LTC2444/LTC2445/
LTC2448/LTC2449
1
2444589fc
For more information www.linear.com/LTC2444
Typical applicaTion
FeaTures DescripTion
24-Bit High Speed
8-/16-Channel ∆∑ ADCs with
Selectable Speed/Resolution
Simple 24-Bit Variable Speed Data Acquisition System
LTC2444/LTC2448
RMS Noise vs Speed
applicaTions
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
No Latency ∆∑ and SoftSpan are trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
n Up to 8 Differential or 16 Single-Ended Input Channels
n Up to 8kHz Output Rate (External fO)
n Up to 4kHz Multiplexing Rate (External fO)
n Selectable Speed/Resolution
n 2µVRMS Noise at 1.76kHz Output Rate
n 200nVRMS Noise at 13.8Hz Output Rate with
Simultaneous 50Hz/60Hz Rejection
n Guaranteed Modulator Stability and Lock-Up
Immunity for any Input and Reference Conditions
n 0.0005% INL, No Missing Codes
n Autosleep Enables 20µA Operation at 6.9Hz
n <5µV Offset (4.5V < VCC < 5.5V, –40°C to 85°C)
n Differential Input and Differential Reference with
GND to VCC Common Mode Range
n No Latency Mode, Each Conversion is Accurate Even
After a New Channel is Selected
n Internal Oscillator—No External Components
n LTC2445/LTC2449 Include MUXOUT/ADCIN for
External Buffering or Gain
n Tiny QFN 5mm × 7mm Package
n High Speed Multiplexing
n Weight Scales
n Auto Ranging 6-Digit DVMs
n Direct Temperature Measurement
n High Speed Data Acquisition
The LTC
®
2444/LTC2445/LTC2448/LTC2449 are 8-/16-
channel (4-/8-differential) high speed 24-bit No Latency
∆Σ™ ADCs. They use a proprietary delta-sigma architec-
ture enabling variable speed/resolution. Through a simple
4-wire serial interface, ten speed/resolution combinations
6.9Hz/280nVRMS to 3.5kHz/25µVRMS (4kHz with external
oscillator) can be selected with no latency between con-
version results or shift in DC accuracy (offset, full-scale,
linearity, drift). Additionally, a 2X speed mode can be
selected enabling output rates up to 7kHz (8kHz if an
external oscillator is used) with one cycle latency.
Any combination of single-ended or differential inputs
can be selected with a common mode input range from
ground to VCC, independent of VREF. While operating in
the 1X speed mode the first conversion following a new
speed, resolution, or channel selection is valid. Since
there is no settling time between conversions, all 8 dif-
ferential channels can be scanned at a rate of 500Hz.
At the conclusion of each conversion, the converter is
internally reset eliminating any memory effects between
successive conversions and assuring stability of the high
order delta-sigma modulator.
SDI
SCK
SDO
CS
FO
REF+VCC
4.5V TO 5.5V
1µF
COM
REF–
GND
THERMOCOU
PLE
VARIABLE SPEED/
RESOLUTION
DIFFERENTIAL
24-BIT ∆∑ ADC
16-CHANNEL
MUX +
–
2444 TA01a
4-WIRE
SPI INTERFACE
LTC2448
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
(SIMULTANEOUS 50Hz/60Hz
REJECTION AT 6.9Hz OUTPUT RATE)
CH0
CH1
•
•
•
•
•
•
CH7
CH8
CH15
CONVERSION RATE (Hz)
1
0.1
RMS NOISE (µV)
1
10
100
10 100
2440 TA01b
1000
10000
2.8µV AT 880Hz
280nV AT 6.9Hz
(50/60Hz REJECTION)
VCC = 5V
VREF = 5V
VIN+ = VIN– = 0V
2X SPEED MODE
NO LATENCY MODE
LTC2444/LTC2445/
LTC2448/LTC2449
2
2444589fc
For more information www.linear.com/LTC2444
pin conFiguraTion
absoluTe MaxiMuM raTings
Supply Voltage (VCC) to GND ....................... –0.3V to 6V
Analog Input Pins Voltage
to GND ......................................–0.3V to (VCC + 0.3V)
Reference Input Pins Voltage
to GND ......................................–0.3V to (VCC + 0.3V)
Digital Input Voltage to GND .........–0.3V to (VCC + 0.3V)
Digital Output Voltage to GND ....... –0.3V to (VCC + 0.3V)
(Notes 1, 2)
LTC2444 LTC2445
13 14 15 16
TOP VIEW
UHF PACKAGE
38-LEAD (5mm × 7mm) PLASTIC QFN
17 18 19
38 37 36 35 34 33 32
24
25
26
27
28
29
30
31
8
7
6
5
4
3
2
1GND
BUSY
EXT
GND
GND
GND
COM
NC
CH0
CH1
NC
NC
GND
REF
–
REF
+
VCC
NC
NC
NC
NC
NC
CH7
CH6
NC
SCK
SDO
CS
FO
SDI
GND
GND
CH2
CH3
NC
NC
CH4
CH5
NC
23
22
21
20
9
10
11
12
39
TJMAX = 125°C, θJA = 34°C/W
EXPOSED PAD (PIN 39) IS GND, MUST BE SOLDERED TO PCB
13 14 15 16
TOP VIEW
UHF PACKAGE
38-LEAD (5mm × 7mm) PLASTIC QFN
17 18 19
38 37 36 35 34 33 32
24
25
26
27
28
29
30
31
8
7
6
5
4
3
2
1GND
BUSY
EXT
GND
GND
GND
COM
NC
CH0
CH1
NC
NC
GND
REF–
REF+
VCC
MUXOUTN
ADCINN
ADCINP
MUXOUTP
NC
CH7
CH6
NC
SCK
SDO
CS
FO
SDI
GND
GND
CH2
CH3
NC
NC
CH4
CH5
NC
23
22
21
20
9
10
11
12
39
TJMAX = 125°C, θJA = 34°C/W
EXPOSED PAD (PIN 39) IS GND, MUST BE SOLDERED TO PCB
LTC2448 LTC2449
13 14 15 16
TOP VIEW
UHF PACKAGE
38-LEAD (5mm × 7mm) PLASTIC QFN
17 18 19
38
39
37 36 35 34 33 32
24
25
26
27
28
29
30
31
8
7
6
5
4
3
2
1GND
BUSY
EXT
GND
GND
GND
COM
CH0
CH1
CH2
CH3
CH4
GND
REF–
REF+
VCC
NC
NC
NC
NC
CH15
CH14
CH13
CH12
SCK
SDO
CS
FO
SDI
GND
GND
CH5
CH6
CH7
CH8
CH9
CH10
CH11
23
22
21
20
9
10
11
12
TJMAX = 125°C, θJA = 34°C/W
EXPOSED PAD (PIN 39) IS GND, MUST BE SOLDERED TO PCB
13 14 15 16
TOP VIEW
UHF PACKAGE
38-LEAD (5mm × 7mm) PLASTIC QFN
17 18 19
38 37 36 35 34 33 32
24
25
26
27
28
39
29
30
31
8
7
6
5
4
3
2
1GND
BUSY
EXT
GND
GND
GND
COM
CH0
CH1
CH2
CH3
CH4
GND
REF–
REF+
VCC
MUXOUTN
ADCINN
ADCINP
MUXOUTP
CH15
CH14
CH13
CH12
SCK
SDO
CS
FO
SDI
GND
GND
CH5
CH6
CH7
CH8
CH9
CH10
CH11
23
22
21
20
9
10
11
12
TJMAX = 125°C, θJA = 34°C/W
EXPOSED PAD (PIN 39) IS GND, MUST BE SOLDERED TO PCB
Operating Temperature Range
LTC2444C/LTC2445C/
LTC2448C/LTC2449C ............................... 0°C to 70°C
LTC2444I/LTC2445I/
LTC2448I/LTC2449I ............................. –40°C to 85°C
Storage Temperature Range .................. –65°C to 150°C
LTC2444/LTC2445/
LTC2448/LTC2449
3
2444589fc
For more information www.linear.com/LTC2444
orDer inForMaTion
elecTrical characTerisTics
PARAMETER CONDITIONS MIN TYP MAX UNITS
Resolution (No Missing Codes) 0.1V ≤ VREF ≤ VCC, –0.5 • VREF ≤ VIN ≤ 0.5 • VREF, (Note 5) l24 Bits
Integral Nonlinearity VCC = 5V, REF+ = 5V, REF– = GND, VINCM = 2.5V, (Note 6)
REF+ = 2.5V, REF– = GND, VINCM = 1.25V, (Note 6)
l5
3
15 ppm of VREF
ppm of VREF
Offset Error 2.5V ≤ REF+ ≤ VCC, REF– = GND,
GND ≤ IN+ = IN– ≤ VCC
(Note 12)
l2.5 5 µV
Offset Error Drift 2.5V ≤ REF+ ≤ VCC, REF– = GND,
GND ≤ IN+ = IN– ≤ VCC
20 nV/°C
Positive Full-Scale Error REF+ = 5V, REF– = GND, IN+ = 3.75V, IN– = 1.25V
REF+ = 2.5V, REF– = GND, IN+ = 1.875V, IN– = 0.625V
l
l
10
10
50
50
ppm of VREF
ppm of VREF
Positive Full-Scale Error Drift 2.5V ≤ REF+ ≤ VCC, REF– = GND,
IN+ = 0.75 • REF+, IN– = 0.25 • REF+0.2 ppm of VREF/°C
Negative Full-Scale Error REF+ = 5V, REF– = GND, IN+ = 1.25V, IN– = 3.75V
REF+ = 2.5V, REF– = GND, IN+ = 0.625V, IN– = 1.875V
l
l
10
10
50
50
ppm of VREF
ppm of VREF
Negative Full-Scale Error Drift 2.5V ≤ REF+ ≤ VCC, REF– = GND,
IN+ = 0.25 • REF+, IN– = 0.75 • REF+0.2 ppm of VREF/°C
Total Unadjusted Error 5V ≤ VCC ≤ 5.5V, REF+ = 2.5V, REF– = GND, VINCM = 1.25V
5V ≤ VCC ≤ 5.5V, REF+ = 5V, REF– = GND, VINCM = 2.5V
REF+ = 2.5V, REF– = GND, VINCM = 1.25V, (Note 6)
15
15
15
ppm of VREF
ppm of VREF
ppm of VREF
Input Common Mode Rejection DC 2.5V ≤ REF+ ≤ VCC, REF– = GND,
GND ≤ IN– = IN+ ≤ VCC
120 dB
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. (Notes 3, 4)
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC2444CUHF#PBF LTC2444CUHF#TRPBF 2444 38-Lead Plastic QFN 0°C to 70°C
LTC2444IUHF#PBF LTC2444IUHF#TRPBF 2444 38-Lead Plastic QFN –40°C to 85°C
LTC2445CUHF#PBF LTC2445CUHF#TRPBF 2445 38-Lead Plastic QFN 0°C to 70°C
LTC2445IUHF#PBF LTC2445IUHF#TRPBF 2445 38-Lead Plastic QFN –40°C to 85°C
LTC2448CUHF#PBF LTC2448CUHF#TRPBF 2448 38-Lead Plastic QFN 0°C to 70°C
LTC2448IUHF#PBF LTC2448IUHF#TRPBF 2448 38-Lead Plastic QFN –40°C to 85°C
LTC2449CUHF#PBF LTC2449CUHF#TRPBF 2449 38-Lead Plastic QFN 0°C to 70°C
LTC2449IUHF#PBF LTC2449IUHF#TRPBF 2449 38-Lead Plastic QFN –40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
http://www.linear.com/product/LTC2444#orderinfo
LTC2444/LTC2445/
