NC

Junction Temperature .....................................................+150NC

Storage Temperature Range .......................... -65NC to +150NC

Lead Temperature (soldering, 10s) ................................+300NC

Soldering Temperature (reflow) .....................................+260NC

TDFN

Junction-to-Ambient Thermal Resistance (BJA) ..........60NC/W

Junction-to-Case Thermal Resistance (BJC) ...............30NC/W

ABSOLUTE MAXIMUM RATINGS

Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-layer

board. For detailed information on package thermal considerations, refer to www.maximintegrated.com/thermal-tutorial.

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional opera-

tion of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute

maximum rating conditions for extended periods may affect device reliability.

PACKAGE THERMAL CHARACTERISTICS (Note 1)

DC ELECTRICAL CHARACTERISTICS

(TA = -40NC to +125NC, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Supply Voltage VDD Local power (Note 3, 4) 3.0 3.3 3.7 V

Pullup Supply Voltage

(Notes 3, 4) VPU

Parasite power (Note 3) 3.0 3.7 V

Local power (Note 3) 3.0 VDD

Input Logic-Low VIL (Notes 4, 6) -0.3 +0.8 V

Input Logic-High (Note 4) VIH

Local power 2.2

lower

of 3.7V

(VDD +

0.3V)

Parasite power 3.0

lower

of 3.7V

(VDD +

0.3V)

Output Sink Current ILVI/O = 0.4V (Note 4) 4.0 mA

Standby Supply Current IDDS (Notes 7, 8) 280 1000 nA

Active Supply Current IDD VDD = 3.7V (Note 9) 900 1750 FA

DQ Input Current IDQ (Note 10) 5FA

Power-Supply Rejection -0.3 °C/V

Input Leakage Current (AD0–AD3 Pins) (Note 3) -1 +1 FA

Thermocouple Input Bias Current IBTC TA = -40°C to +125°C, 100mV across

the thermocouple inputs (Note 3) -100 +100 nA

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

3Maxim Integrated

THERMAL CHARACTERISTICS

(3.0V P VDD P 3.6V, TA = -40NC to +125NC, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

MAX31850K Thermocouple

Temperature Gain and Offset

Error (41.276FV/NC nominal

sensitivity) (Note 11)

TTHERMOCOUPLE = -100NC to +100NC,

TA = 0NC to +70NC (Note 12) -1 +1

TTHERMOCOUPLE = -200NC to +700NC,

TA = -20NC to +85NC (Note 12) -2 +2

TTHERMOCOUPLE = +700NC to +1372NC,

TA = -20NC to +85NC (Note 12) -4 +4

TTHERMOCOUPLE = -270NC to +1372NC,

TA = -40NC to +125NC (Note 12) -6 +6

MAX31850J Thermocouple

Temperature Gain and Offset

Error (57.953FV/NC nominal

sensitivity) (Note 11)

TTHERMOCOUPLE = -100NC to +100NC,

TA = 0NC to +70NC (Note 12) -1 +1

TTHERMOCOUPLE = -210NC to +750NC,

TA = -20NC to +85NC (Note 12) -2 +2

TTHERMOCOUPLE = -210NC to +1200NC,

TA = -40NC to +125NC (Note 12) -4 +4

MAX31850N Thermocouple

Temperature Gain and Offset

Error (36.256FV/NC nominal

sensitivity) (Note 11)

TTHERMOCOUPLE = -100NC to +100NC,

TA = 0NC to +70NC (Note 12) -1 +1

TTHERMOCOUPLE = -200NC to +700NC,

TA = -20NC to +85NC (Note 12) -2 +2

TTHERMOCOUPLE = +700NC to +1300NC,

TA = -20NC to +85NC (Note 12) -4 +4

TTHERMOCOUPLE = -270NC to +1300NC,

TA = -40NC to +125NC (Note 12) -6 +6

MAX31850T Thermocouple

Temperature Gain and Offset

Error (52.18FV/NC nominal

sensitivity) (Note 11)

TTHERMOCOUPLE = -100NC to +100NC,

TA = 0NC to +70NC (Note 12) -1 +1

TTHERMOCOUPLE = -270NC to +400NC,

TA = -20NC to +85NC (Note 12) -2 +2

TTHERMOCOUPLE = -270NC to +400NC,

TA = -40NC to +125NC (Note 12) -4 +4

MAX31850E Thermocouple

Temperature Gain and Offset

Error (76.373FV/NC nominal

sensitivity) (Note 11)

TTHERMOCOUPLE = -120NC to +100NC,

TA = -20NC to +85NC (Note 12) -1 +1

TTHERMOCOUPLE = -200NC to +700NC,

TA = -20NC to +85NC (Note 12) -2 +2

TTHERMOCOUPLE = +700NC to +1000NC,

TA = -20NC to +85NC (Note 12) -4 +4

TTHERMOCOUPLE = -270NC to +1000NC,

TA = -40NC to +125NC (Note 12) -5 +5

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

4Maxim Integrated

THERMAL CHARACTERISTICS (continued)

(3.0V P VDD P 3.6V, TA = -40NC to +125NC, unless otherwise noted.) (Note 2)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

MAX31851R Thermocouple

Temperature Gain and Offset

Error (10.506FV/NC nominal

sensitivity) (Note 11)

TTHERMOCOUPLE = -50NC to +100NC,

TA = 0NC to +70NC (Note 12) -2 +2

TTHERMOCOUPLE = -50NC to +700NC,

TA = -20NC to +85NC (Note 12) -3 +3

TTHERMOCOUPLE = +700NC to +1768NC,

TA = -20NC to +85NC (Note 12) -5 +5

TTHERMOCOUPLE = -50NC to +1768NC,

TA = -40NC to +125NC (Note 12) -7 +7

MAX31851S Thermocouple

Temperature Gain and Offset

Error (9.587FV/NC nominal

sensitivity) (Note 11)

TTHERMOCOUPLE = -50NC to +150NC,

TA = 0NC to +70NC (Note 12) -2 +2

TTHERMOCOUPLE = -50NC to +700NC,

TA = -20NC to +85NC (Note 12) -3 +3

TTHERMOCOUPLE = +700NC to +1768NC,

TA = -20NC to +85NC (Note 12) -5 +5

TTHERMOCOUPLE = -50NC to +1768NC,

TA = -40NC to +125NC (Note 12) -7 +7

Thermocouple Temperature Data

Resolution 0.25 NC

Thermocouple Temperature Data

Long-Term Drift Hot junction temperature = +400NC ±0.24 NC

Internal Cold-Junction

Temperature Error TA = -40NC to +100NC (Note 13) -2 +2 NC

Cold-Junction Temperature Data

Resolution TA = -40NC to +125NC 0.0625 NC

Temperature Conversion Time

(Thermocouple, Cold Junction,

Fault Detection)

tCONV (Note 14) 72 100 ms

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

5Maxim Integrated

Note 2: Limits are 100% production tested at TA = +25NC. Limits over the operating temperature range and relevant supply volt-

age range are guaranteed by design and characterization.

