MAX31856MUD+ [MAXIM]
Analog Circuit, 1 Func, PDSO14, ROHS COMPLIANT, TSSOP-14;
| 型号： | MAX31856MUD+ |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Analog Circuit, 1 Func, PDSO14, ROHS COMPLIANT, TSSOP-14 光电二极管 |
| 文件： | 总30页 (文件大小：509K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
EVALUATION KIT AVAILABLE
MAX31856
Precision Thermocouple to Digital Converter
with Linearization
General Description
Benefits and Features
● Provides High-Accuracy Thermocouple Temperature
The MAX31856 performs cold-junction compensation
and digitizes the signal from any type of thermocouple.
The output data is formatted in degrees Celsius. This
converter resolves temperatures to 0.0078125°C, allows
readings as high as +1800°C and as low as -210°C
(depending on thermocouple type), and exhibits thermo-
couple voltage measurement accuracy of ±0.15%. The
thermocouple inputs are protected against overvoltage
conditions up to ±45V.
Readings
• Includes Automatic Linearization Correction for 8
Thermocouple Types
• ±0.15% (max, -20°C to +85°C) Thermocouple Full-
Scale and Linearity Error
• 19-Bit, 0.0078125°C Thermocouple Temperature
Resolution
● Internal Cold-Junction Compensation Minimizes
A lookup table (LUT) stores linearity correction data for
several types of thermocouples (K, J, N, R, S, T, E, and
B). Line frequency filtering of 50Hz and 60Hz is included,
as is thermocouple fault detection. A SPI-compatible inter-
face allows selection of thermocouple type and setup of
the conversion and fault detection processes.
System Components
• ±0.7°C (max, -20°C to +85°C) Cold-Junction
Accuracy
● ±45V Input Protection Provides Robust System
Performance
● Simplifies System Fault Management and
Troubleshooting
Applications
● Temperature Controllers
● Industrial Ovens, Furnaces, and Environmental
Chambers
• Detects Open Thermocouples
• Over- and Undertemperature Fault Detection
● 50Hz/60Hz Noise Rejection Filtering Improves
● Industrial Equipment
System Performance
Ordering Information appears at end of data sheet.
● 14-Pin TSSOP Package
Typical Application Circuit
AGND
DGND
BIAS
FAULT
0.01µF
SDI
T-
MAX31856
0.1µF
T+
SDO
0.01µF
TO MICROCONTROLLER
3.3V
AVDD
DNC
SCK
CS
0.1µF
DRDY
DVDD
3.3V
0.1µF
19-7534; Rev 0; 2/15
MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Absolute Maximum Ratings
AVDD, DVDD........................................................-0.3V to +4.0V
T+, T-, Bias..........................................................................±45V
T+, T-, Bias.......................................................................±20mA
Operating Temperature Range......................... -55°C to +125°C
Junction Temperature......................................................+150°C
Storage Temperature Range ........................... -65°C to +150°C
All Other Pins .......................................-0.3V to (V
+ 0.3V)
Lead Temperature (soldering, 10s) .................................+300°C
Soldering Temperature
DVDD
Continuous Power Dissipation (T = +70°C)
A
TSSOP (derate 9.1mW/°C above +70°C)..................727.3mW
ESD Protection (All pins, Human Body Model).................2000V
(reflow) ................See IPC/JEDEC J-STD-020A Specification
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation 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.
(Note 1)
Package Thermal Characteristics
TSSOP
Junction-to-Ambient Thermal Resistance (θ ) ........110°C/W
Junction-to-Case Thermal Resistance (θ )...............30°C/W
JC
JA
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.
Recommended DC Operating Conditions
(T = -55°C to +125°C, unless otherwise noted.)(Notes 2 and 4)
A
PARAMETER
SYMBOL
CONDITIONS
MIN
3.0
TYP
MAX
UNITS
V
,
AVDD
Power-Supply Voltage
3.3
3.6
V
V
DVDD
AVDD-DVDD
Cable Resistance
Input Logic 0
Input Logic 1
-100
+100
40
mV
kΩ
V
R
Per lead
CABLE
V
0.8
IL
V
2.1
V
IH
Electrical Characteristics
(3.0V ≤ V
≤ 3.6V, T = -55°C to +125°C, unless otherwise noted.)(Notes 2, 3, and 4)
A
DD
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
10
UNITS
µA
Standby
Active conversion
5.25
1.2
19
Supply Current
I
DD
2
mA
Bits
°C
Thermocouple Temperature
Resolution
0.0078125
Cold-Junction Temperature Data
Resolution
0.015625
°C
T
T
T
T
= +25°C
-10
-10
-20
-20
+10
+65
A
A
A
A
= -40°C to +85°C
= -55°C to +105°C
= -55°C to +125°C
Thermocouple Input Bias Current
I
TCBIAS
nA
+110
+400
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Electrical Characteristics (continued)
(3.0V ≤ V
≤ 3.6V, T = -55°C to +125°C, unless otherwise noted.)(Notes 2, 3, and 4)
A
DD
PARAMETER
SYMBOL
CONDITIONS
= +25°C
MIN
TYP
MAX
UNITS
T
T
T
T
±0.2
A
A
A
A
= -40°C to +85°C
= -55°C to +105°C
= -55°C to +125°C
-4
+4
Thermocouple Input Differential
Bias Current (Note 4)
I
nA
TCIDBIAS
-5.5
-10
+5.5
+10
AV = 8
1.3
0.4
Input-Referred Noise
V
µV
RMS
N
AV = 32
Power-Supply Rejection
PSR
Cold-junction sensor
0.15
°C/V
Power-On-Reset Voltage
Threshold
V
2.7
0.1
2.85
V
POR
Power-On-Reset Voltage
Hysteresis
V
V
V
HYST
Bias Voltage
V
R
0.735
2
BIAS
BIAS Output Resistance
Input Common-Mode Range
kΩ
BIAS
0.5
-0.05
-0.15
-0.2
1.4
+0.05
+0.15
+0.2
V
T = +25°C
A
T = -20°C to +85°C
A
Full-Scale and INL Error (Note 6)
Input Offset Voltage (Note 7)
T = -40°C to +105°C
%FS
%FS
A
T = -40°C to +125°C
-0.3
+0.3
A
T
= -55°C to +125°C
-0.35
-0.01
-0.015
-0.017
-0.02
-7.8
+0.35
+0.01
+0.015
+0.017
+0.02
+7.8
A
T = +25°C
A
T = -20°C to +85°C
A
T = -40°C to +105°C
A
T = -55°C to +125°C
A
T = +25°C
A
T = -20°C to +85°C
-11.7
-13.3
-15.6
-2.0
+11.7
+13.3
+15.6
+2.0
A
AV = 8
T = -40°C to +105°C
A
T = -55°C to +125°C
A
Input Offset Voltage
µV
°C
T = +25°C
A
T = -20°C to +85°C
-2.9
+2.9
A
AV = 32
T = -40°C to +105°C
-3.3
+3.3
A
T = -55°C to +125°C
-3.9
+3.9
A
T = -20°C to +85°C
A
-0.7
+0.7
T = -40°C to +105°C
A
-1
+1
Cold-Junction Temperature Error
T = -55°C to +125°C
-2
+2
A
Overvoltage Rising Threshold
(Note 8)
V
AVDD
0.1
–
V
+
V
AVDD
+ 0.35
AVDD
0.17
V
V
Overvoltage Hysteresis
0.09
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Electrical Characteristics (continued)
(3.0V ≤ V
≤ 3.6V, T = -55°C to +125°C, unless otherwise noted.)(Notes 2, 3, and 4)