LTC2448/LTC2449
4
2444589fc
For more information www.linear.com/LTC2444
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
IN+Absolute/Common Mode IN+ Voltage lGND – 0.3V VCC + 0.3V V
IN–Absolute/Common Mode IN– Voltage lGND – 0.3V VCC + 0.3V V
VIN Input Differential Voltage Range
(IN+ – IN–)
l–VREF/2 VREF/2 V
REF+Absolute/Common Mode REF+ Voltage l0.1 VCC V
REF–Absolute/Common Mode REF– Voltage lGND VCC – 0.1V V
VREF Reference Differential Voltage Range
(REF+ – REF–)
l0.1 VCC V
CS(IN+)IN+ Sampling Capacitance 2 pF
CS(IN–)IN– Sampling Capacitance 2 pF
CS(REF+)REF+ Sampling Capacitance 2 pF
CS(REF–)REF– Sampling Capacitance 2 pF
IDC_LEAK(IN+, IN–,
REF+, REF–)
Leakage Current, Inputs and Reference CS = VCC, IN+ = GND, IN– = GND,
REF+ = 5V, REF– = GND
l–15 1 15 nA
ISAMPLE(IN+, IN–,
REF+, REF–)
Average Input/Reference Current
During Sampling
Varies, See Applications Section nA
tOPEN MUX Break-Before-Make 50 ns
QIRR MUX Off Isolation VIN = 2VP-P DC to 1.8MHz 120 dB
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VIH High Level Input Voltage, CS, FO, SDI 4.5V ≤ VCC ≤ 5.5V l2.5 V
VIL Low Level Input Voltage, CS, FO, SDI 4.5V ≤ VCC ≤ 5.5V l0.8 V
VIH High Level Input Voltage SCK 4.5V ≤ VCC ≤ 5.5V (Note 8) l2.5 V
VIL Low Level Input Voltage SCK 4.5V ≤ VCC ≤ 5.5V (Note 8) l0.8 V
IIN Digital Input Current, CS, FO, EXT, SDI 0V ≤ VIN ≤ VCC l–10 10 µA
IIN Digital Input Current, SCK 0V ≤ VIN ≤ VCC (Note 8) l–10 10 µA
CIN Digital Input Capacitance, CS, FO, SDI 10 pF
CIN Digital Input Capacitance, SCK (Note 8) 10 pF
VOH High Level Output Voltage, SDO, BUSY IO = –800µA lVCC – 0.5V V
VOL Low Level Output Voltage, SDO, BUSY IO = 1.6mA l0.4V V
VOH High Level Output Voltage, SCK IO = –800µA (Note 9) lVCC – 0.5V V
VOL Low Level Output Voltage, SCK IO = 1.6mA (Note 9) l0.4V V
IOZ Hi-Z Output Leakage, SDO l–10 10 µA
analog inpuT anD reFerence
DigiTal inpuTs anD DigiTal ouTpuTs
The l denotes the specifications which apply over the full
operating temperature range, otherwise specifications are at TA = 25°C. (Note 3)
The l denotes the specifications which apply over the
full operating temperature range, otherwise specifications are at TA = 25°C. (Note 3)
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VCC Supply Voltage l4.5 5.5 V
ICC Supply Current
Conversion Mode
Sleep Mode
CS = 0V (Note 7)
CS = VCC (Note 7)
l
l
8
8
11
30
mA
µA
power requireMenTs
The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 3)
LTC2444/LTC2445/
LTC2448/LTC2449
5
2444589fc
For more information www.linear.com/LTC2444
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: VCC = 4.5V to 5.5V unless otherwise specified.
VREF = REF+ – REF–, VREFCM = (REF+ + REF–)/2;
VIN = IN+ – IN–, VINCM = (IN+ + IN–)/2.
Note 4: FO pin tied to GND or to external conversion clock source with
fEOSC = 10MHz unless otherwise specified.
Note 5: Guaranteed by design, not subject to test.
Note 6: Integral nonlinearity is defined as the deviation of a code from a
straight line passing through the actual endpoints of the transfer curve.
The deviation is measured from the center of the quantization band.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
fEOSC External Oscillator Frequency Range l0.1 12 MHz
tHEO External Oscillator High Period l25 10000 ns
tLEO External Oscillator Low Period l25 10000 ns
tCONV Conversion Time OSR = 256 (SDI = 0)
OSR = 32768 (SDI = 1)
External Oscillator, 1X Mode
(Notes 10, 13)
l
l
l
0.99
126
1.13
145
1.33
170
ms
ms
ms
f ISCK Internal SCK Frequency Internal Oscillator (Note 9)
External Oscillator (Notes 9, 10)
l0.8 0.9
fEOSC/10
1 MHz
Hz
DISCK Internal SCK Duty Cycle (Note 9) l45 55 %
fESCK External SCK Frequency Range (Note 8) l20 MHz
tLESCK External SCK Low Period (Note 8) l25 ns
tHESCK External SCK High Period (Note 8) l25 ns
tDOUT_ISCK Internal SCK 32-Bit Data Output Time Internal Oscillator (Notes 9, 11)
External Oscillator (Notes 9, 10)
l
l
41.6 35.3
320/fEOSC
30.9 µs
s
tDOUT_ESCK External SCK 32-Bit Data Output Time (Note 8) l32/fEOSC s
t1CS ↓ to SDO Low Z (Note 12) l0 25 ns
t2CS ↑ to SDO High Z (Note 12) l0 25 ns
t3CS ↓ to SCK ↓(Note 9) 5 µs
t4CS ↓ to SCK ↑(Notes 8, 12) l25 ns
tKQMAX SCK ↓ to SDO Valid l25 ns
tKQMIN SDO Hold After SCK ↓(Note 5) l15 ns
t5SCK Set-Up Before CS ↓l50 ns
t6SCK Hold After CS ↓l50 ns
t7SDI Set-Up Before SCK ↑(Note 5) l10 ns
t8SDI Hold After SCK ↑(Note 5) l10 ns
TiMing characTerisTics
The l denotes the specifications which apply over the full operating temperature
range, otherwise specifications are at TA = 25°C. (Note 3)
Note 7: The converter uses the internal oscillator.
Note 8: The converter is in external SCK mode of operation such that the
SCK pin is used as a digital input. The frequency of the clock signal driving
SCK during the data output is fESCK and is expressed in Hz.
Note 9: The converter is in internal SCK mode of operation such that the
SCK pin is used as a digital output. In this mode of operation, the SCK pin
has a total equivalent load capacitance of CLOAD = 20pF.
Note 10: The external oscillator is connected to the FO pin. The external
oscillator frequency, fEOSC, is expressed in Hz.
Note 11: The converter uses the internal oscillator. FO = 0V.
Note 12: Guaranteed by design and test correlation.
Note 13: There is an internal reset that adds an additional 5 to 15 fO cycles
to the conversion time.
40 • OSR +178
fEOSC (kHz)
LTC2444/LTC2445/
LTC2448/LTC2449
6
2444589fc
For more information www.linear.com/LTC2444
pin FuncTions
GND (Pins 1, 4, 5, 6, 31, 32, 33): Ground. Multiple ground
pins internally connected for optimum ground current flow
and VCC decoupling. Connect each one of these pins to
a common ground plane through a low impedance con-
nection. All seven pins must be connected to ground for
proper operation.
BUSY (Pin 2): Conversion in Progress Indicator. This pin
is HIGH while the conversion is in progress and goes LOW
indicating the conversion is complete and data is ready.
It remains LOW during the sleep and data output states.
At the conclusion of the data output state, it goes HIGH
indicating a new conversion has begun.
EXT (Pin 3): Internal/External SCK Selection Pin. This
pin is used to select internal or external SCK for output-
ting/inputting data. If EXT is tied low, the device is in the
external SCK mode and data is shifted out of the device
under the control of a user applied serial clock. If EXT is
tied high, the internal serial clock mode is selected. The
device generates its own SCK signal and outputs this on
the SCK pin. A framing signal BUSY (Pin 2) goes low
indicating data is being output.
COM (Pin 7): The common negative input (IN–) for all
single ended multiplexer configurations. The voltage on
CH0 to CH15 and COM pins can have any value between
GND – 0.3V to VCC + 0.3V. Within these limits, the two
selected inputs (IN+ and IN–) provide a bipolar input range
(VIN = IN+ – IN–) from –0.5 • VREF to 0.5 • VREF. Outside
this input range, the converter produces unique overrange
and underrange output codes.
CH0 to CH15 (Pins 8 to 23): LTC2448/LTC2449 Analog
Inputs. May be programmed for single-ended or differ-
ential mode.
CH0 to CH7 (Pins 9, 10, 13, 14, 17, 18, 21, 22): LTC2444/
LTC2445 Analog Inputs. May be programmed for single-
ended or differential mode.
NC (Pins 8, 11, 12, 15, 16, 19, 20, 23): LTC2444/LTC2445
No Connect/Channel Isolation Shield. May be left floating
or tied to any voltage 0 to VCC in order to provide isolation
for pairs of differential input channels.
NC (Pins 24, 25, 26, 27): LTC2444/LTC2448 No Connect.
These pins can either be tied to ground or left floating.
MUXOUTP (Pin 24): LTC2445/LTC2449 Positive Mul-
tiplexer Output. Used to drive the input to an external
buffer/amplifier.
ADCINP (Pin 25): LTC2445/LTC2449 Positive ADC Input.
Tie to output of buffer/amplifier driven by MUXOUTP.