Note 3: Limits are 100% production tested at TA = +25NC and +85NC. Limits over the operating temperature range and relevant sup-

ply voltage are guaranteed by design and characterization.

Note 4: All voltages are referenced to GND. Currents entering the IC are specified positive and currents exiting the IC are negative.

Note 5: The pullup supply voltage specification assumes that the pullup device is ideal, and therefore the high level of the pullup

is equal to VPU. To meet the device’s VIH specification, the actual supply rail for the strong pullup transistor must include

margin for the voltage drop across the transistor when it is turned on. Thus: VPU_ACTUAL = VPU_IDEAL + VTRANSISTOR.

Note 6: To guarantee a presence pulse under low-voltage parasite power conditions, VILMAX, may have to be reduced to as low

as 0.5V.

Note 7: Standby current specified up to +70NC.

Note 8: To minimize IDDS, DQ should be within the following ranges: VGND P VDQ P VGND + 0.3V or VDD - 0.3V P VDQ P VDD.

Note 9: Active current refers to supply current during active temperature conversions.

Note 10: DQ is high (high-impedance state with external pullup).

Note 11: Not including cold-junction temperature error or thermocouple nonlinearity.

Note 12: Guaranteed by design. These limits represent six sigma distribution for TA = +25NC to +85NC. Outside this temperature

range, these limits are three sigma distribution.

Note 13: Guaranteed by design. These limits represent a three sigma distribution.

Note 14: After minimum VDD has been reached during power-up, wait 10ms before initiating temperature conversions.

Note 15: See the 1-Wire Timing Diagrams.

Note 16: Under parasite power, if tRSTL > 960Fs, a power-on reset (POR) may occur.

Note 17: Represents the maximum capacitive load that may be applied to the pins and still maintain timing and logic state.

1-Wire TIMING CHARACTERISTICS

(3.0V P VDD P 3.6V, TA = -40NC to +125NC, unless otherwise noted.) (Notes 2, 3)

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Time to Strong Pullup On tSPON Start Convert T command issued 8 µs

Time Slot tSLOT (Note 15) 60 120 µs

Recovery Time tREC (Note 15) 1 µs

Write-0 Low Time tLOW0 (Note 15) 60 120 µs

Write-1 Low Time tLOW1 (Note 15) 1 15 µs

Read Data Valid tRDV (Note 15) 15 µs

Reset Time High tRSTH (Note 15) 480 µs

Reset Time Low tRSTL (Notes 15, 16) 480 µs

Presence Detect High tPDHIGH (Note 15) 15 60 µs

Presence Detect Low tPDLOW (Note 15) 60 240 µs

Capacitance: DQ CIN/OUT (Note 17) 25 pF

Capacitance: AD0–AD3 CIN_ADD (Note 17) 50 pF

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

6Maxim Integrated

1-Wire Timing Diagrams

1-Wire WRITE-ZERO TIME SLOT

1-Wire READ-ZERO TIME SLOT

1-Wire RESET PULSE

1-Wire PRESENCE DETECT

RESET PULSE FROM HOST

PRESENCE DETECT

tSLOT

tRSTL tRSTH

START OF NEXT CYCLE

tLOW0

tREC

tPDHIGH

tPDLOW

tRDV

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

7Maxim Integrated

Typical Operating Characteristics

(VCC = +3.3V, TA = +25NC, unless otherwise noted.)

ADC ACCURACY vs. ADC INPUT

VOLTAGE ACROSS VCC

MAX31850 toc05

ADC INPUT VOLTAGE (mV)

ADC ACCURACY (°C)

4020

-0.25

-0.20

-0.15

-0.10

-0.05

0.05

0.10

0.15

0.20

-0.30

0 60

INTERNAL TEMPERATURE = +25°C

VCC = 3.7V

VCC = 3.3V

VCC = 3.0V

ADC ACCURACY vs. ADC INPUT VOLTAGE

ACROSS TEMPERATURE

MAX31850 toc04

ADC INPUT VOLTAGE (mV)

ADC ACCURACY (°C)

4020

0.5

1.0

1.5

2.0

-0.5

0 60

VCC = 3.3V

TA = +125°C

TA = -40°C

TA = +85°C

TA = +25°C

STANDBY SUPPLY CURRENT

vs. TEMPERATURE

MAX31850 toc02

TEMPERATURE (°C)

STANDBY SUPPLY CURRENT (nA)

12010060 800 20 40-20

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

-40

VCC = 3.7V

INTERNAL TEMPERATURE

SENSOR ACCURACY

MAX31850 toc03

TEMPERATURE (°C)

MEASUREMENT ERROR (°C)

1008040 600 20-20

-0.3

-0.2

-0.1

0.1

0.2

0.3

0.4

0.5

-0.4

-40 120

VCC = 3.3V

ACTIVE SUPPLY CURRENT

vs. TEMPERATURE

MAX31850 toc01

TEMPERATURE (°C)

ACTIVE SUPPLY CURRENT (µA)

100 120806040200-20

200

400

600

800

1000

1200

1400

-40

VCC = 3.7V

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

8Maxim Integrated

Pin Description

Pin Configuration

PIN NAME FUNCTION

1 GND Ground

2 T- Thermocouple Input. See Table 1. Do not connect to GND.

3 T+ Thermocouple Input. See Table 1.

4 VDD Power-Supply Voltage

5 DQ Data Input/Output. Open-drain 1-Wire interface pin. Also provides power to the device when used in

parasite-power mode (see the Parasite Power section.)

6 AD0 Location Address Input (Least Significant Bit)

7 AD1 Location Address Input

8 AD2 Location Address Input

9 AD3 Location Address Input (Most Significant Bit)

10 DNC Do Not Connect

— EP Exposed Pad. No internal connection. Connect to GND or leave unconnected.

+EP

AD0

AD1

DNC

AD3

AD2

DQVDD

GND T- T+

1 2 3

TDFN-EP

(3mm x 4mm)

TOP VIEW

MAX31850

MAX31851

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

9Maxim Integrated

Detailed Description

The MAX31850/MAX31851 are sophisticated thermo-

couple-to-digital converters with a built-in 14-bit analog-

to-digital converter (ADC), cold-junction compensation

sensing and correction, a digital controller, a 1-Wire

data interface, and associated control logic. The devices

are available in several versions, each optimized and

trimmed for a specific thermocouple type (K, J, N, T, S,

R, or E.). The thermocouple type is indicated in the suffix

of the part number (e.g., MAX31850K). See the Ordering

Information table for all options.

The 1-Wire bus by definition requires only one data line

(and ground) for communication with a central microcon-

troller. The data line requires a weak pullup resistor since

all devices are linked to the bus through a three-state or

open-drain port (i.e., the DQ pin). Four location address

inputs simplify mapping of individual devices to specific

locations.

Each device has a unique 64-bit serial code, allowing

multiple devices to function on the same 1-Wire bus.