A
DD
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
-0.17
0.09
MAX
UNITS
Undervoltage Falling Edge
Threshold (Note 8)
-0.3
0
V
V
Undervoltage Hysteresis
Type B,
T
T
= 0 to 125°C,
-0.24
-0.14
-0.11
-0.13
-0.09
-0.19
-0.16
-0.07
+0.25
+0.06
+0.10
+0.12
+0.08
+0.17
+0.20
+0.07
A
= 95°C to +1798°C
TC
Type E,
T
T
= -55°C to +125°C
= -200°C to +1000°C
A
TC
Type J,
T
T
= -55°C to +125°C
= -210°C to +1200°C
A
TC
Type K,
T
T
= -55°C to +125°C
= -200°C to +1372°C
A
TC
Thermocouple Linearity
Correction Error
°C
Type N,
T
T
= -55°C to +125°C
= -200°C to +1300°C
A
TC
Type R,
T
T
= -50°C to +125°C
= -50°C to +1768°C
A
TC
Type S,
T
T
= -50°C to +125°C
= -50°C to +1768°C
A
TC
Type T,
T
T
= -55°C to +125°C
= -200°C to +400°C
A
TC
1-Shot conversion or first
conversion in auto-conversion
mode (60Hz)
143
169
155
185
1-Shot conversion or first
conversion in auto-conversion
mode (50Hz)
Temperature Conversion Time
(Thermocouple + Cold Junction)
t
ms
CONV
Auto conversion mode,
conversions 2 through n (60Hz)
82
98
90
Auto conversion mode,
conversions 2 through n (50Hz)
110
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Electrical Characteristics (continued)
(3.0V ≤ V
≤ 3.6V, T = -55°C to +125°C, unless otherwise noted.)(Notes 2, 3, and 4)
A
DD
PARAMETER
SYMBOL
CONDITIONS
0.5V ≤ V ≤ 1.4V
CM
MIN
TYP
70
MAX
UNITS
dB
Common-Mode Rejection
50/60Hz Noise Rejection
SERIAL INTERFACE
Input Leakage Current
Output High Voltage
Output Low Voltage
CMR
Fundamental and harmonics
91
dB
I
(Note 5)
-1
+1
0.4
5
µA
V
LEAK
V
I
I
= -1.6mA
= 1.6mA
V
- 0.4
OH
OUT
OUT
CC
V
V
OL
Input Capacitance
C
8
pF
MHz
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
IN
Serial Clock Frequency
SCK Pulse High Width
SCK Pulse Low Width
SCK Rise and Fall Time
CS Fall to SCK Rise
SCK to CS Hold
f
SCL
t
100
100
CH
t
CL
t , t
R
C = 10pF
L
200
40
F
t
C = 10pF
L
100
100
CC
t
C = 10pF
L
CCH
CS Rise to Output Disable
Data to SCLK Setup
SCLK to Data Hold
t
C = 10pF
L
CDZ
t
35
35
DC
t
t
CDH
CDD
SCK Fall to Output Data Valid
CS Inactive Time
C = 10pF
L
80
t
(Note 3)
400
CWH
Note 2: All voltages are referenced to GND. Currents entering the IC are specified positive, and currents exiting the IC are negative.
Note 3: All Serial Interface timing specifications are guaranteed by design.
Note 4: Specification is 100% tested at T = +25°C. Specification limits over temperature (T = T
to T
) are guaranteed by
A
A
MIN
MAX
design and characterization; not production tested.
Note 5: For all pins except T+ and T- (see the Thermocouple Input Bias Current parameter in the Electrical Characteristics table.
Note 6: Using a common-mode voltage other than V
will change this specification. See the Typical Operating Characteristics
BIAS
for details.
Note 7: Input-referred full-scale voltage is 78.125mV when AV = 8 and is 19.531mV when AV = 32.
Note 8: Overvoltage and undervoltage limits apply to T+, T-, and BIAS pins.
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
CS
t
CC
SCLK
t
CDD
t
t
CDD
CDH
t
DC
A7
SDI
A6
A0
t
CDZ
D0
SDO
D7
D6
D1
WRITE ADDRESS BYTE
READ DATA BYTE
NOTE: SCLK CAN BE EITHER POLARITY, TIMING SHOWN FOR CPOL = 1.
Figure 1. Timing Diagram: SPI Read Data Transfer
CS
t
CWH
t
CC
t
R
t
CCH
t
t
F
CL
SCLK
SDI
t
CDH
t
CH
t
CDH
t
DC
A7
A6
A0
D7
D0
WRITE ADDRESS BYTE
WRITE DATA BYTE
NOTE: SCLK CAN BE EITHER POLARITY, TIMING SHOWN FOR CPOL = 1.
Figure 2. Timing Diagram: SPI Write Data Transfer
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Typical Operating Characteristics
(V
= 3.3V and T = +25°C, unless otherwise noted.)
CC
A
ACTIVE SUPPLY CURRENT
STANDBY SUPPLY CURRENT
vs. TEMPERATURE
THERMOCOUPLE INPUT BIAS CURRENT
vs. TEMPERATURE
toc03
vs. TEMPERATURE
toc01
toc02
1.5
1.45
10
9
250
200
150
100
50
VDD = 3.0V, 3.3V, 3.6V
VDD = 3.6V
1.4
1.35
1.3
VDD = 3.6V
8
1.25
1.2
7
VDD = 3.3V
6
VDD = 3.3V
1.15
1.1
5
0
1.05
1
VDD = 3.0V
25 45 65 85 105 125
VDD = 3.0V
4
-50
-55 -35 -15
5
-55 -35 -15
5
25 45 65 85 105 125
-55 -35 -15
5
25 45 65 85 105 125
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
THERMOCOUPLE INPUT
DIFFERENTIAL BIAS CURRENT
vs. TEMPERATURE
THERMOCOUPLE INPUT
DIFFERENTIAL BIAS CURRENT
vs. TEMPERATURE
toc05
toc04
5
4
5
4
Differential Input Voltage = 0.1V
VDD = 3.0V
Differential Input Voltage = 0V
VDD = 3.0V, 3.3V, 3.6V
VDD = 3.3V
VDD = 3.6V
3
3
2
2
1
1
0
0
-1
-1
-55 -35 -15
5
25 45 65 85 105 125
-55 -35 -15
5
25 45 65 85 105 125
TEMPERATURE (°C)
TEMPERATURE (°C)
AV = 8 FULL-SCALE ERROR
vs. TEMPERATURE
AV = 32 FULL-SCALE ERROR
vs. TEMPERATURE
toc06
toc07
0.15
0.1
0.15
0.1
VDD = 3.0V
VDD = 3.0V
VDD = 3.3V
0.05
0
0.05
0
VDD = 3.3V
-0.05
-0.1
-0.15
-0.05
-0.1
-0.15
VDD = 3.6V
VDD = 3.6V
-55 -35 -15
5
25 45 65 85 105 125
-55 -35 -15
5
25 45 65 85 105 125
TEMPERATURE (°C)
TEMPERATURE (°C)
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Typical Operating Characteristics (continued)
(V
= 3.3V and T = +25°C, unless otherwise noted.)
A
CC
AV = 8 OFFSET ERROR
vs. TEMPERATURE
AV = 32 OFFSET ERROR
vs. TEMPERATURE
toc08
toc09
10
10
8
VDD = 3.0V, 3.3V, 3.6V
8
6
6
VDD = 3.6V
4
4
VDD = 3.3V
2
0
2
0
-2
-4
-2
-4
-6
-8
-10
-6
VDD = 3.0V
-8
-10
-55 -35 -15
5
25 45 65 85 105 125
-55 -35 -15
5
25 45 65 85 105 125
TEMPERATURE (°C)
TEMPERATURE (°C)
COLD-JUNCTION TEMPERATURE
ERROR vs. TEMPERATURE
FULL-SCALE ERROR
vs. COMMON-MODE VOLTAGE
toc10
toc11
0.08
0.06
0.04
0.02
0
0.2
0.15
0.1
VDD = 3.6V
0.05
0
VDD = 3.6V
VDD = 3.3V
-0.02
-0.04
-0.06
-0.08
-0.05
-0.1
-0.15
-0.2
VDD = 3.0V
VDD = 3.3V
VDD = 3.0V
0.5 0.6 0.7 0.8 0.9
1
1.1 1.2 1.3 1.4
-50
0
50
100
COMMON-MODE VOLTAGE (V)
TEMPERATURE (°C)
B-TYPE LINEARIZATION ERROR
vs. THERMOCOUPLE TEMPERATURE
E-TYPE LINEARIZATION ERROR
vs. THERMOCOUPLE TEMPERATURE
toc13
toc12
0.5
0.4
0.3
0.2
0.1
0
0.5
0.4
0.3
0.2
0.1
0
CJ Temp = -40°C, 25°C, 85°C,125°C
CJ Temp = 0°C, 25°C, 85°C,125°C
-0.1
-0.2
-0.3
-0.4
-0.5
-0.1
-0.2
-0.3
-0.4
-0.5
-200
200
600
1000
1400
1800
-200
200
600
1000
1400
1800
THERMOCOUPLE TEMPERATURE (°C)
THERMOCOUPLE TEMPERATURE (°C)
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Typical Operating Characteristics (continued)
(V
= 3.3V and T = +25°C, unless otherwise noted.)