ADCINN (Pin 26): LTC2445/LTC2449 Negative ADC Input.
Tie to output of buffer/amplifier driven by MUXOUTN.
MUXOUTN (Pin 27): LTC2445/LTC2449 Negative Mul-
tiplexer Output. Used to drive the input to an external
buffer/amplifier.
VCC (Pin 28): Positive Supply Voltage. Bypass to GND with
a 10µF tantalum capacitor in parallel with a 0.1µF ceramic
capacitor as close to the part as possible.
REF+ (Pin 29), REF– (Pin 30): Differential Reference Input.
The voltage on these pins can have any value between
GND and VCC as long as the reference positive input, REF+,
is maintained more positive than the negative reference
input, REF+, by at least 0.1V.
SDI (Pin 34): Serial Data Input. This pin is used to select
the speed, 1X or 2X mode, resolution, and input channel,
for the next conversion cycle. At initial power up, the
default mode of operation is CH0 to CH1, OSR of 256,
and 1X mode. The serial data input contains an enable
bit which determines if a new channel/speed is selected.
If this bit is low the following conversion remains at the
same speed and selected channel. The serial data input
is applied to the device under control of the serial clock
(SCK) during the data output cycle. The first conversion
following a new channel/speed is valid.
FO (Pin 35): Frequency Control Pin. Digital input that con-
trols the internal conversion clock. When FO is connected
to VCC or GND, the converter uses its internal oscillator.
LTC2444/LTC2445/
LTC2448/LTC2449
7
2444589fc
For more information www.linear.com/LTC2444
pin FuncTions
CS (Pin 36): Active Low Chip Select. A LOW on this pin
enables the SDO digital output and wakes up the ADC.
Following each conversion the ADC automatically enters
the sleep mode and remains in this low power state as
long as CS is HIGH. A LOW-to-HIGH transition on CS dur-
ing the Data Output aborts the data transfer and starts a
new conversion.
SDO (Pin 37): Three-State Digital Output. During the data
output period, this pin is used as serial data output. When
the chip select CS is HIGH (CS = VCC) the SDO pin is in
a high impedance state. During the conversion and sleep
periods, this pin is used as the conversion status output.
The conversion status can be observed by pulling CS LOW.
This signal is HIGH while the conversion is in progress
and goes LOW once the conversion is complete.
SCK (Pin 38): Bidirectional Digital Clock Pin. In internal
serial clock operation mode, SCK is used as a digital
output for the internal serial interface clock during the
data output period. In the external serial clock operation
mode, SCK is used as the digital input for the external
serial interface clock during the data output period. The
serial clock operation mode is determined by the logic
level applied to the EXT pin.
Exposed Pad (Pin 39): Ground. The exposed pad on the
bottom of the package must be soldered to the PCB ground.
For prototyping purposes, this pin may remain floating.
LTC2444/LTC2445/
LTC2448/LTC2449
8
2444589fc
For more information www.linear.com/LTC2444
AUTOCALIBRATION
AND CONTROL
DIFFERENTIAL
3RD ORDER
∆∑ MODULATOR
DECIMATING FIR
ADDRESS
INTERNAL
OSCILLATOR
SERIAL
INTERFACE
GND
VCC
CH0
CH1
•
•
•
CH15
COM
IN+
IN–
MUX
SDO
SCK
REF+
REF–
CS
SDI
FO
(INT/EXT)
2444589 F01
+
–
TesT circuiT
FuncTional block DiagraM
1.69k
2444589 TC01
Hi-Z TO VOH
VOL TO VOH
V
TO Hi-Z
CLOAD
1.69k
S
DO
2444589 TC02
Hi-Z TO VOL
VOH TO VOL
VOL TO Hi-Z
CLOAD
= 20pF
V
CC
Figure 1. Functional Block Diagram
LTC2444/LTC2445/
LTC2448/LTC2449
9
2444589fc
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CONVERTER OPERATION
Converter Operation Cycle
The LTC2444/LTC2445/LTC2448/LTC2449 are multi-
channel, high speed, delta-sigma analog-to-digital convert-
ers with an easy to use 3- or 4-wire serial interface (see
Figure 1). Their operation is made up of three states. The
converter operating cycle begins with the conversion, fol-
lowed by the low power sleep state and ends with the data
output/input (see Figure 2). The 4-wire interface consists
of serial data input (SDI), serial data output (SDO), serial
clock (SCK) and chip select (CS). The interface, timing,
operation cycle and data out format is compatible with
Linear’s entire family of ∆Σ converters.
corresponds to the conversion just performed. This result
is shifted out on the serial data out pin (SDO) under the
control of the serial clock (SCK). Data is updated on the
falling edge of SCK allowing the user to reliably latch data
on the rising edge of SCK (see Figure 3). The data output
state is concluded once 32 bits are read out of the ADC
or when CS is brought HIGH. The device automatically
initiates a new conversion and the cycle repeats.
Through timing control of the CS, SCK and EXT pins, the
LTC2444/LTC2445/LTC2448/LTC2449 offer several flex-
ible modes of operation (internal or external SCK). These
various modes do not require programming configuration
registers; moreover, they do not disturb the cyclic operation
described above. These modes of operation are described
in detail in the Serial Interface Timing Modes section.
Ease of Use
The LTC2444/LTC2445/LTC2448/LTC2449 data output
has no latency, filter settling delay or redundant data
associated with the conversion cycle while operating
in the 1X mode. There is a one-to-one correspondence
between the conversion and the output data. Therefore,
multiplexing multiple analog voltages is easy. Speed/
resolution adjustments may be made seamlessly between
two conversions without settling errors.
The LTC2444/LTC2445/LTC2448/LTC2449 perform offset
and full-scale calibrations every conversion cycle. This
calibration is transparent to the user and has no effect
on the cyclic operation described above. The advantage
of continuous calibration is extreme stability of offset and
full-scale readings with respect to time, supply voltage
change and temperature drift.
Power-Up Sequence
The LTC2444/LTC2445/LTC2448/LTC2449 automatically
enter an internal reset state when the power supply volt-
age VCC drops below approximately 2.2V. This feature
guarantees the integrity of the conversion result and of
the serial interface mode selection.
When the VCC voltage rises above this critical threshold, the
converter creates an internal power-on-reset (POR) signal
with a duration of approximately 0.5ms. The POR signal
CONVERT
SLEEP
CHANNEL SELECT
SPEED SELECT
DATA OUTPUT
POWER UP
IN+=CH0, IN–=CH1
OSR=256,1X MODE
2444589 F02
CS = LOW
AND
SCK
Figure 2. LTC2444/LTC2445/LTC2448/LTC2449
State Transition Diagram
Initially, the LTC2444/LTC2445/LTC2448/LTC2449 per-
form a conversion. Once the conversion is complete, the
device enters the sleep state. While in this sleep state, power
consumption is reduced below 10µA. The part remains
in the sleep state as long as CS is HIGH. The conversion
result is held indefinitely in a static shift register while the
converter is in the sleep state.
Once CS is pulled LOW, the device begins outputting the
conversion result. There is no latency in the conversion
result while operating in the 1x mode. The data output
LTC2444/LTC2445/
LTC2448/LTC2449
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clears all internal registers. The conversion immediately
following a POR is performed on the input channel IN+
= CH0, IN– = CH1 at an OSR = 256 in the 1X mode. Fol-
lowing the POR signal, the LTC2444/LTC2445/LTC2448/
LTC2449 start a normal conversion cycle and follow the
succession of states described above. The first conversion
result following POR is accurate within the specifications
of the device if the power supply voltage is restored within
the operating range (4.5V to 5.5V) before the end of the
POR time interval.
Reference Voltage Range
These converters accept a truly differential external
reference voltage. The absolute/common mode voltage
specification for the REF+ and REF– pins covers the entire
range from GND to VCC. For correct converter operation, the
REF+ pin must always be more positive than the REF– pin.
The LTC2444/LTC2445/LTC2448/LTC2449 can accept a
differential reference voltage from 0.1V to VCC. The con-
verter output noise is determined by the thermal noise of
the front-end circuits, and as such, its value in microvolts
is nearly constant with reference voltage. A decrease in
reference voltage will not significantly improve the con-
verter’s effective resolution. On the other hand, a reduced
reference voltage will improve the converter’s overall INL
performance.
Input Voltage Range
Refer to Figure 4. The analog input is truly differential with an
absolute/common mode range for the CH0 to CH15 and COM
input pins extending from GND – 0.3V to VCC + 0.3V. Outside
these limits, the ESD protection devices begin to turn on
and the errors due to input leakage current increase rap-
idly. Within these limits, the LTC2444/LTC2445/LTC2448/
LTC2449 convert the bipolar differential input signal, VIN =
IN+ – IN – (where IN+ and IN– are the selected input chan-
nels), from – FS = –0.5 • VREF to +FS = 0.5 • VREF where
VREF = REF+ – REF–. Outside this range, the converter
indicates the overrange or the underrange condition using
distinct output codes.
MUXOUT/ADCIN
There are two differences between the LTC2444/LTC2448
and the LTC2445/LTC2449. The first is the RMS noise
performance. For a given OSR, the LTC2445/LTC2449
noise level is approximately √2 times lower (0.5 effective
bits)than that of the LTC2444/LTC2448.
The second difference is the LTC2445/LTC2449 includes
MUXOUT/ADCIN pins. These pins enable an external buf-
fer or gain block to be inserted between the output of the
multiplexer and the input to the ADC. Since the buffer is
driven by the output of the multiplexer
, only one circuit is
Figure 3. SDI Speed/Resolution, Channel Selection, and Data Output Timing
MSB
BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 BIT 20 BIT 19 BIT 0
LSB
Hi-Z
2444589 F03
SIG
BIT 29
“0”
BIT 30
EOC
Hi-Z
CS
SCK
SDI
SDO
BUSY
BIT 31
1 0 EN SGL A2 A1 A0 OSR3 OSR2 OSR1 OSR0 TWOXODD
1 2 3 4 5 6 7 8 9 10 11 12 13 14 32
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required for all 16 input channels. Additionally, the transpar-
ent calibration feature of the LTC244X family automatically
removes the offset errors of the external buffer.
In order to achieve optimum performance, the MUXOUT
and ADCIN pins should not be shorted together. In applica-
tions where the MUXOUT and ADCIN need to be shorted
together
, the LTC2444/LTC2448 should be used because
the MUXOUT and ADCIN are internally connected for
optimum performance.
Output Data Format
The LTC2444/LTC2445/LTC2448/LTC2449 serial output
data stream is 32 bits long. The first 3 bits represent sta-
tus information indicating the sign and conversion state.