Therefore, it is simple to use one microcontroller to con-

trol many devices distributed over a large area. In this

bus system, the microcontroller identifies and addresses

devices on the bus using each device’s unique 64-bit

code. Because each device has a unique code, the

number of devices that can be addressed on one bus

is virtually unlimited. The 1-Wire bus protocol, including

detailed explanations of the commands and time slots, is

described in the 1-Wire Bus System section.

The scratchpad memory contains the 2-byte temperature

mocouple temperature data. A second 2-byte register

stores the local cold-junction temperature. Both of these

registers also store fault data for open thermocouple as

well as shorts to supply and ground.

Power may be obtained either from a power supply con-

nected to VDD, or from the 1-Wire pullup resistor through

the DQ pin when the bus is high. The high bus signal also

charges an internal capacitor (CPP), which then supplies

power to the device when the bus is low. This method

of deriving power from the 1-Wire bus is referred to as

“parasite power.”

Temperature Conversion

The devices include signal-conditioning hardware to

convert the thermocouple’s signal into a voltage com-

patible with the input channels of the ADC. The T+

and T- inputs connect to internal circuitry that reduces

the introduction of noise errors from the thermocouple

wires.

Before converting the thermoelectric voltages into equiva-

lent temperature values, it is necessary to compensate for

the difference between the thermocouple cold-junction

side (device ambient temperature) and a 0NC virtual refer-

ence. For a K-type thermocouple, the voltage changes by

approximately 41FV/NC, which approximates the thermo-

couple characteristic with the following linear equation:

VOUT = (41.276FV/NC) x (TR - TAMB)

where VOUT is the thermocouple output voltage (FV), TR

is the temperature of the remote thermocouple junction

(NC), and TAMB is the temperature of the device (NC).

Other thermocouple types use a similar straight-line

approximation but with different gain terms. Note that

the MAX31850/MAX31851 assume a linear relationship

between temperature and voltage. Because all thermo-

couples exhibit some level of nonlinearity, apply appro-

priate correction to the device’s output data.

Cold-Junction Compensation

The function of the thermocouple is to sense a difference

in temperature between two ends of the thermocouple

wires. The thermocouple’s “hot” junction can be read

across the operating temperature range (Table 1). The

reference junction, or “cold” end (which should be at

the same temperature as the board on which the device

is mounted) can range from -55NC to +125NC. While the

temperature at the cold end fluctuates, the device con-

tinues to accurately sense the temperature difference at

the opposite end.

The device senses and corrects for the changes in

the reference junction temperature with cold-junction

compensation. It does this by first measuring its internal

die temperature, which should be held at the same tem-

perature as the reference junction. It then measures the

voltage from the thermocouple’s output at the reference

junction and converts this to the noncompensated ther-

mocouple temperature value. This value is then added

to the device’s die temperature to calculate the thermo-

couple’s “hot junction” temperature. Note that the “hot

junction” temperature can be lower than the cold junction

(or reference junction) temperature.

Optimal performance from the device is achieved when

the thermocouple cold junction and the device are at

the same temperature. Avoid placing heat-generating

devices or components near the MAX31850/MAX31851

because this could produce cold-junction-related errors.

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

10Maxim Integrated

Conversion Functions

During the conversion time, tCONV, three functions are

performed: the temperature conversion of the internal

cold-junction temperature, the temperature conversion of

the external thermocouple, and the detection of thermo-

couple faults.

When executing the temperature conversion for the inter-

nal cold-junction compensation circuit, the connection to

signal from the external thermocouple is opened (switch

S4) and the connection to the cold-junction compensa-

tion circuit is closed (switch S5). The internal T- reference

to ground is still maintained (switch S3 is closed) and

the connections to the fault-detection circuit are open

(switches S1 and S2).

When executing the temperature conversion of the

external thermocouple, the connections to the internal

fault-detection circuit are opened (switches S1 and S2 in

the Block Diagram) and the switch connecting the cold-

junction compensation circuit is opened (switch S5). The

internal ground reference connection (switch S3) and

the connection to the ADC (switch S4) are closed. This

allows the ADC to process the voltage detected across

the T+ and T- terminals. If T+ and T- are unconnected,

the thermocouple temperature sign bit is 0 (MSbit of

Scratchpad Byte 1), and the remainder of the thermo-

couple temperature value is 1.

During fault detection, the connections from the exter-

nal thermocouple and cold-junction compensation

circuit to the ADC are opened (switches S4 and S5,

see the Block Diagram). The internal ground reference

on T- is also opened (switch S3). The connections to

the internal fault-detection circuit are closed (switch

S1 and S2). The fault-detection circuit tests for shorted

connections to VDD or GND on the T+ and T- inputs,

as well as looking for an open thermocouple condi-

tion. Bits 0, 1, and 2 of the internal (cold junction)

temperature data are normally low. Bit 2 goes high to

indicate a thermocouple short to VDD, bit 1 goes high

to indicate a thermocouple short to GND, and bit 0

goes high to indicate a thermocouple open circuit. If

any of these conditions exists, bit 0 of the cold-junction

compensated thermacouple temperature data, which

is normally low, also goes high to indicate that a fault

has occurred.

Table 1. Thermocouple Wire Connections and Nominal Sensitivities

TYPE T- WIRE T+ WIRE TEMP RANGE (°C) SENSITIVITY (µV/°C)

COLD-JUNCTION

SENSITIVITY (µV/°C)

(0NC TO +70NC)

KAlumel Chromel -270 to +1372 41.276

(0NC to +1000NC) 40.73

JConstantan Iron -210 to +1200 57.953

(0NC to +750NC) 52.136

NNisil Nicrosil -270 to +1300 36.256

(0NC to +1000NC) 27.171

RPlatinum Platinum/Rhodium -50 to +1768 10.506

(0NC to +1000NC) 6.158

SPlatinum Platinum/Rhodium -50 to +1768 9.587

(0NC to +1000NC) 6.181

TConstantan Copper -270 to +400 52.18

(0NC to +400NC) 41.56

EConstantan Chromel -270 to +1000 76.373

(0NC to +1000NC) 44.123

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

11Maxim Integrated

Powering the MAX31850/MAX31851

The MAX31850/MAX31851 can be powered by an

external supply on the VDD pin, or they can operate

in “parasite power” mode, which allows the device to

function without a local external supply. Parasite power

is useful for applications that require remote tempera-

ture sensing or those that are very space-constrained.

Figure 1 shows the device’s parasite-power control cir-

cuitry, which “steals” power from the 1-Wire bus through

DQ when the bus is high. The stolen charge powers the

device while the bus is high, and some of the charge is

stored on the internal parasite-power capacitor (CPP) to

provide power when the bus is low. When the device is

used in parasite-power mode, VDD must be connected

to ground.