A
CC
J-TYPE LINEARIZATION ERROR
K-TYPE LINEARIZATION ERROR
vs. THERMOCOUPLE TEMPERATURE
vs. THERMOCOUPLE TEMPERATURE
toc15
toc14
0.5
0.4
0.3
0.2
0.1
0
0.5
0.4
0.3
0.2
0.1
0
CJ Temp = -40°C, 25°C, 85°C,125°C
CJ Temp = -40°C, 25°C, 85°C,125°C
-0.1
-0.2
-0.3
-0.4
-0.5
-0.1
-0.2
-0.3
-0.4
-0.5
-200
200
600
1000
1400
1800
-200
200
600
1000
1400
1800
THERMOCOUPLE TEMPERATURE (°C)
THERMOCOUPLE TEMPERATURE (°C)
R-TYPE LINEARIZATION ERROR
vs. THERMOCOUPLE TEMPERATURE
N-TYPE LINEARIZATION ERROR
vs. THERMOCOUPLE TEMPERATURE
toc16
toc17
0.5
0.4
0.3
0.2
0.1
0
0.5
0.4
0.3
0.2
0.1
0
CJ Temp = -40°C, 25°C, 85°C,125°C
CJ Temp = -40°C, 25°C, 85°C,125°C
-0.1
-0.2
-0.3
-0.4
-0.5
-0.1
-0.2
-0.3
-0.4
-0.5
-200
200
600
1000
1400
1800
-200
200
600
1000
1400
1800
THERMOCOUPLE TEMPERATURE (°C)
THERMOCOUPLE TEMPERATURE (°C)
S-TYPE LINEARIZATION ERROR
vs. THERMOCOUPLE TEMPERATURE
T-TYPE LINEARIZATION ERROR
vs. THERMOCOUPLE TEMPERATURE
toc19
toc18
0.5
0.4
0.3
0.2
0.1
0
0.5
0.4
0.3
0.2
0.1
0
CJ Temp = -40°C, 25°C, 85°C,125°C
CJ Temp = -40°C, 25°C, 85°C,125°C
-0.1
-0.2
-0.3
-0.4
-0.5
-0.1
-0.2
-0.3
-0.4
-0.5
-200
200
600
1000
1400
1800
-200
200
600
1000
1400
1800
THERMOCOUPLE TEMPERATURE (°C)
THERMOCOUPLE TEMPERATURE (°C)
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Pin Configuration
TOP VIEW
+
1
2
3
4
5
6
7
14 DGND
AGND
BIAS
T-
13
FAULT
12
11
10
9
SDI
MAX31856
T+
SDO
SCK
CS
AVDD
DNC
DRDY
8
DVDD
TSSOP
Pin Description
PIN
1
NAME
AGND
BIAS
T-
FUNCTION
Analog Ground
2
Bias Voltage Source. Nominally 0.735V. This pin is floating when no conversions are taking place.
Thermocouple Negative Input. See Table 1.
Thermocouple Positive Input. See Table 1.
Analog Positive Supply. Bypass with a 0.1µF capacitor to AGND.
Do Not Connect
3
4
T+
5
AVDD
DNC
DRDY
DVDD
CS
6
7
Data Ready Output
8
Digital Positive Supply. Bypass with a 0.1µF capacitor to DGND.
Chip Select. Set CS low to enable the serial interface.
Serial Clock Input
9
10
11
12
13
14
SCK
SDO
SDI
Serial Data Output
Serial Data Input
FAULT
DGND
Cable, thermocouple, or temperature fault output
Digital Ground
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Block Diagram
T-
INPUT
PROTECTION
AND FAULT
DETECTION
LINEARIZATION AND
COLD-JUNCTION
COMPENSATION
CONTROL
AND
INTERFACE
19-BIT
ADC
PGA
T+
TEMPERATURE
SENSOR
tion code are summed to produce the code corresponding
to the cold-junction compensated thermocouple tempera-
ture. Finally, the LUT is used to produce a cold-junction
compensated output code in units of °C.
Detailed Description
The MAX31856 is a sophisticated thermocouple-to-digital
converter with a built-in 19-bit analog-to-digital converter
(ADC). Internal functions include correction for thermo-
couple nonlinearity, input protection, cold-junction com-
pensation sensing and correction, a digital controller, a
SPI-compatible interface, and associated control logic.
Thermocouple Voltage Conversion
T+ and T- are the thermocouple inputs. T- is biased to
approximately 0.735V by the BIAS output. The amplifier
provides gain to the μV- and mV-level thermocouple sig-
nals to make the amplitude appropriate for the ADC’s full-
scale input range. Two amplifier gains provide full-scale
input ranges of ±78.125mV and ±19.531mV to accommo-
date higher- and lower-sensitivity thermocouples.
In the simplest configuration, the thermocouple wires con-
nect directly to inputs T- and T+, with a common-mode
bias voltage provided by the BIAS output. Additional
filtering and/or protection components may be added if
needed, as discussed in the Applications Information
section. Operation is controlled by two configuration bytes
and four bytes that contain over- and undertemperature
detection thresholds.
Because long thermocouple wires may pick up noise
from a variety of sources, including AC power cables, the
amplified signal is lowpass filtered before being applied
to the ADC. The ADC provides further digital lowpass and
notch filtering to attenuate input noise. The notch frequen-
cies are either 50Hz and its harmonics or 60Hz and its
harmonics, selectable using bit 0 of the Configuration 0
register (00h). In addition, bits D6:4 of the Configuration
1 register (01h) enable an averaging mode that provides
additional filtering with an associated increase in conver-
sion time. 2, 4, 8, or 16 samples may be averaged using
this mode.
Temperature Conversion
The temperature conversion process consists of five
steps as described in the sections below. The input ampli-
fier and ADC amplify and digitize the thermocouple’s volt-
age output. The internal temperature sensor measures
the cold-junction temperature. Using the internal lookup
table (LUT), the ADC code corresponding to the cold-
junction temperature for the selected thermocouple type
is determined. The thermocouple code and the cold-junc-
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
The conversion mode can be either continuous or “nor-
mally off”, as selected by bit 7 of the Configuration 0
register (00h). When in the normally off mode, a single
“1-shot” conversion may be selected using bit 6 of the
Configuration 0 register (00h).
the cold junction must be measured. This is done with the
internal precision temperature sensor, which has accuracy
better than ±0.7°C from -20°C to +85°C. By placing the
MAX31856 near the cold junction, the cold-junction tem-
perature can be measured and used to compensate for
cold-junction effects.
Thermocouple type is user-selectable using bits D3:0 of
the Configuration 1 register (01h). Thermocouple types
K, J, N, R, S, T, B, and E are supported by automatic
cold-junction compensation and linearization. (To use
a different thermocouple type, use bits D3:0 to select a
gain of either 8 or 32. The linearization and cold-junction
compensation calculations may then be done externally
using the cold-junction temperature and thermocouple
voltage data.)
The MAX31856 stores the cold-junction temperature data
in registers 0Ah and 0Bh. When the cold-junction tem-
perature sensor is enabled, these registers are read-only
and contain the measured cold-junction temperature plus
the value in the Cold-Junction Offset register. Reading
the register with the cold-junction temperature sensor
enabled will reset the DRDY pin high. Both bytes of this
register should be read as a multibyte transfer to ensure
both bytes are from the same temperature update. When
the cold-junction temperature sensor is disabled, these
registers become read-write registers that contain the
most recent measured temperature value. If desired, data
from an external temperature sensor may be written to
these registers when the internal cold-junction sensor
is disabled. The maximum cold-junction temperature is
clamped at 128°C and the minimum is clamped at -64°C.
See Table 2 for the Reference Junction (Cold Junction)
Temperature Data Format.
Cold-Junction Temperature Sensing
The function of the thermocouple is to sense a difference in
temperature between two ends of the thermocouple wires.