The next 24 bits are the conversion result, MSB first. The
remaining 5 bits are sub LSBs beyond the 24-bit level that
may be included in averaging or discarded without loss of
resolution. In the case of ultrahigh resolution modes, more
than 24 effective bits of performance are possible (see
Table 5). Under these conditions, sub LSBs are included
in the conversion result and represent useful information
beyond the 24-bit level. The third and fourth bit together
are also used to indicate an underrange condition (the
differential input voltage is below –FS) or an overrange
condition (the differential input voltage is above +FS).
Bit 31 (first output bit) is the end of conversion (EOC)
indicator. This bit is available at the SDO pin during the
conversion and sleep states whenever the CS pin is LOW.
This bit is HIGH during the conversion and goes LOW
when the conversion is complete.
Bit 30 (second output bit) is a dummy bit (DMY) and is
always LOW.
Bit 29 (third output bit) is the conversion result sign
indicator (SIG). If VIN is >0, this bit is HIGH. If VIN is <0,
this bit is LOW.
Bit 28 (fourth output bit) is the most significant bit (MSB) of
the result. This bit in conjunction with Bit 29 also provides
the underrange or overrange indication. If both Bit 29 and
Bit 28 are HIGH, the differential input voltage is above +FS.
If both Bit 29 and Bit 28 are LOW, the differential input
voltage is below –FS.
applicaTions inForMaTion
V
CC + 0.3V
GND GND
GND
–0.3V
GND
–0.3V
–0.3V
(a) Arbitrary (b) Fully Differential
(d) Pseudo-Differential Unipolar
IN– or COM Grounded
(c) Pseudo Differential Bipolar
IN– or COM Biased
VREF
2
VREF
2
VREF
2
VREF
2
VREF
2
–VREF
2
–VREF
2
–VREF
2
Selected IN+ Ch
Selected IN
–
Ch or COM
VCC
VCC
2444589 F04
VCC
VCC
Figure 4. Input Range
LTC2444/LTC2445/
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The function of these bits is summarized in Table 1.
Table 1. LTC2444/LTC2445/LTC2448/LTC2449 Status Bits
INPUT RANGE
BIT 31
EOC
BIT 30
DMY
BIT 29
SIG
BIT 28
MSB
VIN ≥ 0.5 • VREF 0 0 1 1
0V ≤ VIN < 0.5 • VREF 0 0 1 0
–0.5 • VREF ≤ VIN < 0V 0 0 0 1
VIN < –0.5 • VREF 0 0 0 0
Bits 28 to 5 are the 24-bit conversion result MSB first.
Bit 5 is the least significant bit (LSB).
Bits 4 to 0 are sub LSBs below the 24-bit level. Bits 4 to
0 may be included in averaging or discarded without loss
of resolution.
Data is shifted out of the SDO pin under control of the
serial clock (SCK), see Figure 3. Whenever CS is HIGH,
SDO remains high impedance and SCK is ignored.
In order to shift the conversion result out of the device,
CS must first be driven LOW. EOC is seen at the SDO pin
of the device once CS is pulled LOW. EOC changes real
time from HIGH to LOW at the completion of a conversion.
This signal may be used as an interrupt for an external
microcontroller. Bit 31 (EOC) can be captured on the first
rising edge of SCK. Bit 30 is shifted out of the device on
the first falling edge of SCK. The final data bit (Bit 0) is
shifted out on the falling edge of the 31st SCK and may
be latched on the rising edge of the 32nd SCK pulse. On
the falling edge of the 32nd SCK pulse, SDO goes HIGH
indicating the initiation of a new conversion cycle. This
bit serves as EOC (Bit 31) for the next conversion cycle.
Table 2 summarizes the output data format.
As long as the voltage on the IN+ and IN– pins is main-
tained within the –0.3V to (VCC + 0.3V) absolute maximum
operating range, a conversion result is generated for any
differential input voltage VIN from –FS = –0.5 • VREF to
+FS = 0.5 • VREF. For differential input voltages greater
than +FS, the conversion result is clamped to the value
corresponding to the +FS + 1LSB. For differential input
voltages below –FS, the conversion result is clamped to
the value corresponding to –FS – 1LSB.
SERIAL INTERFACE PINS
The LTC2444/LTC2445/LTC2448/LTC2449 transmit the
conversion results and receive the start of conversion
command through a synchronous 3- or 4-wire interface.
During the conversion and sleep states, this interface can
be used to assess the converter status and during the
data output state it is used to read the conversion result
and program the speed, resolution and input channel.
applicaTions inForMaTion
Table 2. LTC2444/LTC2445/LTC2448/LTC2449 Output Data Format
DIFFERENTIAL INPUT VOLTAGE
VIN*
BIT 31
EOC
BIT 30
DMY
BIT 29
SIG
BIT 28
MSB
BIT 27 BIT 26 BIT 25 … BIT 0
VIN* ≥ 0.5 • VREF** 0 0 1 1 0 0 0 … 0
0.5 • VREF** – 1LSB 0 0 1 0 1 1 1 … 1
0.25 • VREF** 0 0 1 0 1 0 0 … 0
0.25 • VREF** – 1LSB 0 0 1 0 0 1 1 … 1
0 0 0 1 0 0 0 0 … 0
–1LSB 0 0 0 1 1 1 1 … 1
–0.25 • VREF** 0 0 0 1 1 0 0 … 0
–0.25 • VREF** – 1LSB 0 0 0 1 0 1 1 … 1
–0.5 • VREF** 0 0 0 1 0 0 0 … 0
VIN* < –0.5 • VREF** 0 0 0 0 1 1 1 … 1
*The differential input voltage VIN = IN+ – IN–. **The differential reference voltage VREF = REF+ – REF–.
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Serial Clock Input/Output (SCK)
The serial clock signal present on SCK (Pin 38) is used to
synchronize the data transfer. Each bit of data is shifted
out the SDO pin on the falling edge of the serial clock.
In the Internal SCK mode of operation, the SCK pin is an
output and the LTC2444/LTC2445/LTC2448/LTC2449
create their own serial clock. In the External SCK mode
of operation, the SCK pin is used as input. The internal or
external SCK mode is selected by tying EXT (Pin 3) LOW
for external SCK and HIGH for internal SCK.
Serial Data Output (SDO)
The serial data output pin, SDO (Pin 37), provides the
result of the last conversion as a serial bit stream (MSB
first) during the data output state. In addition, the SDO
pin is used as an end of conversion indicator during the
conversion and sleep states.
When CS (Pin 36) is HIGH, the SDO driver is switched
to a high impedance state. This allows sharing the serial
interface with other devices. If CS is LOW during the
convert or sleep state, SDO will output EOC. If CS is LOW
during the conversion phase, the EOC bit appears HIGH
on the SDO pin. Once the conversion is complete, EOC
goes LOW. The device remains in the sleep state until the
first rising edge of SCK occurs while CS = LOW.
Chip Select Input (CS)
The active LOW chip select, CS (Pin 36), is used to test the
conversion status and to enable the data output transfer
as described in the previous sections.
In addition, the CS signal can be used to trigger a new
conversion cycle before the entire serial data transfer
has been completed. The LTC2444/LTC2445/LTC2448/
LTC2449 will abort any serial data transfer in progress
and start a new conversion cycle anytime a LOW-to-HIGH
transition is detected at the CS pin after the converter has
entered the data output state.
Serial Data Input (SDI)
The serial data input (SDI, Pin 34) is used to select the
speed/resolution and input channel of the LTC2444/
LTC2445/LTC2448/LTC2449. SDI is programmed by a
serial input data stream under the control of SCK during
the data output cycle, see Figure 3.
Initially, after powering up, the device performs a conver-
sion with IN+ = CH0, IN– = CH1, OSR = 256 (output rate
nominally 880Hz), and 1X speed mode (no latency). Once
this first conversion is complete, the device enters the
sleep state and is ready to output the conversion result
and receive the serial data input stream programming the
speed/resolution and input channel for the next conversion.
At the conclusion of each conversion cycle, the device
enters this state.
In order to change the speed/resolution or input chan-
nel, the first 3 bits shifted into the device are 101. This
is compatible with the programming sequence of the
LTC2414/LTC2418. If the sequence is set to 000 or 100,
the following input data is ignored (don’t care) and the
previously selected speed/resolution and channel remain
valid for the next conversion. Combinations other than
101, 100, and 000 of the 3 control bits should be avoided.
If the first 3 bits shifted into the device are 101, then the
following 5 bits select the input channel for the following
conversion (see Tables 3 and 4). The next 5 bits select the
speed/resolution and mode 1X (no latency) 2X (double
output rate with one conversion latency), see Table 5. If
these 5 bits are set to all 0’s, the previous speed remains
selected for the next conversion. This is useful in appli-
cations requiring a fixed output rate/resolution but need
to change the input channel. In this case, the timing and
input sequence is compatible with the LTC2414/LTC2418.
When an update operation is initiated (the first 3 bits are
101) the first 5 bits are the channel address. The first
bit, SGL, determines if the input selection is differential
(SGL = 0) or single-ended (SGL = 1). For SGL = 0, two
adjacent channels can be selected to form a differential
input. For SGL = 1, one of 8 channels (LTC2444/LTC2445)
or one of 16 channels (LTC2448/LTC2449) is selected as
the positive input. The negative input is COM for all single
ended operations. The remaining 4 bits (ODD, A2, A1,
A0) determine which channel is selected. The LTC2448/
LTC2449 use all 4 bits to select one of 16 different input
channels (see Table 3) while in the case of the LTC2444/
LTC2445, A2 is always 0, and the remaining 3 bits select
one of 8 different input channels (see Table 4).
