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19-5793; Rev 0; 3/11
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
General Description
The MAX31855 performs cold-junction compensation and digitizes the signal from a K, J, N, T, or E type thermocouple. (Contact the factory for S and R type thermocouples.) The data is output in a signed 14-bit, SPITMcompatible, read-only format. This converter resolves temperatures to 0.25NC, allows readings as high as +1800NC and as low as -270NC, and exhibits thermocouple accuracy of 2NC for temperatures ranging from -200NC to +700NC for K-type thermocouples. For full range accuracies and other thermocouple types, see the Thermal Characteristics specifications. S Cold-Junction Compensation S 14-Bit, 0.25NC Resolution S Versions Available for K, J, N, T, and E Type Thermocouples (Contact Factory for S and R Type Availability) (see Table 1) S Simple SPI-Compatible Interface (Read-Only) S Detects Thermocouple Shorts to GND or VCC S Detects Open Thermocouple
Ordering Information appears at end of data sheet.
Features
Applications
Industrial Appliances HVAC Automotive
For related parts and recommended products to use with this part, refer to: www.maxim-ic.com/MAX31855.related
Typical Application Circuit
VCC
0.1F MAX31855 GND SO T+ TSCK CS MISO SCK SS MICROCONTROLLER
SPI is a trademark of Motorola, Inc.
Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
ABSOLUTE MAXIMUM RATINGS
Supply Voltage Range (VCC to GND) ..................-0.3V to +4.0V All Other Pins............................................ -0.3V to (VCC + 0.3V) Continuous Power Dissipation (TA = +70NC) SO (derate 5.9mW/NC above +70NC) .......................470.6mW ESD Protection (All Pins, Human Body Model).............2000kV Operating Temperature Range ........................ -40NC to +125NC Junction Temperature .....................................................+150NC Storage Temperature Range .......................... -65NC to +150NC Lead Temperature (soldering, 10s) ................................+300NC Soldering Temperature (reflow) .....................................+260NC
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.
PACKAGE THERMAL CHARACTERISTICS (Note 1)
SO Junction-to-Ambient Thermal Resistance (BJA) ........170NC/W Junction-to-Case Thermal Resistance (BJC) ...............40NC/W Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial.
RECOMMENDED OPERATING CONDITIONS
(TA = -40NC to +125NC, unless otherwise noted.) PARAMETER Power-Supply Voltage Input Logic 0 Input Logic 1 SYMBOL VCC VIL VIH (Note 2) CONDITIONS MIN 3.0 -0.3 2.1 TYP 3.3 MAX 3.6 +0.8 VCC + 0.3 UNITS V V V
DC ELECTRICAL CHARACTERISTICS
(3.0V P VCC P 3.6V, TA = -40NC to +125NC, unless otherwise noted.) PARAMETER Power-Supply Current Thermocouple Input Bias Current Power-Supply Rejection Power-On Reset Voltage Threshold Power-On Reset Voltage Hysteresis Output High Voltage Output Low Voltage VOH VOL IOUT = -1.6mA IOUT = 1.6mA VCC 0.4 0.4 VPOR (Note 3) SYMBOL ICC TA = -40NC to +125NC, 100mV across the thermocouple inputs -100 -0.3 2 0.2 2.5 CONDITIONS MIN TYP 900 MAX 1500 +100 UNITS FA nA NC/V V V V V
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*The parametric values (min, typ, max limits) shown in the Electrical Characteristics table supersede values quoted elsewhere in this data sheet.