The thermocouple‘s sensing junction (often called the “hot”
junction regardless of its temperature) can be measured
across its rated operating temperature range (see Table 1
for supported thermocouple temperature ranges).
Additional thermocouples are created where the thermo-
couple wires make contact with different metals, usually
at a connector or at the point where they are soldered to
a PCB (the “cold junction”). To compensate for the errors
due to these additional thermocouples, the temperature at
If desired, a temperature offset may be written to the
Cold-Junction Offset register (09h). The value stored in
registers 0Ah and 0Bh will then be equal to the measured
Table 1. Supported Thermocouples and Temperature Ranges
NOMINAL
SENSITIVITY (µV/°C)
COLD-JUNCTION
TEMP RANGE
TYPE
T-WIRE
Platinum/Rhodium
Constantan
Constantan
Alumel
T+ WIRE
Platinum/Rhodium
Chromel
TEMP RANGE
250oC to 1820oC
-200°C to +1000°C
-210°C to +1200°C
-200°C to +1372°C
-200°C to +1300°C
-50°C to +1768°C
-50°C to +1768°C
-200°C to +400°C
10.086
(+500°C to +1500°C)
B
E
J
0 to 125°C
76.373
(0°C to +1000°C)
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-55°C to +125°C
-50°C to +125°C
-50°C to +125°C
-55°C to +125°C
57.953
(0°C to + 750°C)
Iron
41.276
(0°C to + 1000°C)
K
N
R
S
T
Chromel
36.256
(0°C to +1000°C)
Nisil
Nicrosil
10.506
(0°C to +1000°C)
Platinum
Platinum/Rhodium
Platinum/Rhodium
Copper
9.587
(0°C to +1000°C)
Platinum
52.18
(0°C to +400°C)
Constantan
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
value plus the offset value. The MSB of the offset register
is 4°C and the LSB is 0.0625°C. The resulting range of
the offset value applied to the measured CJ temperature
is -8°C to +7.9375°C. The default offset value is 0°C
(00h).
Thermocouple Linearization and Conversion
of Code to Temperature
Because all thermocouples are nonlinear, the raw cold-
junction-compensated value must be corrected for non-
linearity and converted to a temperature value. This
is done using the LUT to produce the linearized and
cold-junction-compensated temperature value, which is
stored after every conversion as 19 bits in the Linearized
Thermocouple Temperature registers (0Ch, 0Dh, and
0Eh). All three bytes should be read as a multibyte trans-
fer to ensure all are from the same data update. See
Table 3 for the Linearized Thermocouple Temperature
Data Format.
Optimal performance is achieved when the thermocouple
cold junction and the cold-junction sensor are at the same
temperature. Avoid placing heat-generating devices or
components near the cold junction because this may pro-
duce cold-junction-related errors. When a significant tem-
perature differential between the internal sensor and the
cold junction is unavoidable, an external temperature sen-
sor may be used instead. The temperature measured by
the external sensor may be written to the cold-junction tem-
perature register and used for cold-junction compensation.
Bit 3 of Configuration 0 register (00h) disables the internal
cold-junction temperature sensor and allows temperature
values from an external sensor to be written directly into
the Cold-Junction Temperature registers (0Ah and 0Bh).
Linearization accuracy varies by thermocouple type, “hot-
junction” temperature, and cold-junction temperature, with
the largest errors typically occurring near the hot-junction
and cold-junction extremes. Worst-case values for linear-
ization errors are shown in the Electrical Characteristics
table.
Cold-Junction Temperature Translation and
Over-/Undertemperature Fault Detection
Compensation
Over- and undertemperature fault detection are available
for both the cold-junction temperature and the linearized
and cold-junction-compensated temperature reading. Two
registers (03h and 04h) contain the high and low thresh-
olds for the cold-junction temperature. The cold-junction
temperature value in registers 0Ah and 0Bh is compared
to the threshold values. If a threshold is exceeded, the
corresponding bit is set in the Fault Status register (0Fh)
and, if not masked, the FAULT output will assert.
Thermocouple temperature values and corresponding
ADC codes are stored in an internal lookup table. After
measuring the cold-junction temperature, the LUT is
used to convert the temperature value to the equivalent
ADC code for the type of thermocouple being used.
Values between LUT entries are interpolated. The cold-
junction ADC code is added to the conversion result in
the thermocouple voltage register to yield a cold-junction-
compensated value.
Table 2. Reference Junction
(Cold-Junction) Temperature Data Format
Table 3. Linearized Thermocouple
Temperature Data Format
TEMPERATURE (°C)
DIGITAL OUTPUT
0111 1111 1111 1100
0111 1111 0000 0000
0111 1101 0000 0000
0100 0000 0000 0000
0001 1001 0000 0000
0000 0000 1000 0000
0000 0000 0000 0100
0000 0000 0000 0000
1111 1111 1000 0000
1110 0111 0000 0000
1100 1001 0000 0000
TEMPERATURE (°C)
+1600.00
+1000.00
+100.9375
+25.00
DIGITAL OUTPUT
+127.984375
+127
+125
+64
0110 0100 0000 0000 0000 0000
0011 1110 1000 0000 0000 0000
0000 0110 0100 1111 0000 0000
0000 0001 1001 0000 0000 0000
0000 0000 0000 0001 0000 0000
0000 0000 0000 0000 0000 0000
1111 1111 1111 1111 0000 0000
1111 1111 1111 1100 0000 0000
1111 1111 1111 0000 0000 0000
1111 0000 0110 0000 0000 0000
+25
+0.0625
0.00
+0.5
+0.015625
0
-0.0625
-0.25
-0.5
-1.00
-25
-250.00
-55
This format also applies to the High Fault and Low Fault
thresholds.
(Note that the practical temperature range varies with the ther-
mocouple type.)
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Four registers (05h through 08h) contain over- and under-
temperature thresholds for the linearized and cold-junc-
tion-compensated temperature. These threshold register
values are compared to the linearized temperature read-
ing found in registers 0Ch, 0Dh, and 0Eh. If a threshold is
exceeded, the corresponding bit is set in the Fault Status
register (0Fh) and, if not masked, the FAULT output will
assert.
current through the thermocouple wires. The time required
to detect an open circuit depends on the values of the
lead resistances and any filter capacitance at the thermo-
couple input and therefore, bits 4 and 5 also select the
time allowed for open-circuit fault detection. A nominal
detection time of either 10ms, 32ms, or 100ms can be
selected. The Open-Circuit Detection Mode table (Table
4) shows the effect of these two bits on the conversion
time. When the device is in one-shot mode, open-circuit
detection can be disabled or set to occur every one-shot
conversion. When the device is in automatic conversion
mode, open-circuit detection may be disabled, or it may
be set to automatically test for open circuits every 16 con-
version cycles. If on-demand detection is desired, select
“detection disabled” (00), then select the setting for the
desired time constant. An open-circuit detection test will
be performed immediately after the current conversion is
completed. Disabling the open fault detection when in
comparator mode while there is an open fault present
will not clear the fault bit or FAULT pin. If this happens,
to subsequently clear the fault, the MAX31856 must
be placed in interrupt mode and then the fault cleared.
Note that, when cold-junction sensing is enabled, open-
circuit fault detection and cold-junction sensing occur
concurrently. Therefore, cold-junction temperature sens-
ing has no effect on the overall cycle time when open-
circuit fault detection is enabled. An open-circuit fault is
indicated by the Open Fault bit, Bit 0, in the Fault Status
register (0Fh) and the FAULT pin asserts if not masked.
Integrated Input Protection
The internal circuitry is protected from excessive volt-
ages applied to the thermocouple cables by integrated
MOSFETs at the T+ and T- inputs, and the BIAS output.
These MOSFETs turn off when the input voltage is nega-
tive or greater than V . The MOSFETs are capable of
DD
withstanding input voltages up to ±45V. If fault voltages
beyond the ±45V limits are expected, see the Applications
Information section.
When the absolute input voltage at T+ or T- is negative or
greater than V , the Under-/Overvoltage Fault bit, Bit 1,
DD
is set in the Fault Status register (0Fh) and the FAULT pin
asserts if not masked. Conversions are suspended while
the OVUV fault is present and will resume when the fault
is removed.