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Table 3. Channel Selection for the LTC2448/LTC2449
MUX ADDRESS CHANNEL SELECTION
SGL
ODD/
SIGN A2 A1 A0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 COM
0 0 0 0 0 IN+IN–
0 0 0 0 1 IN+IN–
0 0 0 1 0 IN+IN–
0 0 0 1 1 IN+IN–
0 0 1 0 0 IN+IN–
0 0 1 0 1 IN+IN–
0 0 1 1 0 IN+IN–
0 0 1 1 1 IN+IN–
0 1 0 0 0 IN–IN+
0 1 0 0 1 IN–IN+
0 1 0 1 0 IN–IN+
0 1 0 1 1 IN–IN+
0 1 1 0 0 IN–IN+
0 1 1 0 1 IN–IN+
0 1 1 1 0 IN–IN+
0 1 1 1 1 IN–IN+
1 0 0 0 0 IN+IN–
1 0 0 0 1 IN+IN–
1 0 0 1 0 IN+IN–
1 0 0 1 1 IN+IN–
1 0 1 0 0 IN+IN–
1 0 1 0 1 IN+IN–
1 0 1 1 0 IN+IN–
1 0 1 1 1 IN+IN–
1 1 0 0 0 IN+IN–
1 1 0 0 1 IN+IN–
1 1 0 1 0 IN+IN–
1 1 0 1 1 IN+IN–
1 1 1 0 0 IN+IN–
1 1 1 0 1 IN+IN–
1 1 1 1 0 IN+IN–
1 1 1 1 1 IN+IN–
*Default at power up
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Table 4. Channel Selection for the LTC2444/LTC2445 (Bit A2 Should Always Be 0)
MUX ADDRESS CHANNEL SELECTION
SGL ODD/SIGN A2 A1 A0 0 1 2 3 4 5 6 7 COM
0 0 0 0 0 IN+IN–
0 0 0 0 1 IN+IN–
0 0 0 1 0 IN+IN–
0 0 0 1 1 IN+IN–
0 1 0 0 0 IN–IN+
0 1 0 0 1 IN–IN+
0 1 0 1 0 IN–IN+
0 1 0 1 1 IN–IN+
1 0 0 0 0 IN+IN–
1 0 0 0 1 IN+IN–
1 0 0 1 0 IN+IN–
1 0 0 1 1 IN+IN–
1 1 0 0 0 IN+IN–
1 1 0 0 1 IN+IN–
1 1 0 1 0 IN+IN–
1 1 0 1 1 IN+IN–
*Default at power up
Table 5. LTC2444/LTC2445/LTC2448/LTC2449 Speed/Resolution Selection
OSR3 OSR2 OSR1 OSR0 TWOX
RMS NOISE
LTC2444/LTC2448
RMS NOISE
LTC2445/LTC2449
ENOB
LTC2444/LTC2448
ENOB
LTC2445/LTC2449 OSR LATENCY
0 0 0 0 0 Keep Previous Speed/Resolution
0 0 0 1 0 23µV 23µV 17 17 64 none
0 0 1 0 0 4.4µV 3.5µV 20.1 20.1 128 none
0 0 1 1 0 2.8µV 2µV 20.8 21.3 256 none
0 1 0 0 0 2µV 1.4µV 21.3 21.8 512 none
0 1 0 1 0 1.4µV 1µV 21.8 22.4 1024 none
0 1 1 0 0 1.1µV 750nV 22.1 22.9 2048 none
0 1 1 1 0 720nV 510nV 22.7 23.4 4096 none
1 0 0 0 0 530nV 375nV 23.2 24 8192 none
1 0 0 1 0 350nV 250nV 23.8 24.4 16384 none
1 1 1 1 0 280nV 200nV 24.1 24.6 32768 none
0 0 0 0 1 Keep Previous Speed/Resolution
0 0 0 1 1 23µV 23µV 17 17 64 1 cycle
0 0 1 0 1 4.4µV 3.5µV 20.1 20.1 128 1 cycle
0 0 1 1 1 2.8µV 2µV 20.8 21.3 256 1 cycle
0 1 0 0 1 2µV 1.4µV 21.3 21.8 512 1 cycle
0 1 0 1 1 1.4µV 1µV 21.8 22.4 1024 1 cycle
0 1 1 0 1 1.1µV 750nV 22.1 22.9 2048 1 cycle
0 1 1 1 1 720nV 510nV 22.7 23.4 4096 1 cycle
1 0 0 0 1 530nV 375nV 23.2 24 8192 1 cycle
1 0 0 1 1 350nV 250nV 23.8 24.4 16384 1 cycle
1 1 1 1 1 280nV 200nV 24.1 24.6 32768 1 cycle
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Table 6. LTC2444/LTC2445/LTC2448/LTC2449 Interface Timing Modes
CONFIGURATION
SCK
SOURCE
CONVERSION
CYCLE
CONTROL
DATA
OUTPUT
CONTROL
CONNECTION
AND
WAVEFORMS
External SCK, Single Cycle Conversion External CS and SCK CS and SCK Figures 5, 6
External SCK, 2-Wire I/O External SCK SCK Figure 7
Internal SCK, Single Cycle Conversion Internal CS ↓CS ↓Figures 8, 9
Internal SCK, 2-Wire I/O, Continuous Conversion Internal Continuous Internal Figure 10
applicaTions inForMaTion
Speed Multiplier Mode
In addition to selecting the speed/resolution, a speed
multiplier mode is used to double the output rate while
maintaining the selected resolution. The last bit of the
5-bit speed/resolution control word (TWOX, see Table 5)
determines if the output rate is 1X (no speed increase) or
2X (double the selected speed).
While operating in the 1X mode, the device combines two
internal conversions for each conversion result in order
to remove the ADC offset. Every conversion cycle, the
offset and offset drift are transparently calibrated greatly
simplifying the user interface. The resulting conversion
result has no latency. The first conversion following a
newly selected speed/resolution and input channel is
valid. This is identical to the operation of the LTC2440,
LTC2414 and LTC2418.
While operating in the 2X mode, the device performs a
running average of the last two conversion results. This
automatically removes the offset and drift of the device
while increasing the output rate by 2X. The resolution
(noise) remains the same. If a new channel is selected,
the conversion result is valid for all conversions after
the first conversion (one cycle latency). If a new speed/
resolution is selected, the first conversion result is valid
but the resolution (noise) is a function of the running av-
erage. All subsequent conversion results are valid. If the
mode is changed from either 1X to 2X or 2X to 1X without
changing the resolution or channel, the first conversion
result is valid.
If an external buffer/amplifier circuit is used for the
LTC2445/LTC2449, the 2X mode can be used to increase
the settling time of the amplifier between readings. While
operating in the 2X mode, the multiplexer output (input
to the external buffer/amplifier) is switched at the end of
each conversion cycle. Prior to concluding the data out/
in cycle, the analog multiplexer output is switched. This
occurs at the end of the conversion cycle (just prior to
the data output cycle) for auto calibration. The time re-
quired to read the conversion enables more settling time
for the external buffer/amplifier. The offset/offset drift of
the external amplifier is automatically removed by the
converter’s auto calibration sequence for both the 1X and
2X speed modes.
While operating in the 1X mode, if a new input channel
is selected the multiplexer is switched on the falling edge
of the 14th SCK (once the complete data input word is
programmed). The remaining data output sequence time
can be used to allow the external buffer/amplifier to settle.
BUSY
The BUSY output (Pin 2) is used to monitor the state of
conversion, data output and sleep cycle. While the part is
converting, the BUSY pin is HIGH. Once the conversion is
complete, BUSY goes LOW indicating the conversion is
complete and data out is ready. The part now enters the
LOW power sleep state. BUSY remains LOW while data is
shifted out of the device and SDI is shifted into the device. It
goes HIGH at the conclusion of the data input/output cycle
indicating a new conversion has begun. This rising edge
may be used to flag the completion of the data read cycle.
Serial Interface Timing Modes
The LTC2444/LTC2445/LTC2448/LTC2449’s 3- or 4-wire
interface is SPI and MICROWIRE compatible. This interface
offers several flexible modes of operation. These include
internal/external serial clock, 3- or 4-wire I/O, single cycle
conversion and autostart. The following sections describe
each of these serial interface timing modes in detail. In all
these cases, the converter can use the internal oscillator
(FO = LOW) or an external oscillator connected to the FO
pin. Refer to Table6 for a summary.
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MSB
BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 BIT 20 BIT 19 BIT 0
LSB Hi-Z
2444589 F05
SIG
BIT 29
“0”
BIT 30
EOC
Hi-Z
CS
SCK
(EXTERNAL)
SDI
SDO
BUSY
BIT 31
1 0 EN SGL A2 A1 A0 OSR3 OSR2 OSR1 OSR0 TWOXODD
1 2 3 4 5 6 7 8 9 10 11 12 13 14 32
CONVERSION SLEEP DATA OUTPUT
CONVERSION
TEST EOC TEST EOC
VCC FO
REF+
REF–
CH0
CH7
CH8
CH15
COM
SCK
SDI
SDO
CS
GND
28 35
29
30
8
15
16
23
7
38
37
1,4,5,6,31,32,33,39
36
34
REFERENCE
VOLTAGE
0.1V TO VCC
ANALOG
INPUTS 2
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
1µF
4.5V TO 5.5V
LTC2448
4-WIRE
SPI INTERFACE
•
•
•
•
•
•
•
•
•
•
•
•BUSY
Figure 5. External Serial Clock, Single Cycle Operation
External Serial Clock, Single Cycle Operation
(SPI/MICROWIRE Compatible)
This timing mode uses an external serial clock to shift
out the conversion result and a CS signal to monitor and
control the state of the conversion cycle, see Figure 5.
The serial clock mode is selected by the EXT pin. To select
the external serial clock mode, EXT must be tied low.
The serial data output pin (SDO) is Hi-Z as long as CS is
HIGH. At any time during the conversion cycle, CS may be
pulled LOW in order to monitor the state of the converter.
While CS is pulled LOW, EOC is output to the SDO pin.
EOC=1 (BUSY = 1) while a conversion is in progress
and EOC = 0 (BUSY = 0) if the device is in the sleep state.
Independent of CS, the device automatically enters the
low power sleep state once the conversion is complete.
When the device is in the sleep state (
EOC
= 0), its con-
version result is held in an internal static shift register.
The device remains in the sleep state until the first rising
edge of SCK is seen. Data is
shifted out the SDO pin on
each falling edge of SCK. This enables external circuitry
to latch the output on the rising edge of SCK.
EOC
can be
latched on the first rising edge of SCK and the last bit of
the conversion result can be latched on the 32nd rising
edge of SCK. On the 32nd falling edge of SCK, the device
begins a new conversion. SDO goes HIGH (
EOC
= 1) and
BUSY goes HIGH indicating a conversion is in progress.
LTC2444/LTC2445/
LTC2448/LTC2449
18
2444589fc
For more information www.linear.com/LTC2444
At the conclusion of the data cycle, CS may remain LOW
and EOC monitored as an end-of-conversion interrupt.
Alternatively, CS may be driven HIGH setting SDO to Hi-Z
and BUSY monitored for the completion of a conversion.
As described above, CS may be pulled LOW at any time
in order to monitor the conversion status on the SDO pin.
Typically,
CS
remains LOW during the data output state.