In parasite-power mode, the 1-Wire bus and CPP can

provide sufficient current to the device for most opera-

tions as long as the specified timing and voltage require-

ments are met (see the DC Electrical Characteristics

and the 1-Wire Timing Characteristics tables). However,

when the device is performing temperature conversions,

the operating current can be as high as 1.5mA. This

current can cause an unacceptable voltage drop across

the weak 1-Wire pullup resistor and is more current than

can be supplied by CPP. To ensure that the device has

sufficient supply current, it is necessary to provide a

strong pullup on the 1-Wire bus whenever temperature

conversions are taking place. This can be accomplished

by using a MOSFET to pull the bus directly to the rail

as shown in Figure 1. The 1-Wire bus must be switched

to the strong pullup within 10Fs (max) after a Convert T

[44h] command is issued, and the bus must be held high

by the pullup for the duration of the conversion (tCONV).

No other activity can take place on the 1-Wire bus while

the pullup is enabled.

The device can also be powered by the conventional

method of connecting an external power supply to VDD,

as shown in Figure 2. The advantage of this method is

that the MOSFET pullup is not required, and the 1-Wire

bus is free to carry other traffic during the temperature

conversion period.

The use of parasite power is not recommended for tem-

peratures above 100NC because the device may not be

able to sustain communications due to the higher leak-

age currents that can exist at these temperatures. For

applications in which such temperatures are likely, it is

strongly recommended that the device be powered by

an external power supply.

Figure 1. Supplying the Parasite-Powered MAX31850/MAX31851 During Temperature Conversions

Figure 2. Powering the MAX31850/MAX31851 with an External Supply

µP

VPU

4.7kΩ

VPU

1-Wire BUS TO OTHER 1-Wire DEVICES

MAX31850

MAX31851

GND DQ VDD

µP

VPU

4.7kΩ

1-Wire BUS TO OTHER 1-Wire DEVICES

(EXTERNAL SUPPLY)VDD

MAX31850

MAX31851

GND DQ VDD

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

12Maxim Integrated

Figure 3. 64-Bit ROM Code

Figure 4. Memory Map

In some situations the bus master might not know whether

the devices on the bus are parasite powered or powered

by external supplies. The master needs this information

to determine if the strong bus pullup should be used dur-

ing temperature conversions. To get this information, the

master can issue a Skip ROM [CCh] command, followed

by a Read Power Supply [B4h] command, followed by a

read time slot. During the read time slot, parasite-pow-

ered devices pull the bus low, and externally powered

devices let the bus remain high. If the bus is pulled low,

the master knows that it must supply the strong pullup on

the 1-Wire bus during temperature conversions.

64-Bit ROM Code

Each device contains a unique 64-bit code stored in ROM

(Figure 3). The least significant 8 bits of the ROM code

contain the device’s 1-Wire family code, 3Bh. The next 48

bits contain a unique serial number. The most significant

8 bits contain a cyclic redundancy check (CRC) byte that

is calculated from the first 56 bits of the ROM code. See

the CRC Generation section for a detailed explanation

of the CRC bits. The 64-bit ROM code and associated

ROM function control logic allow the device to operate as

a 1-Wire device using the protocol detailed in the 1-Wire

Bus System section.

Scratchpad

The device’s scratchpad is organized as shown

in Figure 4. All memory commands are described

in detail in the MAX31850/MAX31851 Function

Commands section.

Byte 0 and byte 1 of Scratchpad contain the least sig-

nificant byte and the most significant byte of the thermo-

couple temperature register, respectively. Bytes 2 and 3

contain the LSB and MSB of the internal (cold-junction)

temperature value, as well as fault status. Byte 4 con-

tains the configuration information. Bytes 5, 6, and 7 are

reserved for internal use by the device and cannot be

overwritten; these bytes return all ones when read.

Byte 8 of Scratchpad is read-only and contains the CRC

code for bytes 0–7 of the scratchpad. The device gener-

ates this CRC using the method described in the CRC

Generation section.

MSb

8-BIT

CRC CODE 48-BIT SERIAL NUMBER

MSb MSbLSb

LSb

8-BIT FAMILY CODE

(3Bh)

MSbLSb

COLD-JUNCTION-COMPENSATED THERMOCOUPLE TEMPERATURE LSB AND FAULT STATUS (00h)BYTE 0

COLD-JUNCTION-COMPENSATED THERMOCOUPLE TEMPERATURE MSB (00h)BYTE 1

INTERNAL (COLD JUNCTION) TEMPERATURE AND FAULT STATUS LSB (00h)BYTE 2

INTERNAL (COLD JUNCTION) TEMPERATURE MSB (00h)BYTE 3

CONFIGURATION REGISTER*BYTE 4

RESERVED (FFh)BYTE 5

RESERVED (FFh)BYTE 6

RESERVED (FFh)BYTE 7

CRCBYTE 8

*THE LOWER 4 BITS (AD[3:0]) OF THE CONFIGURATION REGISTER ARE HARDWIRED THROUGH AD0–AD3.

SCRATCHPAD (POWER-UP STATE SHOWN IN PARENTHESES)

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

13Maxim Integrated

Configuration Register

Byte 4 of Scratchpad contains the configuration reg-

ister, which is organized as shown in Configuration

user to read the programmed value of the address pins.

The AD[3:0] bits report the pin-programmed location

information. Pins connected to DQ are reported with

logic 1, and pins connected to GND are reported as

logic 0. Pins connected to DQ or GND through a resis-

tor are valid logic 1s or logic 0s if the resistor is less

than 10kI. Unconnected or high-impedance ( > 10kI)

connections are indeterminate. Bits [7:4] are reserved

for internal use and cannot be overwritten; they return

a 1 when read.

Table 2. Temperature Data Format

Configuration Register Format

Note: Bits [3:0] are programmed through the four location programming address pins, AD[3:0]. Reading the configuration register

provides location information on up to 16 individual devices.

Table 3. Thermocouple Temperature Data

Format

Table 4. Internal (Cold-Junction)

Temperature Data Format

Cold-Junction-Compensated Thermocouple Temperature Data (Bytes 0 and 1)

BIT 76543210

LSByte (Scratchpad

Byte 0) 232221202-1 2-2 Reserved 1 = Fault

BIT 15 14 13 12 11 10 9 8

MSByte (Scratchpad

Byte 1) Sign 210 292827262524

Internal (Cold-Junction) Temperature Data (Bytes 2 and 3)

BIT 76543210

LSByte (Scratchpad

Byte 2) 2-1 2-2 2-3 2-4 Reserved 1 = Short

to VDD

1 = Short

to GND

1 = Open

Circuit

BIT 15 14 13 12 11 10 9 8

MSByte (Scratchpad

Byte 3) Sign 26252423222120

BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0

— — — — AD3 AD2 AD1 AD0

TEMPERATURE

(NC)

DIGITAL OUTPUT

(D[31:18])

+1600.00 0110 0100 0000 00

+1000.00 0011 1110 1000 00

+100.75 0000 0110 0100 11

+25.00 0000 0001 1001 00

0.00 0000 0000 0000 00

-0.25 1111 1111 1111 11

-1.00 1111 1111 1111 00

-250.00 1111 0000 0110 00

TEMPERATURE

(NC)

DIGITAL OUTPUT

(D[15:4])

+127.0000 0111 1111 0000

+100.5625 0110 0100 1001

+25.0000 0001 1001 0000

0.0000 0000 0000 0000

-0.0625 1111 1111 1111

-1.0000 1111 1111 0000

-20.0000 1110 1100 0000

-55.0000 1100 1001 0000

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

14Maxim Integrated

CRC Generation

CRC bytes are provided as part of the device’s 64-bit

ROM code, in the 9th byte of Scratchpad. The ROM code

CRC is calculated from the first 56 bits of the ROM code

and is contained in the most significant byte of the ROM.