Open-Circuit Fault Detection
Detection of open-circuit faults, such as those caused
by broken thermocouple wires, can be enabled or dis-
abled using bits 4 and 5 in the Configuration 0 register
(00h). Fault detection is accomplished by forcing a small
Table 4. Open-Circuit Detection Mode
FAULT TEST TIME (ms)
BITS 5:4
OCFAULT1:
OCFAULT0 (Config
Byte 0)
FAULT TEST
INPUT NETWORK
CJ SENSE ENABLED
CJ SENSE DISABLED
TYP
MAX
TYP
MAX
00
01
Disabled
N/A
0
0
0
0
Enabled
(Once every 16
conversions)
R
< 5kΩ
13.3
33.4
15
37
40
60
44
66
S
Enabled
(Once every 16
conversions)
40kΩ > R > 5kΩ;
S
10
11
Time constant <
2ms
Enabled
(Once every 16
conversions)
40kΩ > R > 5kΩ;
S
Time constant >
113.4
125
140
154
2ms
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
edge (see Table 5 and Figure 3). There is one clock for
each bit transferred. Address and data bits are transferred
in groups of eight, MSB first.
Cold-Junction and Thermocouple
Out-of-Range Detection
Thermocouple characteristics, the measurement circuitry,
and the linearization calculations limit the optimum tem-
perature ranges for both the cold junction and the mea-
surement junction (“hot junction”). Bit D7 of the Fault
Status register indicates when the cold-junction tempera-
ture falls outside of the optimum range, and bit D6 indi-
cates when the hot-junction temperature is out of range.
Table 1 shows the temperature limits that apply for the
supported thermocouple types. These values are rounded
to the nearest °C. When the temperature falls outside of
the limit for a given measurement, the reported thermo-
couple temperature is clamped at the limit value. Note
that the FAULT pin never asserts for an out-of-range fault.
Address and Data Bytes
Address and data bytes are shifted MSB-first into the
serial-data input (SDI) and out of the serial-data output
(SDO). Any transfer requires the address of the byte to
specify a write or a read, followed by one or more bytes of
data. Data is transferred out of the SDO for a read opera-
tion and into the SDI for a write operation. The address
byte is always the first byte transferred after CS is driven
low. The MSB (A7) of this byte determines whether the
following byte will be written or read. If A7 is 0, one or
more byte reads will follow the address byte. If A7 is 1,
one or more byte writes will follow the address byte.
For a single-byte transfer, 1 byte is read or written and
then CS is driven high (see Figure 4 and Figure 5). For
a multiple-byte transfer, multiple bytes can be read or
written after the address has been written (see Figure 6).
The address continues to increment through all memory
locations as long as CS remains low. If data continues to
be clocked in or out, the address will loop from 7Fh/FFh to
00h/80h. Invalid memory addresses report an FFh value.
Attempting to write to a read-only register will result in no
change to that register’s contents.
Serial Interface
Four pins are used for SPI-compatible communications:
SDO (serial-data out), SDI (serial-data in), CS (chip
select), and SCLK (serial clock). SDI and SDO are the
serial-data input and output pins, respectively. The CS
input initiates and terminates a data transfer. SCLK syn-
chronizes data movement between the master (microcon-
troller) and the slave (MAX31856).
The serial clock (SCLK), which is generated by the
microcontroller, is active only when CS is low and dur-
ing address and data transfer to any device on the SPI
bus. The inactive clock polarity is programmable in some
microcontrollers. The MAX31856 automatically accom-
modates either clock polarity by sampling SCLK when CS
becomes active to determine the polarity of the inactive
clock. Input data (SDI) is latched on the internal strobe
edge and output data (SDO) is shifted out on the shift
DRDY
The DRDY output goes low when a new conversion result
is available in the Linearized Thermocouple Temperature
register. When a read-operation of the Linearized
Thermocouple Temperature register or the Cold-Junction
Temperature Register (if enabled) completes, DRDY
returns high.
Table 5. Serial Interface Function
MODE
SCLK
SDI
SDO
CS
High
Disable Reset
Input Disabled
Input disabled
High impedance
CPOL = 1*, SCLK rising
CPOL = 0, SCLK falling
CPOL = 1, SCLK falling
CPOL = 0, SCLK rising
Write
Read
Low
Low
Data bit latch
X
High impedance
Next data bit shift**
Note: CPHA bit polarity must be set to 1.
*CPOL is the clock polarity bit that is set in the control register of the microcontroller.
**SDO remains at high impedance until 8 bits of data are ready to be shifted out during a read.
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
CS
SHIFT
SHIFT
INTERNAL STROBE
INTERNAL STROBE
CPOL = 1
SCLK
CS
CPOL = 0
SCLK
NOTE: CPOL IS A BIT THAT IS SET IN THE MICROCONTROLLER’S CONTROL REGISTER.
Figure 3. Serial Clock as a Function of Microcontroller Clock Polarity (CPOL)
CS
SCLK
SDI
A7
A6
A5
A4
A3
A2
A1
A0
SDO
HIGH-Z
D7
D6
D5
D4
D3
D2
D1
D0
Figure 4. SPI Single-Byte Read
CS
SCLK
SDI
A7
A6
A5
A4
A3
A2
A1
A0
D7
D6
D5
D4
D3
D2
D1
D0
SDO
HIGH-Z
Figure 5. SPI Single-Byte Write
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
CS
SCLK
ADDRESS
BYTE
DATA
BYTE 0
DATA
BYTE 1
DATA
BYTE N
WRITE
SDI
SDI
ADDRESS
BYTE
READ
DATA
BYTE 0
DATA
BYTE 1
DATA
BYTE N
SDO
Figure 6. SPI Multibyte Transfer
DRDY
REGISTER
CONTENTS
CONVERSION n
CONVERSION n+1
CONVERSION n+2
CS
SDO
SDI
Figure 7. DRDY Operation
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Internal Registers
Communication with the MAX31856 is accomplished through 16 8-bit registers that contain conversion, status, and
configuration data. All programming is done by selecting the appropriate address of the desired register location. The
Register Memory Map (Table 6) illustrates the addresses for the temperature, status, and configuration registers.
The registers are accessed using the 0Xh addresses for reads and the 8Xh addresses for writes. Data is read from or
written to the registers MSB first. Attempts to write to a read-only register results in no change in the data.
Table 6. Register Memory Map
FACTORY
DEFAULT
ADDRESS
READ/WRITE
NAME
FUNCTION
Configuration 0 Register
00h/80h
01h/81h
02h/82h
03h/83h
04h/84h
05h/85h
06h/86h
07h/87h
08h/88h
09h/89h
0Ah/8Ah
0Bh/8Bh
0Ch
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read Only
Read Only
Read Only
Read Only
CR0
CR1
00h
03h
Configuration 1 Register
MASK
CJHF
FFh
7Fh
C0h
7Fh
FFh
80h
Fault Mask Register
Cold-Junction High Fault Threshold
Cold-Junction Low Fault Threshold
Linearized Temperature High Fault Threshold MSB
Linearized Temperature High Fault Threshold LSB
Linearized Temperature Low Fault Threshold MSB
Linearized Temperature Low Fault Threshold LSB
Cold-Junction Temperature Offset Register
Cold-Junction Temperature Register, MSB
Cold-Junction Temperature Register, LSB
Linearized TC Temperature, Byte 2
Linearized TC Temperature, Byte 1
Linearized TC Temperature, Byte 0
Fault Status Register
CJLF
LTHFTH
LTHFTL
LTLFTH
LTLFTL
CJTO
CJTH
00h
00h
00h
CJTL
00h
LTCBH
LTCBM
LTCBL
SR
00h
0Dh
00h
0Eh
00h
0Fh
00h
Register 00h/80h: Configuration 0 Register (CR0)
The Configuration 0 register selects the conversion mode (automatic or triggered by the 1-shot command), selects open-
circuit fault detection timing, enables the cold-junction sensor, clears the fault status register, and selects the filter notch
frequencies. The effects of the configuration bits are described below.
Default Value: 00h
MEMORY AC-
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
CESS
00h/80h
CMODE
Bit 7
1SHOT
OCFAULT1
OCFAULT0
CJ
FAULT
FAULTCLR
50/60Hz
Bit 0
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Register 00h/80h: Configuration 0 Register (CR0) (continued)
BIT
NAME
DESCRIPTION
Conversion Mode
7
CMODE
0 = Normally Off mode (default)
1 = Automatic Conversion mode. Conversions occur continuously every 100ms (nominal).