However, the data output state may be aborted by pull-
ing
CS
HIGH anytime between the fifth falling edge and
the 32nd falling edge of SCK, see Figure 6. On the rising
edge of
CS
, the device aborts the data output state and
immediately initiates a new conversion.
Thirteen serial
input data bits are required in order to properly program
the speed/resolution and input channel. If the data output
sequence is aborted prior to the 13th rising edge of SCK,
the new input data is ignored, and the previously selected
speed/resolution and channel are used for the next conver-
sion cycle.
This is useful for systems not requiring all 32
bits of output data, aborting an invalid conversion cycle or
synchronizing the start of a conversion. If a new channel
is being programmed, the rising edge of
CS
must come
after the 14th falling edge of SCK in order to store the
data input sequence.
applicaTions inForMaTion
CS
SCK
(EXTERNAL)
SDI
SDO
BUSY
1 2 3 4 5 6 1 5
MSB
BIT 28 BIT 27 BIT 26 BIT 25
SIG
BIT 29
“0”
BIT 30
EOC
Hi-Z Hi-Z
BIT 31
2444589 F06
CONVERSION
SLEEP
SLEEP
DATA OUTPUT DATA OUTPUT
CONVERSION
CONVERSION
TEST EOC
DON'T CARE DON'T CARE
VCC FO
REF+
REF–
CH0
CH7
CH8
CH15
COM
SCK
SDI
SDO
CS
GND
28 35
29
30
8
15
16
23
7
38
37
1,4,5,6,31,32,33,39
36
34
REFERENCE
VOLTAGE
0.1V TO VCC
ANALOG
INPUTS 2
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
1µF
4.5V TO 5.5V
LTC2448
4-WIRE
SPI INTERFACE
•
•
•
•
•
•
•
•
•
•
•
•BUSY
DON'T CARE
Figure 6. External Serial Clock, Reduced Output Data Length
LTC2444/LTC2445/
LTC2448/LTC2449
19
2444589fc
For more information www.linear.com/LTC2444
External Serial Clock, 2-Wire I/O
This timing mode utilizes a 2-wire serial I/O interface.
The conversion result is shifted out of the device by an
externally generated serial clock (SCK) signal, see Figure
7. CS may be permanently tied to ground, simplifying the
user interface or isolation barrier. The external serial clock
mode is selected by tying EXT LOW.
Since CS is tied LOW, the end-of-conversion (EOC) can be
continuously monitored at the SDO pin during the convert
and sleep states. Conversely, BUSY (Pin 2) may be used
to monitor the status of the conversion cycle. EOC or
BUSY may be used as an interrupt to an external control-
applicaTions inForMaTion
ler
indicating the conversion result is ready.
EOC
= 1
(BUSY = 1) while the conversion is in progress and
EOC
=0 (BUSY = 0) once the conversion enters the low
power sleep state. On the falling edge of
EOC
/BUSY, the
conversion result is loaded into an internal static shift
register. The device remains in the sleep state until the
first rising edge of SCK. Data is shifted out the SDO pin
on each falling edge of SCK enabling external circuitry to
latch data on the rising edge of SCK.
EOC
can be latched
on the first rising edge of SCK. On the 32nd falling edge
of SCK, SDO and BUSY go HIGH (
EOC
=1) indicating a
new conversion has begun.
Figure 7. External Serial Clock, CS = 0 Operation (2-Wire)
CS
SCK
(EXTERNAL)
SDI
SDO
BUSY
2444589 F07
CONVERSION SLEEP DATA OUTPUT
CONVERSION
MSB
BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 BIT 20 BIT 19 BIT 0
LSB
SIG
BIT 29
“0”
BIT 30
EOC
BIT 31
1 0 EN SGL A2 A1 A0 OSR3 OSR2 OSR1 OSR0 TWOXODD
1 2 3 4 5 6 7 8 9 10 11 12 13 14 32
DON'T CAREDON'T CARE
VCC FO
REF+
REF–
CH0
CH7
CH8
CH15
COM
SCK
SDI
SDO
CS
GND
28 35
29
30
8
15
16
23
7
38
37
1,4,5,6,31,32,33,39
36
34
REFERENCE
VOLTAGE
0.1V TO VCC
ANALOG
INPUTS 2
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
1µF
4.5V TO 5.5V
LTC2448
4-WIRE
SPI INTERFACE
•
•
•
•
•
•
•
•
•
•
•
•BUSY
LTC2444/LTC2445/
LTC2448/LTC2449
20
2444589fc
For more information www.linear.com/LTC2444
applicaTions inForMaTion
Internal Serial Clock, Single Cycle Operation
This timing mode uses an internal serial clock to shift
out the conversion result and a CS signal to monitor and
control the state of the conversion cycle, see Figure 8.
In order to select the internal serial clock timing mode,
the EXT pin must be tied HIGH.
The serial data output pin (SDO) is Hi-Z as long as CS is
HIGH. At any time during the conversion cycle, CS may be
pulled LOW in order to monitor the state of the converter.
Once CS is pulled LOW, SCK goes LOW and EOC is output
to the SDO pin. EOC = 1 while a conversion is in progress
and EOC = 0 if the device is in the sleep state. Alterna-
tively, BUSY (Pin 2) may be used to monitor the status
of the conversion in progress. BUSY is HIGH during the
conversion and goes LOW at the conclusion. It remains
LOW until the result is read from the device.
When testing EOC, if the conversion is complete (EOC =
0), the device will exit the sleep state and enter the data
output state if CS remains LOW. In order to prevent the
device from exiting the low power sleep state, CS must
be pulled HIGH before the first rising edge of SCK. In the
internal SCK timing mode, SCK goes HIGH and the device
begins outputting data at time tEOCtest after the falling edge
of CS (if EOC = 0) or tEOCtest after EOC goes LOW (if CS is
LOW during the falling edge of EOC). The value of tEOCt-
est is 500ns. If CS is pulled HIGH before time tEOCtest, the
device remains in the sleep state. The conversion result
is held in the internal static shift register.
Figure 8. Internal Serial Clock, Single Cycle Operation
MSB
BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 BIT 20 BIT 19 BIT 0
LSB Hi-Z
2444589 F08
SIG
BIT 29
“0”
BIT 30
EOC
Hi-Z
CS
SCK
SDI
SDO
BUSY
BIT 31
1 0 EN SGL A2 A1 A0 OSR3 OSR2 OSR1 OSR0 TWOXODD
1 2 3 4 5 6 7 8 9 10 11 12 13 14 32
CONVERSION SLEEP DATA OUTPUT
CONVERSION
TEST EOC TEST EOC
DON'T CARE DON'T CARE
<tEOC(TEST)
VCC FO
REF+
REF–
CH0
CH7
CH8
CH15
COM
SCK
SDI
SDO
CS
GND
28 35
29
30
8
15
16
23
7
38
37
1,4,5,6,31,32,33,39
36
34
REFERENCE
VOLTAGE
0.1V TO VCC
ANALOG
INPUTS 2
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
1µF
4.5V TO 5.5V
LTC2448
4-WIRE
SPI INTERFACE
•
•
•
•
•
•
•
•
•
•
•
•BUSY
LTC2444/LTC2445/
LTC2448/LTC2449
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2444589fc
For more information www.linear.com/LTC2444
If CS remains LOW longer than tEOCtest, the first rising
edge of SCK will occur and the conversion result is serially
shifted out of the SDO pin. The data output cycle begins
on this first rising edge of SCK and concludes after the
32nd rising edge. Data is shifted out the SDO pin on each
falling edge of SCK. The internally generated serial clock
is output to the SCK pin. This signal may be used to shift
the conversion result into external circuitry. EOC can be
latched on the first rising edge of SCK and the last bit of
the conversion result on the 32nd rising edge of SCK.
After the 32nd rising edge, SDO goes HIGH (EOC = 1),
SCK stays HIGH and a new conversion starts.
Typically, CS remains LOW during the data output state.
However, the data output state may be aborted by pulling
CS HIGH anytime between the first and 32nd rising edge
of SCK, see Figure 9. On the rising edge of CS, the device
aborts the data output state and immediately initiates a
new conversion. This is useful for systems not requiring
all 32 bits of output data, aborting an invalid conversion
cycle, or synchronizing the start of a conversion. Thirteen
serial input data bits are required in order to properly pro-
gram the speed/resolution and input channel. If the data
output sequence is aborted prior to the 13th rising edge
of SCK, the new input data is ignored, and the previously
selected speed/resolution and channel are used for the next
conversion cycle.
If a new channel is being programmed,
the rising edge of
CS
must come after the 14th falling
edge of SCK in order to store the data input sequence.
applicaTions inForMaTion
Figure 9. Internal Serial Clock, Reduced Data Output Length
CS
SCK
SDI
SDO
BUSY
1 2 3 4 5 6 1 5
MSB
BIT 28 BIT 27 BIT 26 BIT 25
SIG
BIT 29
“0”
BIT 30
EOC
Hi-Z Hi-Z
BIT 31
2444589 F09
CONVERSION
SLEEP
SLEEP
DATA OUTPUT DATA OUTPUT
CONVERSION
CONVERSION
TEST EOC
DON'T CARE DON'T CARE DON'T CARE
<t
EOC(TEST)
<tEOC(TEST)
VCC FO
REF+
REF–
CH0
CH7
CH8
CH15
COM
SCK
SDI
SDO
CS
GND
28 35
29
30
8
15
16
23
7
38
37
1,4,5,6,31,32,33,39
36
34
REFERENCE
VOLTAGE
0.1V TO VCC
ANALOG
INPUTS 2
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
1µF
4.5V TO 5.5V
LTC2448
4-WIRE
SPI INTERFACE
•
•
•
•
•
•
•
•
•
•
•
•BUSY
LTC2444/LTC2445/
LTC2448/LTC2449
22
2444589fc
For more information www.linear.com/LTC2444
applicaTions inForMaTion
Internal Serial Clock, 2-Wire I/O, Continuous
Conversion
This timing mode uses a 2-wire, all output (SCK and SDO)
interface. The conversion result is shifted out of the device
by an internally generated serial clock (SCK) signal, see
Figure 10. CS may be permanently tied to ground, sim-
plifying the user interface or isolation barrier. The internal
serial clock mode is selected by tying EXT HIGH.