The scratchpad CRC is calculated from the data in the

scratchpad, and therefore changes when the data in it

associated scratchpad changes. The CRC provides the

bus master with a method of data validation when data is

read from the device. To verify that data has been read

correctly, the bus master must recalculate the CRC from

the received data and then compare this value to either

the ROM code CRC (for ROM reads) or to the scratch-

pad CRC (for scratchpad reads). If the calculated CRC

matches the read CRC, the data has been received

error-free. The comparison of CRC values and the deci-

sion to continue with an operation are determined entirely

by the bus master. There is no circuitry inside the device

that prevents a command sequence from proceeding if

the device CRC (ROM or scratchpad) does not match the

value generated by the bus master.

The equivalent polynomial function of the CRC (ROM or

scratchpad) is:

CRC = X8 + X5 + X4 + 1

The bus master can recalculate the CRC and compare it

to the CRC values from the device using the polynomial

generator shown in Figure 5. This circuit consists of a

shift register and XOR gates, and the shift register bits

are initialized to 0. Starting with the least significant bit

of the ROM code or the least significant bit of byte 0 in

the scratchpad, one bit at a time should shifted into the

shift register. After shifting in the 56th bit from the ROM

or the most significant bit of byte 7 from the scratchpad,

the polynomial generator contains the recalculated CRC.

Next, the 8-bit ROM code or scratchpad CRC from the

device must be shifted into the circuit. At this point, if the

recalculated CRC was correct, the shift register contains

all zeros. Additional information about the Maxim 1-Wire

CRC is available in Application Note 27: Understanding

and Using Cyclic Redundancy Checks with Maxim iBut-

ton® Products.

1-Wire Bus System

The 1-Wire bus system uses a single bus master to

control one or more slave devices. The MAX31850/

MAX31851 are always a slave. When there is only one

slave on the bus, the system is referred to as a single-

drop system; the system is multidrop if there are multiple

slaves on the bus. All data and commands are transmit-

ted least significant bit first over the 1-Wire bus.

The following discussion of the 1-Wire bus system is

broken down into three topics: hardware configuration,

transaction sequence, and 1-Wire signaling (signal types

and timing).

Hardware Configuration

The 1-Wire bus has by definition only a single data line.

Each device (master or slave) interfaces to the data line

by using an open-drain or three-state port. This allows

each device to “release” the data line when the device

is not transmitting data so the bus is available for use by

another device. The device’s 1-Wire port (DQ) is open

drain with an internal circuit equivalent to that shown in

Figure 6.

Figure 5. CRC Generator

iButton is a registered trademark of Maxim Integrated Products, Inc.

1ST

STAGE

2ND

STAGE

3RD

STAGE

4TH

STAGE

7TH

STAGE

8TH

STAGE

6TH

STAGE

5TH

STAGE

X0X1X2X3X4

POLYNOMIAL = X8 + X5 + X4 + 1

INPUT DATA

X5X6X7X8

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

15Maxim Integrated

Figure 6. Hardware Configuration

The 1-Wire bus requires an external pullup resistor of

approximately 5kI; thus, the idle state for the 1-Wire bus

is high. If for any reason a transaction needs to be sus-

pended, the bus must be left in the idle state if the transac-

tion is to resume. Infinite recovery time can occur between

bits so long as the 1-Wire bus is in the inactive (high) state

during the recovery period. If the bus is held low for more

than 480Fs, all components on the bus are reset.

Transaction Sequence

The transaction sequence for accessing the device is as

follows:

1) Step 1: Initialization

2) Step 2: ROM Command (followed by any required

data exchange)

3) Step 3: MAX31850/MAX31851 Function Command

(followed by any required data exchange)

It is very important to follow this sequence every time the

device is accessed, as the device does not respond if

any steps in the sequence are missing or out of order. An

exception to this rule is the Search ROM [F0h] command.

After issuing this ROM command, the master must return

to step 1 in the sequence.

Initialization

All transactions on the 1-Wire bus begin with an initializa-

tion sequence. The initialization sequence consists of a

reset pulse transmitted by the bus master followed by

presence pulse(s) transmitted by the slave(s). The pres-

ence pulse lets the bus master know that slave devices

(such as the MAX31850/MAX31851) are on the bus and

are ready to operate. Timing for the reset and presence

pulses is detailed in the 1-Wire Signaling section.

ROM Commands

After the bus master has detected a presence pulse, it

can issue a ROM command. These commands operate

on the unique 64-bit ROM codes of each slave device

and allow the master to single out a specific device if

many are present on the 1-Wire bus. These commands

also allow the master to determine how many and what

types of devices are present on the bus. There are four

ROM commands, and each command is 8 bits long. The

master device must issue an appropriate ROM command

before issuing a MAX31850/MAX31851 function com-

mand. Figure 7 shows a flowchart for operation of the

ROM commands.

Search ROM [F0h]

When a system is initially powered up, the master must

identify the ROM codes of all slave devices on the

bus, which allows the master to determine the number

of slaves and their device types. The master learns

the ROM codes through a process of elimination that

requires the master to perform a Search ROM cycle (i.e.,

Search ROM command followed by data exchange) as

many times as necessary to identify all the slave devices.

If there is only one slave on the bus, the simpler Read

ROM command can be used in place of the Search ROM

process. For a detailed explanation of the Search ROM

command procedure, refer to Application Note 937: Book

of iButton® Standards. After every Search ROM cycle,

the bus master must return to step 1 (initialization) in the

transaction sequence.

4.7kΩ

5µA

TYP

VPU

BUS MASTER

OPEN-DRAIN

PORT PIN 100Ω MOSFET

MAX31850/MAX31851 1-Wire PORT

Rx = RECEIVE

Tx = TRANSMIT

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

16Maxim Integrated

Read ROM [33h]

This command can be used only when there is one slave

on the bus. It allows the bus master to read the slave’s

64-bit ROM code without using the Search ROM com-

mand procedure. If this command is used when there is

more than one slave present on the bus, a data collision

occurs when all the slaves attempt to respond at the

same time.

Match ROM [55h]

The Match ROM command followed by a 64-bit ROM

code sequence allows the bus master to address a

specific slave device on a multidrop or single-drop bus.