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
THERMAL CHARACTERISTICS
(3.0V P VCC P 3.6V, TA = -40NC to +125NC, unless otherwise noted.) (Note 4) PARAMETER MAX31855K Thermocouple Temperature Gain and Offset Error (41.276FV/NC nominal sensitivity) (Note 4) SYMBOL CONDITIONS TTHERMOCOUPLE = -200NC to +700NC, TA = -20NC to +85NC (Note 3) TTHERMOCOUPLE = +700NC to +1350NC, TA = -20NC to +85NC (Note 3) TTHERMOCOUPLE = -200NC to +1350NC, TA = -40NC to +125NC (Note 3) TTHERMOCOUPLE = -40NC to +750NC, TA = -20NC to +85NC (Note 3) TTHERMOCOUPLE = -40NC to +750NC, TA = -40NC to +125NC (Note 3) TTHERMOCOUPLE = -200NC to +700NC, TA = -20NC to +85NC (Note 3) TTHERMOCOUPLE = +700NC to +1300NC, TA = -20NC to +85NC (Note 3) TTHERMOCOUPLE = -200NC to +1300NC, TA = -40NC to +125NC (Note 3) TTHERMOCOUPLE = -250NC to +400NC, TA = -20NC to +85NC (Note 3) TTHERMOCOUPLE = -250NC to +400NC, TA = -40NC to +125NC (Note 3) TTHERMOCOUPLE = -40NC to +700NC, TA = -20NC to +85NC (Note 3) TTHERMOCOUPLE = +700NC to +900NC, TA = -20NC to +85NC (Note 3) TTHERMOCOUPLE = -40NC to +900NC, TA = -40NC to +125NC (Note 3) MIN -2 -4 -6 -2 -4 -2 -4 -6 -2 -4 -2 -3 -5 0.25 TA = -20NC to +85NC (Note 3) TA = -40NC to +125NC (Note 3) TA = -40NC to +125NC tCONV tCONV_PU (Note 5) -2 -3 0.0625 +2 +3 TYP MAX +2 +4 +6 +2 NC +4 +2 +4 +6 +2 NC +4 +2 +3 +5 NC NC NC NC NC NC UNITS
MAX31855J Thermocouple Temperature Gain and Offset Error (57.953FV/NC nominal sensitivity) (Note 4)
MAX31855N Thermocouple Temperature Gain and Offset Error (36.256FV/NC nominal sensitivity) (Note 4)
MAX31855T Thermocouple Temperature Gain and Offset Error (52.18FV/NC nominal sensitivity) (Note 4)
MAX31855E Thermocouple Temperature Gain and Offset Error (76.373FV/NC nominal sensitivity) (Note 4) Thermocouple Temperature Data Resolution Internal Cold-Junction Temperature Error Cold-Junction Temperature Data Resolution Temperature Conversion Time (Thermocouple, Cold Junction, Fault Detection) Thermocouple Conversion Power-Up Time
70
100
ms
(Note 6)
200
ms
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*The parametric values (min, typ, max limits) shown in the Electrical Characteristics table supersede values quoted elsewhere in this data sheet.
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
SERIAL-INTERFACE TIMING CHARACTERISTICS
(See Figure 1 and Figure 2.) PARAMETER Input Leakage Current Input Capacitance 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 CS Fall to Output Enable CS Rise to Output Disable SCK Fall to Output Data Valid CS Inactive Time Note 2: Note 3: Note 4: Note 5: tDV tTR tDO (Note 3) 200 tCSS 100 100 100 40 40 SYMBOL ILEAK CIN fSCL tCH tCL 100 100 200 (Note 7) CONDITIONS MIN -1 8 5 TYP MAX +1 UNITS A pF MHz ns ns ns ns ns ns ns ns ns
All voltages are referenced to GND. Currents entering the IC are specified positive, and currents exiting the IC are negative. Guaranteed by design; not production tested. Not including cold-junction temperature error or thermocouple nonlinearity. Specification is 100% tested at TA = +25NC. Specification limits over temperature (TA = TMIN to TMAX) are guaranteed by design and characterization; not production tested. Note 6: Because the thermocouple temperature conversions begin at VPOR, depending on VCC slew rates, the first thermocouple temperature conversion may not produce an accurate result. Therefore, the tCONV_PU specification is required after VCC is greater than VCCMIN to guarantee a valid thermocouple temperature conversion result. Note 7: For all pins except T+ and T- (see the Thermocouple Input Bias Current parameter in the DC Electrical Characteristics table).
Maxim Integrated Products 4
*The parametric values (min, typ, max limits) shown in the Electrical Characteristics table supersede values quoted elsewhere in this data sheet.
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
Serial-Interface Diagrams
CS
SCK
SO
D31
D8
D7
D6
D5
D4
D3
D2
D1
D0
Figure 1. Serial-Interface Protocol
tCSS CS tCH SCK tDV SO D31 D3 D2 D1 D0 tDO tTR tCL
Figure 2. Serial-Interface Timing
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MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
Typical Operating Characteristics
(VCC = +3.3V, TA = +25NC, unless otherwise noted.)