During the conversion, the SCK and the serial data output
pin (SDO) are HIGH (EOC = 1) and BUSY = 1. Once the
conversion is complete, SCK, BUSY and SDO go LOW (EOC
= 0) indicating the conversion has finished and the device
has entered the low power sleep state. The part remains in
the sleep state a minimum amount of time (≈500ns) then
immediately begins outputting data. The data output cycle
begins on the first rising edge of SCK and ends after the
32nd rising edge. Data is shifted out the SDO pin on each
falling edge of SCK. The internally generated serial clock
is output to the SCK pin. This signal may be used to shift
the conversion result into external circuitry. EOC can be
latched on the first rising edge of SCK and the last bit of
the conversion result can be latched on the 32nd rising
edge of SCK. After the 32nd rising edge, SDO goes HIGH
(EOC = 1) indicating a new conversion is in progress. SCK
remains HIGH during the conversion.
Figure 10. Internal Serial Clock, Continuous Operation
CS
SCK
SDI
SDO
BUSY
2444589 F10
CONVERSION
SLEEP
DATA OUTPUT
CONVERSION
MSB
BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 BIT 20 BIT 19 BIT 0
LSB
SIG
BIT 29
“0”
BIT 30
EOC
BIT 31
1 0 EN SGL A2 A1 A0 OSR3 OSR2 OSR1 OSR0 TWOXODD
1 2 3 4 5 6 7 8 9 10 11 12 13 14 32
DON'T CAREDON'T CARE
VCC FO
REF+
REF–
CH0
CH7
CH8
CH15
COM
SCK
SDI
SDO
CS
GND
28 35
29
30
8
15
16
23
7
38
37
1,4,5,6,31,32,33,39
36
34
REFERENCE
VOLTAGE
0.1V TO VCC
ANALOG
INPUTS 2
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
1µF
4.5V TO 5.5V
LTC2448
4-WIRE
SPI INTERFACE
•
•
•
•
•
•
•
•
•
•
•
•BUSY
LTC2444/LTC2445/
LTC2448/LTC2449
23
2444589fc
For more information www.linear.com/LTC2444
applicaTions inForMaTion
Normal Mode Rejection and Antialiasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital filtering. Combined
with a large oversampling ratio, the LTC2444/LTC2445/
LTC2448/LTC2449 significantly simplify antialiasing filter
requirements.
The LTC2444/LTC2445/LTC2448/LTC2449’s speed/
resolution is determined by the over sample ratio (OSR)
of the on-chip digital filter. The OSR ranges from 64 for
3.5kHz output rate to 32,768 for 6.9Hz (in No Latency
mode) output rate. The value of OSR and the sample rate
fS determine the filter characteristics of the device. The first
NULL of the digital filter is at fN and multiples of fN where
fN = fS/OSR, see Figure 11 and Table 7. The rejection at
the frequency fN ±14% is better than 80dB, see Figure12.
If FO is grounded, fS is set by the on-chip oscillator at
1.8MHz (over supply and temperature variations). At an
OSR of 32,768, the first NULL is at fN = 55Hz and the
no latency output rate is fN/8 = 6.9Hz. At the maximum
OSR, the noise performance of the device is 280nVRMS
(LTC2444/LTC2448) and 200nVRMS (LTC2445/LTC2449)
Table 7. OSR vs Notch Frequency (fN) (with Internal Oscillator
Running at 9MHz)
OSR NOTCH (fN)
64 28.16kHz
128 14.08kHz
256 7.04kHz
512 3.52kHz
1024 1.76kHz
2048 880Hz
4096 440Hz
8192 220Hz
16384 110Hz
32768* 55Hz
*Simultaneous 50/60Hz rejection
Figure 11. LTC2444/LTC2445/LTC2448/LTC2449 Normal
Mode Rejection (Internal Oscillator)
Figure 12. LTC2444/LTC2445/LTC2448/LTC2449 Normal
Mode Rejection (Internal Oscillator)
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–60
–40
0
180
2444589 F11
–80
–100
60 120
240
–120
–140
–20
NORMAL MODE REJECTION (dB)
SINC4 ENVELOPE
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
47
–140
NORMAL MODE REJECTION (dB)
–130
–120
–110
–100
51 55 59 63
2440 F12
–90
–80
49 53 57 61
LTC2444/LTC2445/
LTC2448/LTC2449
24
2444589fc
For more information www.linear.com/LTC2444
applicaTions inForMaTion
clock applied to FO results in a NULL at 0.6Hz plus all
harmonics up to 20kHz, see Figure 14. This is useful in
applications requiring digitalization of the DC component
of a noisy input signal and eliminates the need of placing
a 0.6Hz filter in front of the ADC.
An external oscillator operating from 100kHz to 12MHz can
be implemented using the LTC1799 (resistor set SOT-23
oscillator), see Figure 17. By floating pin 4 (DIV) of the
LTC1799, the output oscillator frequency is:
f
MHz
k
R
OSC
SET
=
10
10
10
•
•
The normal mode rejection characteristic shown in
Figure13 is achieved by applying the output of the LTC1799
(with RSET = 100k) to the FO pin on the LTC2444/LTC2445/
LTC2448/LTC2449 with SDI tied HIGH (OSR = 32768).
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–60
–40
0
2444589 F13
–80
–100
1000000
2000000
–120
1.8MHz
–140
–20
NORMAL MODE REJECTION (dB)
REJECTION > 120dB
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
–40
–20
0
8
2444589 F14
–60
–80
246
10
–100
–120
–140
NORMAL MODE REJECTION (dB)
Figure 13. LTC2444/LTC2445/LTC2448/LTC2449 Normal
Mode Rejection (Internal Oscillator) Figure 14. LTC2444/LTC2445/LTC2448/LTC2449 Normal
Mode Rejection (External Oscillator at 90kHz)
with better than 80dB rejection of 50Hz ±2% and 60Hz ±2%.
Since the OSR is large (32,768) the wide band rejection
is extremely large and the antialiasing requirements are
simple. The first multiple of fS occurs at 55Hz • 32,768 =
1.8MHz, see Figure 13.
The first NULL becomes fN = 7.04kHz with an OSR of 256
(an output rate of 880Hz) and FO grounded. While the NULL
has shifted, the sample rate remains constant. As a result
of constant modulator sampling rate, the linearity, offset
and full-scale performance remains unchanged as does
the first multiple of fS.
The sample rate fS and NULL fN, may also be adjusted by
driving the FO pin with an external oscillator. The sample
rate is fS = fEOSC/5, where fEOSC is the frequency of the
clock applied to FO. Combining a large OSR with a reduced
sample rate leads to notch frequencies fN near DC while
maintaining simple antialiasing requirements. A 100kHz
LTC2444/LTC2445/
LTC2448/LTC2449
25
2444589fc
For more information www.linear.com/LTC2444
Reduced Power Operation
In addition to adjusting the speed/resolution of the
LTC2444/LTC2445/LTC2448/LTC2449, the speed/reso-
lution/power dissipation may also be adjusted using the
automatic sleep mode. During the conversion cycle, the
LTC2444/LTC2445/LTC2448/LTC2449 draw 8mA supply
current independent of the programmed speed. Once the
conversion cycle is completed, the device automatically
enters a low power sleep state drawing 8µA. The device
remains in this state as long as CS is HIGH and data is not
shifted out. By adjusting the duration of the sleep state
(hold CS HIGH longer) and the duration of the conversion
cycle (programming OSR) the DC power dissipation can
be reduced, see Figure 15.
Average Input Current
The LTC2444/LTC2448 switch the input and reference to
a 2pF capacitor at a frequency of 1.8MHz. A simplified
equivalent circuit is shown in Figure 16. The sample ca-
pacitor for the LTC2445/LTC2449 is 4pF, and its average
input current is externally buffered from the input source.
The average input and reference currents can be expressed
in terms of the equivalent input resistance of the sample
capacitor, where: Req = 1/(fSW • Ceq)
When using the internal oscillator, fSW is 1.8MHz and the
equivalent resistance is approximately 110kΩ.
Input Bandwidth and Frequency Rejection
The combined effect of the internal SINC4 digital filter and
the digital and analog autocalibration circuits determines
the LTC2444/LTC2445/LTC2448/LTC2449 input bandwidth
Figure 16. LTC2444/LTC2448 Input Structure
Figure 15. Reduced Power Timing Mode
SLEEP CONVERT SLEEP CONVERT SLEEP
8µA
2444589 F15
8mA8µA8mA8µA
DATA
OUT
DATA
OUT
CONVER
TER
STATE
SUPPLY
CURRENT
CS
VREF+
VIN+
V
CC
RSW (TYP)
500Ω
ILEAK
ILEAK
VCC
ILEAK
ILEAK
VCC
RSW (TYP)
500Ω CEQ
5pF
(TYP)
(CEQ = 2pF
SAMPLE CAP
+ PARASITICS)
RSW (TYP)
500Ω
ILEAK
IIN+
VIN–
IIN–
IREF+
IREF–
2444589 F16
ILEAK
VCC
ILEAK
ILEAK SWITCHING FREQUENCY
fSW = 1.8MHz INTERNAL OSCILLATOR
f
SW
= f
EOSC
/5 EXTERNAL OSCILLATOR
VREF–
RSW (TYP)
500Ω
MUX
MUX
applicaTions inForMaTion
LTC2444/LTC2445/
LTC2448/LTC2449
26
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For more information www.linear.com/LTC2444
Table 8. Performance vs Oversample Ratio
OVER-
SAMPLE
RATIO
(OSR)
*RMS
NOISE
LTC2444/
LTC2448
*RMS
NOISE
LTC2445/
LTC2449
ENOB
(VREF = 5V)
MAXIMUM CONVERSION
RATE (sps)
FIRST NOTCH
FREQUENCY (Hz)
EFFECTIVE
NOISE BW (Hz)
–3dB POINT (Hz)
LTC2444
LTC2449
LTC2445/
LTC2449
Internal
Clock
External fO
(1X Mode)
[fO/x]
External fO
(2X Mode)
[fO/x]
Internal
Clock
External fO
[fO/x]
Internal
9MHz
Clock
External fO
[fO/x]
Internal
Clock
External fO
[fO/x]
64 23µV 23µV 17 17 2816.35 fø/2738 fo/1458 28125 fo/320 3148 fo/2860 1696 fo/5310
128 4.5µV 3.5µV 20.1 20 1455.49 fo/5298 fo/2738 14062.5 fo/640 1574 fo/5720 848 fo/10600
256 2.8µV 2µV 20.8 21.3 740.18 fo/10418 fo/5298 7031.3 fo/1280 787 fo/11440 424 fo/21200
512 2µV 1.4µV 21.3 21.8 373.28 fo/20658 fo/10418 3515.6 fo/2560 394 fo/22840 212 fo/42500
1024 1.4µV 1µV 21.8 22.4 187.45 fo/41138 fo/20658 1757.8 fo/5120 197 fo/45690 106 fo/84900
2048 1.1µV 750nV 22.1 22.9 93.93 fo/82098 fo/41138 878.9 fo/10200 98.4 fo/91460 53 fo/170000
4096 720nV 510nV 22.7 23.4 47.01 fo/164018 fo/82098 439.5 fo/20500 49.2 fo/183000 26.5 fo/340000
8192 530nV 375nV 23.2 24 23.52 fo/327858 fo/164018 219.7 fo/41000 24.6 fo/366000 13.2 fo/679000
16384 350nV 250nV 23.8 24.4 11.76 fo/655538 fo/327858 109.9 fo/81900 12.4 fo/731000 6.6 fo/1358000
32768 280nV 200nV 24.1 24.6 5.88 fo/1310898 fo/655538 54.9 fo/163800 6.2 fo/1463000 3.3 fo/2717000
*ADC noise increases by approximately √2 when OSR is decreased by a factor of 2 for OSR 32768 to OSR 256. The ADC noise at OSR 128 and OSR 64
include effects from internal modulator quantization noise.
and rejection characteristics. The digital filter’s response
can be adjusted by setting the oversample ratio (OSR)
through the SPI interface or by supplying an external
conversion clock to the FO pin.