Only the slave that exactly matches the 64-bit ROM code

sequence responds to the function command issued by

the master; all other slaves on the bus wait for a reset

pulse.

Skip ROM [CCh]

The master can use this command to address all devices

on the bus simultaneously without sending out any ROM

code information. For example, the master can make all

devices on the bus perform simultaneous temperature

conversions by issuing a Skip ROM command followed

by a Convert T [44h] function command.

Note that the Read Scratchpad command can follow the

Skip ROM command only if there is a single slave device

on the bus. In this case, time is saved by allowing the

master to read from the slave without sending the device’s

64-bit ROM code. A Skip ROM command followed by a

Read Scratchpad command causes a data collision on

the bus if there is more than one slave because multiple

devices attempt to transmit data simultaneously.

MAX31850/MAX31851 Function Commands

After the bus master has used a ROM command

to address the MAX31850/MAX31851 with which it

wishes to communicate, the master can issue one of

the MAX31850/MAX31851 function commands. These

commands allow the master to read from the device’s

scratchpad memories, initiate temperature conversions,

and determine the power-supply mode. The MAX31850/

MAX31851 function commands are summarized in

Table 5 and illustrated by the flowchart in Figure 8.

Convert T [44h]

This command initiates a single thermocouple tempera-

ture conversion, which consists of measuring the internal

(cold junction) temperature, measuring the thermocouple

temperature, and detecting any faults. Following the con-

version, the resulting cold-junction-compensated thermo-

couple data, internal temperature data, and fault status

are stored in the two 2-byte temperature registers in

the scratchpad memory, and the MAX31850/MAX31851

returns to its low-power idle state. If the device is being

used in parasite-power mode, within 10Fs (max) after this

command is issued the master must enable a strong pul-

lup on the 1-Wire bus for the duration of the conversion

(tCONV) as described in the Powering the MAX31850/

MAX31851 section. If the device is powered by an exter-

nal supply, the master can issue read time slots after the

Convert T command and the device responds by trans-

mitting 0 while the temperature conversion is in progress

and 1 when the conversion is done. In parasite-power

mode, this notification technique cannot be used because

the bus is pulled high by the strong pullup during the

conversion.

Table 5. MAX31850/MAX31851 Function Command Set

Note 1: For parasite-powered devices, the master must enable a strong pullup on the 1-Wire bus during temperature conversions.

No other bus activity can take place during this time.

Note 2: The master can interrupt the transmission of data at any time by issuing a reset.

Note 3: During the read time slot, parasite-powered devices pull the DQ bus low, and externally powered units let the bus remain

high.

COMMAND DESCRIPTION PROTOCOL 1-Wire BUS ACTIVITY AFTER COMMAND IS

ISSUED

Convert T

(Note 1) Initiates temperature conversion. 44h The device transmits conversion status to master

(not applicable for parasite-powered devices).

Read Scratchpad

(Note 2)

Reads the 9-byte scratchpad

including the CRC byte. BEh The device transmits up to 9 data bytes to master.

The 9th byte is the CRC byte.

Read Power Supply

(Note 3)

Signals the device’s power-supply

mode to the master. B4h The device transmits supply status to the master.

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

17Maxim Integrated

Figure 7. ROM Commands Flowchart

F0h

ROM?

BIT 0

MATCH?

BIT 0

MATCH?

CCh

SKIP

ROM?

55h

MATCH

ROM?

33h

READ

ROM?

YY Y

MASTER Tx

BIT 0

DEVICE Tx

FAMILY CODE

1 BYTE

MASTER Tx

FUNCTION COMMAND

DEVICE Tx

SERIAL NUMBER

6 BYTES

DEVICE Tx

CRC BYTE

MASTER Tx

ROM COMMAND

DEVICE Tx

PRESENCE PULSE

MASTER Tx

RESET PULSE

INITIALIZATION

SEQUENCE

DEVICE Tx BIT 0

MASTER Tx BIT 0

MASTER Tx

BIT 63

DEVICE Tx BIT 63

MASTER Tx BIT 63

BIT 1

MATCH?

BIT 1

MATCH?

BIT 63

MATCH?

BIT 63

MATCH?

Y Y

MASTER Tx

BIT 1

DEVICE Tx BIT 1

MASTER Tx BIT 1

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

18Maxim Integrated

Figure 8. MAX31850/MAX31851 Function Commands Flowchart

44h

CONVERT T?

PARASITE

POWER?

MASTER Tx

FUNCTION COMMAND

DEVICE BEGINS

CONVERSION

MASTER Rx

“1s”

MASTER Rx

“0s”

MASTER ENABLES

STRONG PULLUP ON DQ

DEVICE CONVERTS

TEMPERATURE

MASTER DISABLES

STRONG PULLUP

DEVICE

CONVERTING

TEMPERATURE?

MASTER Rx

“0s”

MASTER Rx

“1s”

MASTER Rx DATA BYTE

FROM SCRATCHPAD

RETURN TO INITIALIZATION SEQUENCE

FOR NEXT TRANSACTION

MASTER Rx

SCRATCHPAD CRC BYTE

BEh

READ

SCRATCHPAD?

B4h

READ

POWER SUPPLY?

PARASITE

POWER?

MASTER Tx

RESET?

HAVE 8 BYTES

BEEN READ?

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

19Maxim Integrated

Read Scratchpad [BEh]

This command allows the master to read the contents of

Scratchpad. The data transfer starts with the least signifi-

cant bit of byte 0 and continues through the scratchpad

until the 9th byte (byte 8, CRC) is read. The master can

issue a reset to terminate reading at any time if only part

of the scratchpad data is needed. The CRC is computed

while data is read from bytes 0–7, and is shifted out as

byte 8.

Read Power Supply [B4h]

The master device issues this command followed by a read

time slot to determine if any devices on the bus are using

parasite power. During the read time slot, parasite-powered

devices pull the bus low, and externally powered devices

do not pull the bus low. See the Powering the MAX31850/

MAX31851 section for more information.

Building a Cross-Reference Table

The procedure in Figure 9 uses the Search ROM com-

mand to find all MAX31850/MAX31851s on the 1-Wire bus

(16 maximum) and then reads each configuration register

to match the ROM IDs to the hardwired addresses.

1-Wire Signaling

The device uses a strict 1-Wire communication protocol

to ensure data integrity. Several signal types are defined

by this protocol: reset pulse, presence pulse, write-zero,

write-one, read-zero, and read-one. The bus master

initiates all these signals except the presence pulse. All

MAX31850/MAX31851s on the bus are slaves.

Initialization Procedure:

Reset and Presence Pulses

All communication with the device begins with an initial-

ization sequence that consists of a reset pulse from the

master followed by a presence pulse from the device.

This is illustrated in Figure 10. When the device sends the

presence pulse in response to the reset, it is indicating

to the master that it is on the bus and ready to operate.