SUPPLY CURRENT vs. TEMPERATURE
MAX31855 toc01
INTERNAL TEMPERATURE SENSOR ACCURACY
0.6 MEASUREMENT ERROR (C) 0.5 0.4 0.3 0.2 0.1 0 -0.1
VCC = 3.3V
1.2 SUPPLY CURRENT (mA) 1.0 0.8 0.6 0.4 0.2 0
VCC = 3.6V
VCC = 3.3V VCC = 3.0V
NOTE: THIS DATA WAS TAKEN IN PRECISION BATH SO HIGH TEMPERATURE LIMIT IS 90C
-40 -20
0
20
40
60
80
100 120
-0.2 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
TEMPERATURE (C)
ADC ACCURACY vs. ADC INPUT VOLTAGE ACROSS TEMPERATURE
MAX31855 toc03
ADC ACCURACY vs. ADC INPUT VOLTAGE ACROSS VCC
-0.1 -0.2 ADC ACCURACY (C) -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0
0.2 0.1 ADC ACCURACY (C) 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 0
AT -40C
VCC = 3.0V
AT +85C
VCC = 3.3V
AT +25C
VCC = 3.6V
VCC = 3.3V
20 40 60 ADC INPUT VOLTAGE (mV)
INTERNAL TEMPERATURE = +25C
0 20 40 60 ADC INPUT VOLTAGE (mV)
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MAX31855 toc04
0.3
0
MAX31855 toc02
1.4
0.7
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
Pin Configuration
PIN
TOP VIEW +
GND 1 T- 2 T+ 3 MAX31855 8 DNC 7 SO 6 CS
Pin Description
NAME GND TT+ VCC SCK CS SO DNC Ground Thermocouple Input. See Table 1. Do not connect to GND. Thermocouple Input. See Table 1. Power-Supply Voltage Serial-Clock Input Active-Low Chip Select. Set CS low to enable the serial interface. Serial-Data Output Do Not Connect FUNCTION 1 2 3 4 5 6 7 8
VCC 4
5 SCK
SO
Block Diagram
VCC S5 VCC SCK SO CS
MAX31855
COLD-JUNCTION COMPENSATION
DIGITAL CONTROL
T+ TS1 S2 FAULT DETECTION
S4 ADC
GND S3 REFERENCE VOLTAGE
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MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
Detailed Description
The MAX31855 is a sophisticated thermocouple-todigital converter with a built-in 14-bit analog-to-digital converter (ADC). The device also contains cold-junction compensation sensing and correction, a digital controller, an SPI-compatible interface, and associated control logic. The device is designed to work in conjunction with an external microcontroller (FC) in thermostatic, processcontrol, or monitoring applications. The device is available in several versions, each optimized and trimmed for a specific thermocouple type (K, J, N, T, or E; contact the factory for S and R types). The thermocouple type is indicated in the suffix of the part number (e.g., MAX31855K). See the Ordering Information table for all options. The device includes signal-conditioning hardware to convert the thermocouple's signal into a voltage compatible with the input channels of the ADC. The T+ and T- inputs connect to internal circuitry that reduces the introduction of noise errors from the thermocouple wires. Before converting the thermoelectric voltages into equivalent temperature values, it is necessary to compensate for the difference between the thermocouple coldjunction side (device ambient temperature) and a 0NC virtual reference. For a K type thermocouple, the voltage changes by about 41FV/NC, which approximates the thermocouple characteristic with the following linear equation: VOUT = (41.276FV/NC) x (TR - TAMB) where VOUT is the thermocouple output voltage (FV), TR is the temperature of the remote thermocouple junction (NC), and TAMB is the temperature of the device (NC). Other thermocouple types use a similar straight-line approximation but with different gain terms. Note that the MAX31855 assumes a linear relationship between temperature and voltage. Because all thermocouples exhibit some level of nonlinearity, apply appropriate correction to the device's output data. The function of the thermocouple is to sense a difference in temperature between two ends of the thermocouple wires. The thermocouple's "hot" junction can be read across the operating temperature range (Table 1). The reference junction, or "cold" end (which should be at
Temperature Conversion
Cold-Junction Compensation
Table 1. Thermocouple Wire Connections and Nominal Sensitivities
TYPE T- WIRE T+ WIRE TEMP RANGE (C) -200 to +1350 -40 to +750 -200 to + 1300 +50 to +1600 -250 to +400 -40 to +900 -50 to +1770 SENSITIVITY (V/C) 41.276 (0NC to +1000NC) 57.953 (0NC to +750NC) 36.256 (0NC to +1000NC) 9.587 (0NC to +1000NC) 52.18 (0NC to +400NC) 76.373 (0NC to +1000NC) 10.506 (0NC to +1000NC) COLD-JUNCTION SENSITIVITY (V/C) (0NC TO +70NC) 40.73 52.136 27.171 6.181 41.56 44.123 6.158
K J N S* T E R*
Alumel Constantan Nisil Platinum/ Rhodium Constantan Constantan Platinum/ Rhodium
Chromel Iron Nicrosil Platinum Copper Chromel Platinum
*Contact factory.
