Datasheet

ManualsBrandsMAXIM INTEGRATED ManualsLinear ICsMAX31855KASA+

General Description

The MAX31855 performs cold-junction compensation

and digitizes the signal from a K-, J-, N-, T-, S-, R-, or

E-type thermocouple. The data is output in a signed

14-bit, SPI-compatible, read-only format. This converter

resolves temperatures to 0.25°C, allows readings as

high as +1800°C and as low as -270°C, and exhibits

thermocouple accuracy of ±2°C for temperatures ranging

from -200°C to +700°C for K-type thermocouples. For full

range accuracies and other thermocouple types, see the

Thermal Characteristics specifications.

Applications

● Industrial

● Appliances

● HVAC

Features

● Cold-Junction Compensation

● 14-Bit, 0.25°C Resolution

● Versions Available for K-, J-, N-, T-, S-, R-, and

E-Type Thermocouples (see Table 1)

● Simple SPI-Compatible Interface (Read-Only)

● Detects Thermocouple Shorts to GND or VCC

● Detects Open Thermocouple

Ordering Information appears at end of data sheet.

For related parts and recommended products to use with this part, refer

to www.maximintegrated.com/MAX31855.related.

19-5793; Rev 4; 11/14

GND

T+

T-

SCK

MICROCONTROLLER

MISO

SCK

0.1µF

MAX31855

MAX31855 Cold-Junction Compensated

Thermocouple-to-Digital Converter

Typical Application Circuit

Summary of content (13 pages)

PAGE 1
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-, S-, R-, or E-type thermocouple. The data is output in a signed 14-bit, SPI-compatible, read-only format. This converter resolves temperatures to 0.
PAGE 2
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 = +70°C) SO (derate 5.9mW/°C above +70°C).......................470.6mW ESD Protection (All Pins, Human Body Model)....................±2kV Operating Temperature Range......................... -40°C to +125°C Junction Temperature....
PAGE 3
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter Thermal Characteristics (3.0V ≤ VCC P 3.6V, TA = -40°C to +125°C, unless otherwise noted.) (Note 4) PARAMETER MAX31855K Thermocouple Temperature Gain and Offset Error (41.276µV/°C nominal sensitivity) (Note 4) MAX31855J Thermocouple Temperature Gain and Offset Error (57.953µV/°C nominal sensitivity) (Note 4) MAX31855N Thermocouple Temperature Gain and Offset Error (36.
PAGE 4
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter Thermal Characteristics (continued) (3.0V ≤ VCC P 3.6V, TA = -40°C to +125°C, unless otherwise noted.) (Note 4) PARAMETER SYMBOL CONDITIONS MIN Thermocouple Temperature Data Resolution TA = -20°C to +85°C (Note 3) TA = -40°C to +125°C (Note 3) Cold-Junction Temperature Data Resolution Thermocouple Conversion Power-Up Time MAX 0.
PAGE 5
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter Serial-Interface Diagrams CS SCK SO D31 D8 D7 D6 D5 D4 D2 D3 D1 D0 Figure 1. Serial-Interface Protocol tCSS CS tCH SCK tDV tCL tDO tTR SO D31 D3 D2 D1 D0 Figure 2. Serial-Interface Timing www.maximintegrated.
PAGE 6
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter Typical Operating Characteristics (VCC = +3.3V, TA = +25°C, unless otherwise noted.) 1.0 VCC = 3.3V 0.8 VCC = 3.0V 0.6 0.4 -40 -20 0 20 40 60 80 MAX31855 toc02 NOTE: THIS DATA WAS TAKEN IN PRECISION BATH SO HIGH TEMPERATURE LIMIT IS 90°C 0.3 0.2 0.1 0 -0.2 100 120 -40 -20 0 20 40 60 80 100 ADC ACCURACY vs. ADC INPUT VOLTAGE ACROSS TEMPERATURE ADC ACCURACY vs. ADC INPUT VOLTAGE ACROSS VCC 0 -0.1 AT +85°C -0.
PAGE 7
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter Pin Configuration Pin Description TOP VIEW GND 1 T- 2 T+ + 8 DNC 7 SO MAX31855 3 6 VCC 4 CS 5 SCK SO PIN NAME 1 GND FUNCTION 2 T- Thermocouple Input. See Table 1. Do not connect to GND. 3 T+ Thermocouple Input. See Table 1. 4 VCC Power-Supply Voltage 5 SCK Serial-Clock Input 6 CS Active-Low Chip Select. Set CS low to enable the serial interface.
PAGE 8
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter Detailed Description The MAX31855 is a sophisticated thermocouple-to-digital 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 SPIcompatible interface, and associated control logic.
PAGE 9
MAX31855 same temperature as the board on which the device is mounted) can range from -55°C to +125°C. 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.
PAGE 10
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter Table 2. Memory Map—Bit Weights and Functions 14-BIT THERMOCOUPLE TEMPERATURE DATA RES FAULT BIT 12-BIT INTERNAL TEMPERATURE DATA RES SCV BIT SCG BIT OC BIT BIT D31 D30 … D18 D17 D16 D15 D14 … D4 D3 D2 D1 D0 VALUE Sign MSB 210 (1024°C) … LSB 2-2 (0.25°C) Reserved 1= Fault Sign MSB 26 (64°C) … LSB 2-4 (0.0625°C) Reserved 1= Short to VCC 1= Short to GND 1= Open Circuit Table 3.
PAGE 11
MAX31855 Applications Information Noise Considerations 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 0.1µF 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.
PAGE 12
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter Ordering Information PART THERMOCOUPLE TYPE MEASURED TEMP RANGE PIN-PACKAGE MAX31855KASA+ K -200°C to +1350°C 8 SO MAX31855KASA+T K -200°C to +1350°C 8 SO MAX31855JASA+ J -40°C to +750°C 8 SO MAX31855JASA+T J -40°C to +750°C 8 SO MAX31855NASA+ N -200°C to + 1300°C 8 SO MAX31855NASA+T N -200°C to + 1300°C 8 SO MAX31855SASA+ S -50°C to +1600°C 8 SO MAX31855SASA+T S -50°C to +1600°C 8 SO MAX31855TASA+ T
PAGE 13
MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter Revision History REVISION NUMBER REVISION DATE PAGES CHANGED 0 3/11 Initial release 1 11/11 Corrected ESD protection value; added “S” and “R” type specifications 2 2/12 Corrected the thermocouple temperature conditions in the Thermal Characteristics table and Table 1; added clarification to the Serial Interface section to help users better understand how to communicate with the device; added a recommendation to add a 10nF diff