During the initialization sequence, the bus master trans-

mits (Tx) the reset pulse by pulling the 1-Wire bus low for

480Fs (min). The bus master then releases the bus and

goes into receive mode (Rx). When the bus is released,

the 5kI pullup resistor pulls the 1-Wire bus high. When

the device detects this rising edge, it waits 15Fs to 60Fs

Figure 9. Building a Cross-Reference Table

SEARCH ALL

ROM IDs ON BUS

AND

STORE ROM IDs

(F0h COMMAND)

NMAX IS THE NUMBER

OF ROM IDs FOUND

DONE

BUILDING CROSS-REFERENCE TABLE

USING ROM IDs AND 4-BIT ADDRESSES

READ SCRATCHPAD

(USE AD3−AD0 FROM

CONFIG REGISTER)

MASTER Tx

NEXT ROM ID

INCREMENT COUNTER

N = N + 1 N > NMAX?

MATCH ROM ID TO

ADDRESS AND ADD TO

CROSS-REFERENCE

TABLE

ROM ID(0) 0000

ROM ID AD3−AD0

CROSS-REFERENCE TABLE

NOTE: TEMPERATURE SENSORS ARE ADDRESSED

BY ROM ID, NOT BY BINARY ADDRESS.

ROM ID(1) 0001

ROM ID(2) 0010

ROM ID(3) 0011

ROM ID(12) 1100

ROM ID(13) 1101

ROM ID(14) 1110

ROM ID(15) 1111

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

20Maxim Integrated

and then transmits a presence pulse by pulling the

1-Wire bus low for 60Fs to 240Fs.

Read/Write Time Slots

The bus master writes data to the device during write

time slots and reads data from the device during read

time slots. One bit of data is transmitted over the 1-Wire

bus per time slot.

Write Time Slots

There are two types of write time slots: write-one time

slots and write-zero time slots. The bus master uses a

write-one time slot to write a logic 1 to the slave and a

write-zero time slot to write a logic 0 to the slave. All write

time slots must have a 60Fs (min) duration with a 1Fs

(min) recovery time between individual write slots. Both

types of write time slots are initiated by the master pulling

the 1-Wire bus low (Figure 11).

To generate a write-one time slot, after pulling the 1-Wire

bus low, the bus master must release the 1-Wire bus

within 15Fs. When the bus is released, the 5kI pullup

resistor pulls the bus high. To generate a write-zero time

slot, after pulling the 1-Wire bus low, the bus master must

continue to hold the bus low for the duration of the time

slot (at least 60Fs).

The slave samples the 1-Wire bus during a window that

lasts from 15Fs to 60Fs after the master initiates the write

time slot. If the bus is high during the sampling window,

a 1 is written to the slave. If the line is low, a 0 is written

to the device.

Read Time Slots

The slave can only transmit data to the master when

the master issues read time slots. Therefore, the master

must generate read time slots immediately after issuing

a Read Scratchpad [BEh] command or Read Power

Supply [B4h] command, so that the device can provide

the requested data. In addition, the master can gener-

ate read time slots after issuing a Convert T [44h] com-

mand to verify the operation status as explained in the

MAX31850/MAX31851 Function Commands section.

All read time slots must be 60Fs (min) in duration with a

1Fs (min) recovery time between slots. A read time slot is

initiated by the master device pulling the 1-Wire bus low

for a minimum of 1Fs (tINIT) and then releasing the bus

(Figure 11). After the master initiates the read time slot,

the slave begins transmitting a 1 or 0 on bus. The slave

transmits a 1 by leaving the bus high and transmits a 0

by pulling the bus low. When transmitting a 0, the slave

releases the bus by the end of the time slot, and the

pullup resistor pulls the bus back to its high idle state.

Output data from the slave is valid for 15Fs after the fall-

ing edge that initiated the read time slot. Therefore, the

master must release the bus and then sample the bus

state within 15Fs from the start of the slot.

Figure 12 illustrates that the sum of tINIT, tRC, and the

master sample window must be less than 15Fs for a read

time slot. tRC is the rise time due to the resistive and

capacitive characteristics of the bus. Figure 13 shows

that system timing margin is maximized by keeping tINIT

and tRC as short as possible and by locating the master

sample time during read time slots towards the end of

the 15Fs period.

Figure 10. Initialization Timing

VPU

1-Wire BUS

MASTER Tx RESET PULSE

480µs MINIMUM

MASTER Rx

480µs MINIMUM

MAX31850/MAX31851 Tx PRESENCE PULSE

60µs TO 240µs

MAX31850/MAX31851 WAITS

15µs TO 60µs

GND

BUS MASTER PULLING LOW MAX31850/MAX31851 PULLING LOW RESISTOR PULLUP

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

21Maxim Integrated

Figure 11. Read/Write Time Slot Timing Diagram

Figure 12. Detailed Master Read-One Timing

VPU

1-Wire BUS

START

OF SLOT

START

OF SLOT

60µs < Tx “0” < 120µs

1µs < tREC < ∞

> 1µs

MASTER SAMPLES MASTER SAMPLES

> 1µs

15µs 15µs 30µs

MASTER WRITE-ZERO SLOT

MAX31850/MAX31851 SAMPLES

MIN MAXTYP

MAX31850/MAX31851 SAMPLES

MIN MAXTYP

MASTER WRITE-ONE SLOT

MASTER READ-ZERO SLOT MASTER READ-ONE SLOT

GND

VPU

1-Wire BUS

GND

15µs 45µs 15µs

15µs 15µs 30µs

BUS MASTER PULLING LOW MAX31850/MAX31851PULLING LOW RESISTOR PULLUP

VIH OF MASTER

MASTER SAMPLES

VPU

1-Wire BUS

GND

15µs

tINIT > 1µs tRC

BUS MASTER PULLING LOW RESISTOR PULLUP

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

22Maxim Integrated

Operation Example

Table 6 shows an operation example in which there are

multiple devices on the bus using parasite power. The

bus master initiates a temperature conversion in a spe-

cific MAX31850/MAX31851 and then reads Scratchpad

and recalculates the CRC to verify the data.

Figure 13. Recommended Master Read-One Timing

Table 6. Operation Example

MASTER

MODE DATA (LSB FIRST) COMMENTS

Tx Reset Master issues reset pulse.

Rx Presence Devices respond with presence pulse.

Tx F0h Master issues Search ROM command

Tx Reset Master issues reset pulse.

Rx Presence Devices respond with presence pulse.

Tx 55h Master issues Match ROM command for desired address

Tx 64-bit ROM code Master sends device ROM code.

Tx 44h Master issues Convert T command.

Tx DQ line held high by

strong pullup Master applies strong pullup to DQ for the duration of the conversion (tCONV).

Tx Reset Master issues reset pulse.

Rx Presence Devices respond with presence pulse.

Tx 55h Master issues Match ROM command.

Tx 64-bit ROM code Master sends device ROM code.

Tx BEh Master issues Read Scratchpad command.