One-Shot Mode
0 = No conversions requested (default)
1 = This causes a single cold-junction and thermocouple conversion to take place when Conversion
Mode bit =0 (normally off mode). The conversion is triggered when CS goes high after writing a 1 to
this bit. Note that if a multi-byte write is performed, the conversion is triggered when CS goes high
at the end of the transaction. A single conversion requires approximately 143ms in 60Hz filter mode
or 169ms in 50Hz filter mode to complete. This bit self clears to 0.
6
5:4
3
1SHOT
OCFAULT[1:0]
CJ
These bits enable/disable open-circuit fault detection and select fault detection timing.
See Open-Circuit Fault Detection section and Table 4 for operation of these bits.
Cold-Junction Sensor Disable
0 = Cold-junction temperature sensor enabled (default)
1 = Cold-junction temperature sensor disabled. Data from an external temperature sensor may be
written to the cold-junction temperature register. When this bit changes from 0 to 1, the most recent
cold-junction temperature value will remain in the cold-junction temperature register until the internal
sensor is enabled or until a new value is written to the register. The overall temperature conversion
time is reduced by 25ms (typ) when this bit is set to 1.
Fault Mode
0 = Comparator Mode. The FAULT output and respective fault bit reflects the state of any non-
masked faults by asserting when the fault condition is true, and deasserting when the fault condition
is no longer true. There is a 2°C hysteresis when in comparator mode for threshold fault conditions.
(default)
2
FAULT
1 = Interrupt Mode. The FAULT output and respective fault bit asserts when a non-masked fault
condition is true and remain asserted until a 1 is written to the Fault Status Clear bit. This deasserts
FAULT and respective fault bit until a new fault is detected (note that this may occur immediately if
the fault condition is still in place).
Fault Status Clear
0 = Default
1 = When in interrupt mode, returns all Fault Status bits [7:0] in the Fault Status Register (0Fh) to 0
and deasserts the FAULT output. This bit has no effect in comparator mode. Note that the FAULT
output and the fault bit may reassert immediately if the fault persists. To prevent the FAULT output
from reasserting, first set the Fault Mask bits. The fault status clear bit self-clears to 0.
1
0
FAULTCLR
50/60Hz
50Hz/60Hz Noise Rejection Filter Selection
0= Selects rejection of 60Hz and its harmonics (default)
1= Selects rejection of 50Hz and its harmonics
Note: Change the notch frequency only while in the “Normally Off” mode – not in the Automatic
Conversion mode.
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Register 01h/81h: Configuration 1 Register (CR1)
The Configuration 1 register selects the averaging time for the thermocouple voltage conversion averaging mode and
also selects the thermocouple type being monitored.
Default Value: 03h
MEMORY
ACCESS
N/A
R/W
R/W
R/W
R/W
R/W
R/W
R/W
01h/81h
Reserved
Bit 7
AVGSEL
AVGSEL
AVGSEL
TC TYPE
TC TYPE
TC TYPE
TC TYPE
Bit 0
2
1
0
3
2
1
0
BIT
NAME
DESCRIPTION
7
Reserved
Reserved.
Thermocouple Voltage Conversion Averaging Mode
000 = 1 sample (default)
001 = 2 samples averaged
010 = 4 samples averaged
011 = 8 samples averaged
1xx = 16 samples averaged
Adding samples increases the conversion time and reduces noise.
Typical conversion times:
6:4
AVGSEL[2:0]
1-shot or first conversion in Auto mode:
= t
= t
+ (samples -1) x 33.33mS (60Hz rejection)
+ (samples -1) x 40mS (50Hz rejection)
CONV
CONV
2 thru n conversions in Auto mode
= t
= t
+ (samples -1) x 16.67mS (60Hz rejection)
+ (samples -1) x 20mS (50Hz rejection)
CONV
CONV
The Thermocouple Voltage Conversion Averaging Mode settings should not be changed while
conversions are taking place.
Thermocouple Type
0000 = B Type
0001 = E Type
0010 = J Type
0011 = K Type (default)
0100 = N Type
0101 = R Type
0110 = S Type
3:0
TC TYPE[3:0]
0111 = T Type
10xx = Voltage Mode, Gain = 8. Code = 8 x 1.6 x 217 x V
11xx = Voltage Mode, Gain = 32. Code = 32 x 1.6 x 217 x V
IN
IN
Where Code is 19 bit signed number from TC registers and V is thermocouple input voltage
IN
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Register 02h/82h: Fault Mask Register (MASK)
The Fault Mask Register allows the user to mask faults from causing the FAULT output from asserting. Masked faults
will still result in fault bits being set in the Fault Status register (0Fh). Note that the FAULT output is never asserted by
thermocouple and cold-junction out-of-range status.
Default Value: FFh
MEMORY
ACCESS
N/A
N/A
R/W
R/W
R/W
R/W
R/W
R/W
CJ High
FAULT
Mask
CJ Low
FAULT
Mask
TC High
FAULT
Mask
TC Low
FAULT
Mask
OV/UV
FAULT
Mask
Open
FAULT
Mask
02h/82h
Reserved
Bit 7
Reserved
Bit 0
BIT
NAME
DESCRIPTION
7:6
Reserved
Reserved.
Cold-Junction High Fault Threshold Mask
CJ High
FAULT Mask
0 = FAULT output asserted when the Cold-Junction Temperature rises above the Cold-Junction
Temperature high threshold limit value
5
1 = FAULT output masked (default)
Cold-Junction Low Fault Threshold Mask
CJ Low
FAULT Mask
0 = FAULT output asserted when the Cold-Junction Temperature falls below the Cold-Junction
Temperature low threshold limit value
1 = FAULT output masked (default)
4
3
2
Thermocouple Temperature High Fault Threshold Mask
0 = FAULT output asserted when the Thermocouple Temperature rises above the Thermocouple
Temperature high threshold limit value
TC High
FAULT Mask
1 = FAULT output masked (default)
Thermocouple Temperature Low Fault Threshold Mask
0 = FAULT output asserted when the Thermocouple Temperature falls below the Thermocouple
Temperature low threshold limit value
TC Low
FAULT Mask
1 = FAULT output masked (default)
Over-voltage or Undervoltage Input Fault Mask
0 = FAULT output asserted when an over- or undervoltage condition is detected
1 = FAULT output masked (default)
OV/UV FAULT
Mask
1
0
Thermocouple Open-Circuit Fault Mask
0 = FAULT output asserted when a thermocouple open condition is detected
1 = FAULT output masked (default)
Open FAULT
Mask
Maxim Integrated
│ 21
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Register 03h/83h: Cold-Junction High Fault Threshold Register (CJHF)
Write a temperature limit value to this register. When the measured cold-junction temperature is greater than this value,
the CJ High fault status bit will be set and (if not masked) the FAULT output will assert.
Default Value: 7Fh
MEMORY
ACCESS
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
03h/83h
CJHF7
Sign
CJHF6
CJHF5
CJHF4
CJHF3
CJHF2
CJHF1
CJHF0
6
5
4
3
2
1
0
2
2
2
2
2
2
2
Bit 7
Bit 0
Register 04h/84h: Cold-Junction Low Fault Threshold Register (CJLF)
Write a temperature limit value to this register. When the measured cold-junction temperature is less than this value, the
CJ Low fault status bit will be set and (if not masked) the FAULT output will assert.
Default Value: C0h
MEMORY
ACCESS
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
04h/84h
CJLF7
Sign
CJLF6
CJLF5
CJLF4
CJLF3
CJLF2
CJLF1
CJLF0
6
5
4
3
2
1
0
2
2
2
2
2
2
2
Bit 7
Bit 0
Register 05h/85h: Linearized Temperature High Fault Threshold Register, MSB (LTHFTH)
Write the MSB of the two-byte temperature limit value to this register. When the linearized thermocouple temperature is
greater than the two-byte (05h and 06h) limit value, the TC High fault status bit will be set and (if not masked) the FAULT
output will assert.