Table 8 lists the properties of the LTC2444/LTC2445/
LTC2448/LTC2449 with various combinations of overs-
ample ratio and clock frequency. Understanding these
properties is the key to fine tuning the characteristics of the
LTC2444/LTC2445/LTC2448/LTC2449 to the application.
Maximum Conversion Rate
The maximum conversion rate is the fastest possible rate
at which conversions can be performed.
First Notch Frequency
This is the first notch in the SINC4 portion of the digital filter
and depends on the FO clock frequency and the oversample
ratio. Rejection at this frequency and its multiples (up to
the modulator sample rate of 1.8MHz) exceeds 120dB.
This is 8 times the maximum conversion rate.
applicaTions inForMaTion
28 2
29
38
37
8
936
3
2444589 F17
35
1,4,5,6,31,32,33,39
REFERENCE
VOLTAGE
0.1V TO VCC
ANALOG INPUT
–0.5VREF TO
0.5VREF
3-WIRE
SPI INTERFACE
4.5V TO 5.5V
30
VCC BUSY
fO
REF+
SCK
CH0
CH1
SDO
GND
CS
EXT
LTC2448
REF–
1µF
0.1µF
LTC1799
OUT
DIV SET
GND
V+
RSET
NC
•
•
•
Figure 17. Simple External Clock Source
LTC2444/LTC2445/
LTC2448/LTC2449
27
2444589fc
For more information www.linear.com/LTC2444
Effective Noise Bandwidth
The LTC2444/LTC2445/LTC2448/LTC2449 has extremely
good input noise rejection from the first notch frequency
all the way out to the modulator sample rate (typically
1.8MHz). Effective noise bandwidth is a measure of how
the ADC will reject wideband input noise up to the modu-
lator sample rate. The example on the following page
shows how the noise rejection of the LTC2444/LTC2445/
LTC2448/LTC2449 reduces the effective noise of an ampli-
fier driving its input.
Example: If an amplifier (e.g. LT1219) driving the input of
an LTC2444/LTC2445/LTC2448/LTC2449 has wideband
noise of 33nV/√Hz, band-limited to 1.8MHz, the total noise
entering the ADC input is:
33nV/√Hz • √1.8MHz = 44.3µV.
When the ADC digitizes the input, its digital filter filters
out the wideband noise from the input signal. The noise
reduction depends on the oversample ratio which defines
the effective bandwidth of the digital filter.
At an oversample of 256, the noise bandwidth of the ADC
is 787Hz which reduces the total amplifier noise to:
33nV/√Hz • √787Hz = 0.93µV.
The total noise is the RMS sum of this noise with the 2µV
noise of the ADC at OSR=256.
√(0.93µV)2 + (2µV)2 = 2.2µV.
Increasing the oversample ratio to 32768 reduces the
noise bandwidth of the ADC to 6.2Hz which reduces the
total amplifier noise to:
33nV/√Hz • √6.2Hz = 82nV.
The total noise is the RMS sum of this noise with the
200nV noise of the ADC at OSR = 32768.
√(82nV)2 + (200nV)2 = 216nV.
In this way, the digital filter with its variable oversampling
ratio can greatly reduce the effects of external noise sources.
Automatic Offset Calibration of External
Buffers/Amplifiers
The LTC2445/LTC2449 enable an external amplifier to
be inserted between the multiplexer output and the ADC
input. This enables one external buffer/amplifier circuit to
be shared between all 17 analog inputs (16 single-ended
or 8 differential). The LTC2445/LTC2449 perform an
internal offset calibration every conversion cycle in order
to remove the offset and drift of the ADC. This calibration
is performed through a combination of front end switch-
ing and digital processing. Since the external amplifier is
placed between the multiplexer and the ADC, it is inside
the correction loop. This results in automatic offset cor-
rection and offset drift removal of the external amplifier.
The LT1368 is an excellent amplifier for this function. It
has rail-to-rail inputs and outputs, and it operates on a
single 5V supply. Its open-loop gain is 1M and its input
bias current is 10nA. It also requires at least a 0.1µF load
capacitor for compensation. It is this feature that sets it
apart from other amplifiers—the load capacitor attenuates
sampling glitches from the LTC2445/LTC2449 ADCIN
terminals, allowing it to achieve full performance of the
ADC with high impedance at the multiplexer inputs.
Another benefit of the LT1368 is that it can be powered
from supplies equal to or greater than that of the ADC.
This can allow the inputs to span the entire absolute
maximum of GND – 0.3V to VCC + 0.3V. Using a positive
supply of 7.5V to 10V and a negative supply of –2.5 to –5V
gives the amplifier plenty of headroom over the LTC2445/
LTC2449 input range.
applicaTions inForMaTion
LTC2444/LTC2445/
LTC2448/LTC2449
28
2444589fc
For more information www.linear.com/LTC2444
package DescripTion
Please refer to http://www.linear.com/product/LTC2444#packaging for the most recent package drawings.
5.00 ±0.10
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE
OUTLINE M0-220 VARIATION WHKD
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
PIN 1
TOP MARK
(SEE NOTE 6)
37
1
2
38
BOTTOM VIEW—EXPOSED PAD
5.50 REF 5.15 ±0.10
7.00
±0.10
0.75 ±0.05
R = 0.125
TYP
R = 0.10
TYP
0.25 ±0.05
(UH) QFN REF C 1107
0.50 BSC
0.200 REF
0.00 – 0.05
RECOMMENDED SOLDER PAD LAYOUT
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
3.00 REF
3.15 ±0.10
0.40 ±0.10
0.70 ±0.05
0.50 BSC
5.5 REF
3.00 REF 3.15 ±0.05
4.10 ±0.05
5.50 ±0.05 5.15 ±0.05
6.10 ±0.05
7.50 ±0.05
0.25 ±0.05
PACKAGE
OUTLINE
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
PIN 1 NOTCH
R = 0.30 TYP OR
0.35 × 45°
CHAMFER
UHF Package
38-Lead Plastic QFN (5mm × 7mm)
(Reference LTC DWG # 05-08-1701 Rev C)
LTC2444/LTC2445/
LTC2448/LTC2449
29
2444589fc
For more information www.linear.com/LTC2444
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
C 01/17 Updated Max values for fEOSC
Updated formula for tCONV
Updated Note 13
Inserted Figure 4, Input Range
Revised Table 8, Performance vs Oversample Ratio
5
5
5
11
26
(Revision history begins at Rev C)
LTC2444/LTC2445/
LTC2448/LTC2449
30
2444589fc
For more information www.linear.com/LTC2444
LINEAR TECHNOLOGY CORPORATION 2004
LT 0117 REV C • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC2444
relaTeD parTs
Typical applicaTion
PART NUMBER DESCRIPTION COMMENTS
LT
®
1025 Micropower Thermocouple Cold Junction Compensator 80µA Supply Current, 0.5°C Initial Accuracy
LTC1043 Dual Precision Instrumentation Switched Capacitor Building
Block
Precise Charge, Balanced Switching, Low Power
LTC1050 Precision Chopper Stabilized Op Amp No External Components 5µV Offset, 1.6µVP-P Noise
LT1236A-5 Precision Bandgap Reference, 5V 0.05% Max, 5ppm/°C Drift
LT1461 Micropower Series Reference, 2.5V 0.04% Max, 3ppm/°C Max Drift
LTC1592 Ultraprecise 16-Bit SoftSpan™ DAC Six Programmable Output Ranges
LTC1655 16-Bit Rail-to-Rail Micropower DAC ±1LSB DNL, 600µA, Internal Reference, SO-8
LTC1799 Resistor Set SOT-23 Oscillator Single Resistor Frequency Set
LTC2053 Rail-to-Rail Instrumentation Amplifier 10µV Offset with 50nV/°C Drift, 2.5µVP-P Noise 0.01Hz to 10Hz
LTC2412 2-Channel, Differential Input, 24-Bit, No Latency ∆∑ ADC 0.16ppm Noise, 2ppm INL, 200µA
LTC2415 1-Channel, Differential Input, 24-Bit, No Latency ∆∑ ADC 0.23ppm Noise, 2ppm INL, 2X Speed Mode
LTC2414/LTC2418 4-/8-Channel, Differential Input, 24-Bit, No Latency ∆∑ ADC 0.2ppm Noise, 2ppm INL, 200µA
LTC2430/LTC2431 1-Channel, Differential Input, 20-Bit, No Latency ∆∑ ADC 0.56ppm Noise, 3ppm INL, 200µA
LTC2436-1 2-Channel, Differential Input, 16-Bit, No Latency ∆∑ ADC 800nVRMS Noise, 0.12LBS INL, 0.006LBS Offset, 200µA
LTC2440 1-Channel, Differential Input, High Speed/Low Noise, 24-Bit,
No Latency ∆∑ ADC
2µVRMS Noise at 880Hz, 200nVRMS Noise at 6.9Hz,
0.005% INL, Up to 3.5kHz Output Rate
–
+
–
+
5V
0V
1/2 LT1368
1/2 LT1368
1
2
3
4
5
6
7
8
HIGH
SPEED
∆∑ ADC
MUX
MUXOUTN
MUXOUTP
ADCINP
ADCINN
17
2444589 TA02
(EXTERNAL AMPLIFIERS)
LTC2449
CH0-CH15/
COM
SDI
SCK
SDO
CS
0.1µF
0.1µF
External Buffers Provide High Impedance Inputs and Amplifier Offsets are Cancelled