Rx 9 data bytes

Master reads entire scratchpad including CRC. The master then recalculates the CRC of the

first 8 data bytes from the scratchpad and compares the calculated CRC with the read CRC

(byte 9). If they match, the master continues; if not, the read operation is repeated.

VPU

VIH OF MASTER

tINIT =

SMALL

tRC =

SMALL

MASTER SAMPLES

15µs

1-Wire BUS

GND

BUS MASTER PULLING LOW RESISTOR PULLUP

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

23Maxim Integrated

Figure 14. Address Programming Diagram—VDD Powered Figure 15. Address Programming Diagram—Parasite Powered

Applications Information

Open and Shorted Thermocouple Detection

The LSB of Byte 0 in Scratchpad is normally low and

goes high if the thermocouple input is open or shorted

to ground or VDD. Bits 0, 1, and 2 of Byte 2 are normally

low. When bit 2 (SCV) is high, it indicates a thermocouple

short to VDD. When bit 1 (SCG) is high, it indicates a

thermocouple short to ground. When bit 0 (OC) is high, it

indicates a thermocouple open circuit.

Noise Considerations

Because of the small signal levels involved, thermocouple

temperature measurement is susceptible to power supply

coupled noise. The effects of power-supply noise can be

minimized by placing a 0.1FF ceramic bypass capacitor

close to the VDD pin of the device and to GND.

The input amplifier is a low-noise amplifier designed

to enable high-precision input sensing. Keep the

thermocouple and connecting wires away from electrical

noise sources.

It is strongly recommended to add a 10nF ceramic surface-

mount differential capacitor, placed across the T+ and

T- pins, in order to filter noise on the thermocouple lines.

MAX31850

MAX31851

AD0

AD1

AD2

AD3

LOCATION 0

AD0 = GND

AD1 = GND

AD2 = GND

AD3 = GND

VDD

1-Wire BUS

NOTE: AD3–AD0 CANNOT BE LEFT UNCONNECTED; EACH PIN MUST BE

CONNECTED TO EITHER VDD OR GND.

GND

MAX31850

MAX31851

AD0

AD1

AD2

AD3

LOCATION 1

AD0 = VDD

AD1 = GND

AD2 = GND

AD3 = GND

VDD

GND

MAX31850

MAX31851

AD0

AD1

AD2

AD3

LOCATION 2

AD0 = GND

AD1 = VDD

AD2 = GND

AD3 = GND

VDD

GND

MAX31850

MAX31851

AD0

AD1

AD2

AD3

LOCATION 15

AD0 = VDD

AD1 = VDD

AD2 = VDD

AD3 = VDD

VDD

GND

MAX31850

MAX31851

AD0

AD1

AD2

AD3

LOCATION 0

AD0 = GND

AD1 = GND

AD2 = GND

AD3 = GND

VDD

1-Wire BUS

NOTE: AD3–AD0 CANNOT BE LEFT UNCONNECTED; EACH PIN MUST BE

CONNECTED TO EITHER DQ OR GND.

GND

MAX31850

MAX31851

AD0

AD1

AD2

AD3

LOCATION 1

AD0 = DQ

AD1 = GND

AD2 = GND

AD3 = GND

VDD

GND

MAX31850

MAX31851

AD0

AD1

AD2

AD3

LOCATION 2

AD0 = GND

AD1 = DQ

AD2 = GND

AD3 = GND

VDD

GND

MAX31850

MAX31851

AD0

AD1

AD2

AD3

LOCATION 15

AD0 = DQ

AD1 = DQ

AD2 = DQ

AD3 = DQ

VDD

GND

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

24Maxim Integrated

Ordering Information

Note: All devices are specified over the -40°C to +125°C operating temperature range.

+Denotes a lead(Pb)-free/RoHS-compliant package.

T = Tape and reel.

*EP = Exposed pad.

Package Information

For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a

“+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the

drawing pertains to the package regardless of RoHS status.

Thermal Considerations

Self-heating degrades the temperature measurement

accuracy of the MAX31850/MAX31851 in some

applications. The magnitude of the temperature errors

depends on the thermal conductivity of the MAX31850/

MAX31851 package, the mounting technique, and the

effects of airflow. Use a large ground plane to improve the

temperature measurement accuracy of the MAX31850/

MAX31851.

The accuracy of a thermocouple system can also be

improved by following these precautions:

• Usethelargestwirepossiblethatdoesnotshuntheat

away from the measurement area.

• If a small wire is required, use it only in the region

of the measurement, and use extension wire for the

region with no temperature gradient.

• Avoid mechanical stress and vibration, which could

strain the wires.

• When using long thermocouple wires, use a twisted

pair extension wire.

• Avoidsteeptemperaturegradients.

• Trytousethethermocouplewirewellwithinitstem-

perature rating.

• Usethepropersheathingmaterialinhostileenviron-

ments to protect the thermocouple wire.

• Useextensionwireonlyatlowtemperaturesandonly

in regions of small gradients.

• Keepaneventlogandacontinuousrecordofthermo-

couple resistance.

PART THERMOCOUPLE TYPE MEASURED TEMP RANGE PIN-PACKAGE

MAX31850KATB+ K -270NC to +1372NC10 TDFN-EP*

MAX31850KATB+T K -270NC to +1372NC10 TDFN-EP*

MAX31850JATB+ J -210NC to +1200NC10 TDFN-EP*

MAX31850JATB+T J -210NC to +1200NC10 TDFN-EP*

MAX31850NATB+ N -270NC to +1300NC10 TDFN-EP*

MAX31850NATB+T N -270NC to +1300NC10 TDFN-EP*

MAX31850TATB+ T -270NC to +400NC10 TDFN-EP*

MAX31850TATB+T T -270NC to +400NC10 TDFN-EP*

MAX31850EATB+ E -270NC to +1000NC10 TDFN-EP*

MAX31850EATB+T E -270NC to +1000NC10 TDFN-EP*

MAX31851SATB+ S -270NC to +1768NC10 TDFN-EP*

MAX31851SATB+T S -270NC to +1768NC10 TDFN-EP*

MAX31851RATB+ R -270NC to +1768NC10 TDFN-EP*

MAX31851RATB+T R -270NC to +1768NC10 TDFN-EP*

PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO.

10 TDFN-EP T1034N+1 21-0268 90-0247

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent

licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and

max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.

Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000 25

Revision History

REVISION

NUMBER

REVISION

DATE DESCRIPTION PAGES

CHANGED

0 6/13 Initial release —

1 8/13 Clarify that MAX31850 can be used for R- and S- thermocouples 1, 4, 11,

16, 24

2 1/15 Revised the Benefits and Features section 1

Mouser Electronics

Authorized Distributor

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Maxim Integrated:

MAX31850NATB+ MAX31850NATB+T MAX31850KATB+ MAX31851RATB+ MAX31851SATB+T

MAX31850TATB+T MAX31851SATB+ MAX31850EATB+T MAX31850JATB+T MAX31850TATB+

MAX31851RATB+T MAX31850KATB+T MAX31850EATB+ MAX31850JATB+