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MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
the same temperature as the board on which the device is mounted) can range from -55NC to +125NC. While the temperature at the cold end fluctuates, the device continues to accurately sense the temperature difference at the opposite end. The device senses and corrects for the changes in the reference junction temperature with cold-junction compensation. It does this by first measuring its internal die temperature, which should be held at the same temperature as the reference junction. It then measures the voltage from the thermocouple's output at the reference junction and converts this to the noncompensated thermocouple temperature value. This value is then added to the device's die temperature to calculate the thermocouple's "hot junction" temperature. Note that the "hot junction" temperature can be lower than the cold junction (or reference junction) temperature. Optimal performance from the device is achieved when the thermocouple cold junction and the device are at the same temperature. Avoid placing heat-generating devices or components near the MAX31855 because this could produce cold-junction-related errors. During the conversion time, tCONV, three functions are performed: the temperature conversion of the internal cold-junction temperature, the temperature conversion of the external thermocouple, and the detection of thermocouple faults. When executing the temperature conversion for the internal cold-junction compensation circuit, the connection to signal from the external thermocouple is opened (switch S4) and the connection to the cold-junction compensation circuit is closed (switch S5). The internal T- reference to ground is still maintained (switch S3 is closed) and the connections to the fault-detection circuit are open (switches S1 and S2). When executing the temperature conversion of the external thermocouple, the connections to the internal fault-detection circuit are opened (switches S1 and S2 in the Block Diagram) and the switch connecting the coldjunction compensation circuit is opened (switch S5). The internal ground reference connection (switch S3) and the connection to the ADC (switch S4) are closed. This allows the ADC to process the voltage detected across the T+ and T- terminals. During fault detection, the connections from the external thermocouple and cold-junction compensation circuit to the ADC are opened (switches S4 and S5). The internal ground reference on T- is also opened (switch S3). The connections to the internal fault-detection circuit are closed (switch S1 and S2). The fault-detection circuit tests for shorted connections to VCC or GND on the T+ and T- inputs, as well as looking for an open thermocouple condition. Bits D0, D1, and D2 of the output data are normally low. Bit D2 goes high to indicate a thermocouple short to VCC, bit D1 goes high to indicate a thermocouple short to GND, and bit D0 goes high to indicate a thermocouple open circuit. If any of these conditions exists, bit D16 of the SO output data, which is normally low, also goes high to indicate that a fault has occurred.
Serial Interface
The Typical Application Circuit shows the device interfaced with a microcontroller. In this example, the device processes the reading from the thermocouple and transmits the data through a serial interface. Drive CS low and apply a clock signal at SCK to read the results at SO. Conversions are always being performed in the background. The fault and temperature data are only be updated when CS is high. Drive CS low to output the first bit on the SO pin. A complete serial-interface read of the cold-junction compensated thermocouple temperature requires 14 clock cycles. Thirty-two clock cycles are required to read both the thermocouple and reference junction temperatures (Table 2 and Table 3.) Read the output bits on the falling edge of the clock. The first bit, D31, is the thermocouple temperature sign bit. Bits D[30:18] contain the converted temperature in the order of MSB to LSB. Bit D16 is normally low and goes high when the thermocouple input is open or shorted to GND or VCC. The reference junction temperature data begins with D15. CS can be taken high at any point while clocking out conversion data. Figure 1 and Figure 2 show the serial-interface timing and order. Table 2 and Table 3 show the SO output bit weights and functions.
Conversion Functions
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MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
Table 2. Memory Map--Bit Weights and Functions
14-BIT THERMOCOUPLE TEMPERATURE DATA BIT D31 D30 ... D18 RES D17 FAULT BIT D16 D15 D14 MSB Sign 26 (64NC) ... 12-BIT INTERNAL TEMPERATURE DATA ... D4 RES D3 SCV BIT D2 1= LSB 2-4 (0.0625NC) Reserved Short to VCC SCG BIT D1 1= Short to GND OC BIT D0 1= Open Circuit
VALUE
Sign
MSB 210 (1024NC)
...