Default Value: 7Fh
MEMORY
ACCESS
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
05h/85h
LTHFTH7
Sign
LTHFTH6
LTHFTH5
LTHFTH4
LTHFTH3
LTHFTH2
LTHFTH1
LTHFTH0
10
9
8
7
6
5
4
2
2
2
2
2
2
2
Bit 7
Bit 0
Register 06h/86h: Linearized Temperature High Fault Threshold Register, LSB (LTHFTL)
Write the LSB of the two-byte temperature limit value to this register. When the linearized thermocouple temperature is
greater than the two-byte (05h and 06h) limit value, the TC High fault status bit will be set and (if not masked) the FAULT
output will assert.
Default Value: FFh
MEMORY
ACCESS
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
06h/86h
LTHFTL7
LTHFTL6
LTHFTL5
LTHFTL4
LTHFTL3
LTHFTL2
LTHFTL1
LTHFTL0
3
2
1
0
-1
-2
-3
-4
2
2
2
2
2
2
2
2
Bit 7
Bit 0
Maxim Integrated
│ 22
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Register 07h/87h: Linearized Temperature Low Fault Threshold Register, MSB (LTLFTH)
Write the MSB of the two-byte temperature limit value to this register. When the linearized thermocouple temperature is
less than the two-byte (07h and 08h) limit value, the TC Low fault status bit will be set and (if not masked) the FAULT
output will assert.
Default Value: 80h
MEMORY
ACCESS
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
07h/87h
LTLFTH7
Sign
LTLFTH6
LTLFTH5
LTLFTH4
LTLFTH3
LTLFTH2
LTLFTH1
LTLFTH0
10
9
8
7
6
5
4
2
2
2
2
2
2
2
Bit 7
Bit 0
Register 08h/88h: Linearized Temperature Low Fault Threshold Register, LSB (LTLFTL)
Write the LSB of the two-byte temperature limit value to this register. When the linearized thermocouple temperature is
less than the two-byte (07h and 08h) limit value, the TC Low fault status bit will be set and (if not masked) the FAULT
output will assert.
Default Value: 00h
MEMORY
ACCESS
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
08h/88h
LTLFTL7
LTLFTL6
LTLFTL5
LTLFTL4
LTLFTL3
LTLFTL2
LTLFTL1
LTLFTL0
3
2
1
0
-1
-2
-3
-4
2
2
2
2
2
2
2
2
Bit 7
Bit 0
Register 09h/89h: Cold-Junction Temperature Offset Register (CJTO)
When the cold-junction temperature sensor is enabled, this register allows an offset temperature to be applied to the
measured value. See the Cold-Junction Temperature Sensing section of this data sheet for additional information.
Default Value: 00h
MEMORY
ACCESS
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
09h/89h
CJTO7
Sign
CJTO6
CJTO5
CJTO4
CJTO3
CJTO2
CJTO1
CJTO0
2
1
0
-1
-2
-3
-4
2
2
2
2
2
2
2
Bit 7
Bit 0
Maxim Integrated
│ 23
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Register 0Ah/8Ah: Cold-Junction Temperature Register, MSB (CJTH)
This register contains the MSB of the two-byte (0Ah and 0Bh) value used for cold-junction compensation of the ther-
mocouple measurement. When the cold-junction temperature sensor is enabled, this register is read-only and contains
the MSB of the measured cold-junction temperature plus the value in the Cold-Junction Offset register. Also when the
cold-junction temperature sensor is enabled, a read of this register will reset the DRDY pin high. When the cold-junction
temperature sensor is disabled, this register becomes a read-write register that contains the MSB of the most recent
cold-junction conversion result until a new value is written into it. This allows writing the results from an external tem-
perature sensor, if desired. The maximum contained in the two cold-junction temperature bytes is clamped at 128°C and
the minimum is clamped at -64°C.
Default Value: 00h
MEMORY
ACCESS
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
0Ah/8Ah
CJTH7
Sign
CJTH6
CJTH5
CJTH4
CJTH3
CJTH2
CJTH1
CJTH0
6
5
4
3
2
1
0
2
2
2
2
2
2
2
Bit 7
Bit 0
Register 0Bh/8Bh: Cold-Junction Temperature Register, LSB (CJTL)
This register contains LSB of the two-byte (0Ah and 0Bh) value used for cold-junction compensation of the thermocouple
measurement. When the cold-junction temperature sensor is enabled, this register is read-only and contains the LSB of
the measured cold-junction temperature plus the value in the Cold-Junction Offset register. Also when the cold-junction
temperature sensor is enabled, a read of this register will reset the DRDY pin high. When the cold-junction temperature
sensor is disabled, this register becomes a read-write register that contains the LSB of the most recent cold-junction
conversion result until a new value is written into it.
Default Value: 00h
MEMORY
ACCESS
R/W
R/W
R/W
R/W
R/W
R/W
R
R
0Bh/8Bh
CJTL7
CJTL6
CJTL5
CJTL4
CJTL3
CJTL2
CJTL1
0
CJTL0
0
-1
-2
-3
-4
-5
-6
2
2
2
2
2
2
Bit 7
Bit 0
Register 0Ch: Linearized TC Temperature, Byte 2 (LTCBH)
This is the high byte of the 19-bit register that contains the linearized and cold-junction-compensated thermocouple
temperature value.
Default Value: 00h
MEMORY
ACCESS
R
R
R
R
R
R
R
R
0Ch
LTCBH7
Sign
LTCBH6
LTCBH5
LTCBH4
LTCBH3
LTCBH2
LTCBH1
LTCBH0
10
9
8
7
6
5
4
2
2
2
2
2
2
2
Bit 7
Bit 0
Maxim Integrated
│ 24
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Register 0Dh: Linearized TC Temperature, Byte 1 (LTCBM)
This is the middle byte of the 19-bit register that contains the linearized and cold-junction-compensated thermocouple
temperature value.
Default Value: 00h
MEMORY
ACCESS
R
R
R
R
R
R
R
R
0Dh
LTCBM7
LTCBM6
LTCBM5
LTCBM4
LTCBM3
LTCBM2
LTCBM1
LTCBM0
3
2
1
0
-1
-2
-3
-4
2
2
2
2
2
2
2
2
Bit 7
Bit 0
Register 0Eh: Linearized TC Temperature, Byte 0 (LTCBL)
This is the low byte of the 19-bit register that contains the linearized and cold-junction-compensated thermocouple tem-
perature value.
Default Value: 00h
MEMORY
ACCESS
R
R
R
R
R
R
R
R
0Eh
LTCBL7
LTCBL6
LTCBL5
LTCBL4
X
LTCBL3
X
LTCBL2
X
LTCBL1
X
LTCBL0
X
-5
-6
-7
2
2
2
Bit 7
Bit 0
Register 0Fh: Fault Status Register (SR)
The Fault Status Register contains eight bits that indicate the fault conditions (Thermocouple Out-of-Range, Cold
Junction Out-of-Range, Cold Junction High, Cold Junction Low, Thermocouple High Temperature, Thermocouple Low
Temperature, Over-Under Voltage, or Open Thermocouple) that have been detected.
Default Value: 00h
MEMORY
ACCESS
R
R
R
R
R
R
R
R
0Fh
CJ Range
Bit 7
TC Range
CJHIGH
CJLOW
TCHIGH
TCLOW
OVUV
OPEN
Bit 0
Note: When the MAX31856 is set to operate in “comparator” fault mode (set with bit 2 of Configuration 0 register (00h)), the fault
status bits simply reflect the state of any faults by asserting when the fault condition is true, and deasserting when the fault condition
is no longer true.
When in “interrupt” fault mode, the fault status bits assert when a fault condition is true. The bits remain asserted until a 1 is written
to the Fault Status Clear bit. This deasserts the fault bits until a new fault is detected (note that this may occur immediately if the
fault condition is still in place).
Maxim Integrated
│ 25
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Register 0Fh: Fault Status Register (SR) (continued)
BIT
NAME
DESCRIPTION
Cold Junction Out-of-Range
0 = The Cold-Junction temperature is within the normal operating range (-55°C to +125°C for types E,
J, K, N, and T; -50°C to +125°C for types R and S; 0 to 125°C for type B).
7
CJ Range
1 = The Cold-Junction temperature is outside of the normal operating range.
Thermocouple Out-of-Range
0 = The Thermocouple Hot Junction temperature is within the normal operating range (see Table 1).