LSB 2-2 (0.25NC)
Reserved
1= Fault
Table 3. Memory Map--Descriptions
BIT D[31:18] D17 D16 D[15:4] D3 D2 D1 D0 NAME 14-Bit Thermocouple Temperature Data Reserved Fault 12-Bit Internal Temperature Data Reserved SCV Fault SCG Fault OC Fault DESCRIPTION These bits contain the signed 14-bit thermocouple temperature value. See Table 4. This bit always reads 0. This bit reads at 1 when any of the SCV, SCG, or OC faults are active. Default value is 0. These bits contain the signed 12-bit value of the reference junction temperature. See Table 5. This bit always reads 0. This bit is a 1 when the thermocouple is short-circuited to VCC. Default value is 0. This bit is a 1 when the thermocouple is short-circuited to GND. Default value is 0. This bit is a 1 when the thermocouple is open (no connections). Default value is 0.
Table 4. Thermocouple Temperature Data Format
TEMPERATURE (NC) +1600.00 +1000.00 +100.75 +25.00 0.00 -0.25 -1.00 -250.00 DIGITAL OUTPUT (D[31:18]) 0110 0100 0000 00 0011 1110 1000 00 0000 0110 0100 11 0000 0001 1001 00 0000 0000 0000 00 1111 1111 1111 11 1111 1111 1111 00 1111 0000 0110 00
Table 5. Reference Junction Temperature Data Format
TEMPERATURE (NC) +127.0000 +100.5625 +25.0000 0.0000 -0.0625 -1.0000 -20.0000 -55.0000 DIGITAL OUTPUT (D[15:4]) 0111 1111 0000 0110 0100 1001 0001 1001 0000 0000 0000 0000 1111 1111 1111 1111 1111 0000 1110 1100 0000 1100 1001 0000
Note: The practical temperature ranges vary with the thermocouple type.
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MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
Applications Information
Because of the small signal levels involved, thermocouple temperature measurement is susceptible to powersupply coupled noise. The effects of power-supply noise can be minimized by placing a 0.1FF ceramic bypass capacitor close to the VCC pin of the device and to GND. The input amplifier is a low-noise amplifier designed to enable high-precision input sensing. Keep the thermocouple and connecting wires away from electrical noise sources. Self-heating degrades the device's temperature measurement accuracy in some applications. The magnitude of the temperature errors depends on the thermal conductivity of the device package, the mounting technique, and the effects of airflow. Use a large ground plane to improve the device's temperature measurement accuracy. The thermocouple system's accuracy can also be improved by following these precautions: * Usethelargestwirepossiblethatdoesnotshuntheat away from the measurement area.
Noise Considerations
* If a small wire is required, use it only in the region of the measurement, and use extension wire for the region with no temperature gradient. * Avoid mechanical stress and vibration, which could strain the wires. * When using long thermocouple wires, use a twisted pair extension wire. * Avoidsteeptemperaturegradients. * Try to use the thermocouple wire well within its temperature rating. * Usethepropersheathingmaterialinhostileenvironments to protect the thermocouple wire. * Useextensionwireonlyatlowtemperaturesandonly in regions of small gradients. * Keepaneventlogandacontinuousrecordofthermocouple resistance.
Thermal Considerations
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MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
Ordering Information
PART MAX31855KASA+ MAX31855KASA+T MAX31855JASA+ MAX31855JASA+T MAX31855NASA+ MAX31855NASA+T MAX31855SASA+* MAX31855SASA+T* MAX31855TASA+ MAX31855TASA+T MAX31855EASA+ MAX31855EASA+T MAX31855RASA+* MAX31855RASA+T* THERMOCOUPLE TYPE K K J J N N S S T T E E R R MEASURED TEMP RANGE -200NC to +1350NC -200NC to +1350NC -40NC to +750NC -40NC to +750NC -200NC to + 1300NC -200NC to + 1300NC +50NC to +1600NC +50NC to +1600NC -250NC to +400NC -250NC to +400NC -40NC to +900NC -40NC to +900NC -50NC to +1770NC -50NC to +1770NC PIN-PACKAGE 8 SO 8 SO 8 SO 8 SO 8 SO 8 SO 8 SO 8 SO 8 SO 8 SO 8 SO 8 SO 8 SO 8 SO
Note: All devices are specified over the -40C to +125C operating temperature range. +Denotes a lead(Pb)-free/RoHS-compliant package. T = Tape and reel. *Future product--contact factory for availability.
Package Information
For the latest package outline information and land patterns (footprints), go to www.maxim-ic.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. PACKAGE TYPE 8 SO PACKAGE CODE S8+4 OUTLINE NO. 21-0041 LAND PATTERN NO. 90-0096
Maxim Integrated Products 12
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter
Revision History
REVISION NUMBER 0 REVISION DATE 3/11 Initial release DESCRIPTION PAGES CHANGED --
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
(c)
13
2011 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.

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