1 = The Thermocouple Hot Junction temperature is outside of the normal operating range.
Note: The TC Range bit should be ignored in voltage mode.
6
5
TC Range
CJHIGH
Cold-Junction High Fault
0 = The Cold-Junction temperature is at or lower than the cold-junction temperature high threshold
(default).
1 = The Cold-Junction temperature is higher than the cold-junction temperature high threshold. The
FAULT output is asserted unless masked.
Cold-Junction Low Fault
0 = The Cold-Junction temperature is at or higher than the cold-junction temperature low threshold
(default).
1 = The Cold-Junction temperature is lower than the cold-junction temperature low threshold. The
FAULT output is asserted unless masked.
4
3
2
CJLOW
TCHIGH
TCLOW
Thermocouple Temperature High Fault
0 = The Thermocouple Temperature is at or lower than the thermocouple temperature high threshold
(default).
1 = The Thermocouple Temperature is higher than the thermocouple temperature high threshold. The
FAULT output is asserted unless masked.
Thermocouple Temperature Low Fault
0 = Thermocouple temperature is at or higher than the thermocouple temperature low threshold
(default).
1 = Thermocouple temperature is lower than the thermocouple temperature low threshold. The FAULT
output is asserted unless masked.
Overvoltage or Undervoltage Input Fault
0 = The input voltage is positive and less than V
(default).
DD
1
0
OVUV
OPEN
1 = The input voltage is negative or greater than V . The FAULT output is asserted unless masked.
Note: The presence of the OVUV fault will suspend conversions and the ability of the MAX31856 to
detect other faults (or clear faults when in comparator mode) until the fault is no longer present.
DD
Thermocouple Open-Circuit Fault
0 = No open circuit or broken thermocouple wires are detected (default)
1 = An open circuit such as broken thermocouple wires has been detected. The FAULT output is
asserted unless masked.
Maxim Integrated
│ 26
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
These values may need to be modified depending on the
nature of the noise pickup. Other techniques, such as
adding series resistance and shielding the thermocouple
wires and circuit board, may also be necessary in the
presence of larger noise sources. Figure 8 shows the
typical application circuit with input capacitors and input
resistors added.
Applications Information
Thermocouple Temperature Sensing Guide-
lines
Follow these guidelines to get the best results when sens-
ing temperature. The Typical Application Circuit shows a
basic MAX31856 schematic. Connect the thermocouple
wires to inputs T+ and T-; be sure that the wires are con-
nected to the correct input as shown in Figure 8. Connect
the BIAS output to T-. This biases the thermocouple within
the common-mode range of the inputs.
Input Protection
The ±45V input protection circuitry prevents damage to
the IC caused by overvoltage conditions at T+, T-, or
BIAS. If larger input faults are possible, external protec-
tion should be added. Resistors in series with T+, T-, and
BIAS can increase the acceptable fault voltages. For
example, adding 2kΩ in series with these inputs allows
an additional ±40V of overdrive before the 20mA input
current limit is reached. Note, however, that if the input
has 45V across it and 20mA flowing into it, the power dis-
sipation will be 900mW due to the overdrive at that input.
Overdriving other inputs at the same time will further
increase the power dissipation. Always ensure that if a
continuous overdrive voltage greater than ±45V is expect-
ed, any current-limiting resistors are large enough to keep
total power dissipation well under the IC’s absolute maxi-
mum power dissipation. Note also that added resistance
in series with T+ and T- can increase offset voltage, as
mentioned in the Effect of Series Resistance section.
Noise Considerations
Because of the small signal levels involved, thermocouple
temperature measurement is susceptible to power-sup-
ply-coupled noise. The effects of power-supply noise can
be minimized by placing 0.1µF ceramic bypass capacitors
close to the V
pins and to GND.
DD
The input amplifier is a low-noise amplifier designed to
enable high-precision input sensing. Keep the thermocou-
ple and connecting wires away from electrical noise sourc-
es. It is strongly recommended to add a 100nF ceramic
surface-mount differential capacitor, placed across the T+
and T- pins, to filter noise on the thermocouple lines. In
environments with high noise levels, especially significant
RF fields, a 100nF capacitor between T+ and T- should
be supplemented with a 10nF capacitor between T+ and
GND, and another 10nF capacitor between T- and GND.
AGND
DGND
FAULT
SDI
BIAS
0.01µF
T-
100
MAX31856
0.1µF
T+
SDO
0.01µF
100
TO MICROCONTROLLER
3.3V
AVDD
DNC
SCK
CS
0.1µF
DRDY
DVDD
3.3V
0.1µF
Figure 8. Typical Connection to Reduce the Effect of Noise Pickup in the Thermocouple Cable
Maxim Integrated
│ 27
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
AGND
BIAS
DGND
FAULT
SDI
2k
0.01µF
2k
2k
T-
0.1µF
MAX31856
T+
SDO
0.01µF
0.1µF
TO MICROCONTROLLER
3.3V
AVDD
DNC
SCK
CS
DRDY
DVDD
3.3V
0.1µF
Figure 9. When Thermocouple Inputs May Be Exposed to Fault Voltages Greater than ±45V, Resistors Can be Added to Limit
Current into the MAX31856.
To minimize the effect of input resistance on accuracy:
Effect of Series Resistance
Bias and leakage current at the thermocouple inputs will
flow through input resistors and cable resistance, gen-
erating input offset voltage. For the circuits in Figure 8
and Figure 9, assuming that the thermocouple’s source
resistance is negligible, the offset voltage due to series
resistance will be:
● Minimize the values of any external resistors
● When the cable resistance is very low, match the val-
ues of the external resistors as closely as possible.
● If the cable resistance is known, increase the value
of the resistor connected to T- by the value of the
cable resistance. This will minimize the total mis-
match between the two inputs.
I x ∆R + ∆I x R
S
B
S
B
where:
If the cable resistance is excessive, consider using larger-
gauge thermocouple wire.
● R is the series resistance between each input and
S
the bias point
MAX31856 Location
●● ∆R is the difference between the two R values.
S
S
Because the MAX31856 includes an internal cold-junction
temperature sensor, place it in a location whose tempera-
ture is as close as possible to that of the cold junction. If
the thermocouple leads are directly soldered to the PCB,
the MAX31856 should be as close as possible to the
thermocouple lead connections and thermal gradients
between the IC and the thermocouple connections should
be minimized. If the thermocouple leads terminate in a
connector, mount the IC as close as possible to the con-
nector, and again minimize thermal gradients between the
connector and the IC.
This will generally be equal to the tolerance of any
discrete series resistors plus any cable resistance.
● I is the input bias and leakage current
B
●● ∆I is the differential input bias and leakage current
B
As an example, assume that the circuit in Figure 8 will be
used up to a temperature of 85°C, the mismatch between
the 100Ω input resistors is 1Ω, and the external cable
resistance is 50Ω. This yields a worst-case offset voltage
due to the external resistances of:
65nA x (50Ω + 1Ω) + 4nA x 100Ω = 3.7μV
Maxim Integrated
│ 28
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
the transfer functions from the Configuration 1 Register
table. When voltage mode is selected, no linearization is
performed on the conversion data. Use the voltage data
and the cold-junction temperature to calculate the thermo-
couple’s hot-junction temperature.
Using “Unsupported” Thermocouple Types
To use a thermocouple type other than B, E, J, K, N,
R, S, or T, select one of the voltage mode options in
Configuration 1. Selecting “Gain = 8” results in a full-scale
input voltage range of ±78.125mV. “Gain = 32” results
in a full-scale input voltage range of ±19.531mV. See
Ordering Information
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.
PART
TEMP RANGE
-55°C to +125°C
-55°C to +125°C
PIN-PACKAGE
14 TSSOP
MAX31856MUD+
MAX31856MUD+T
14 TSSOP
+Denotes a lead(Pb)-free/RoHS-compliant package.
T = Tape and reel.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
14 TSSOP
U14+2
21-0066
90-0113
Maxim Integrated
│ 29
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MAX31856
Precision Thermocouple to Digital Converter
with Linearization
Revision History
REVISION REVISION
PAGES
DESCRIPTION
CHANGED
NUMBER
DATE
0
2/15
Initial release
—
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.
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 and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
2015 Maxim Integrated Products, Inc.
│ 30
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