Datasheet MAX6674 (Maxim) - 4

MAX6674 Cold-Junction-Compensated K-Thermocouple- to-Digital Converter (0°C to +128°C) For a type-K thermocouple, the voltage changes by
Pin Description
41µV/°C, which approximates the thermocouple charac-
PIN NAME FUNCTION
teristic with the following linear equation: 1 GND Ground VOUT = (41µV/°C) 5 (TR - TAMB) Alumel Lead of Type-K Thermocouple. where: 2 T- Should be connected to ground V externally. OUT is the thermocouple output voltage (µV). T 3 T+ Chromel Lead of Type-K Thermocouple R is the temperature of the remote point (°C). Positive Supply. Bypass with a 0.1µF TAMB is the ambient temperature (°C). 4 VCC capacitor to GND.
Cold-Junction Compensation
5 SCK Serial Clock Input The function of the thermocouple is to sense a difference in temperature between two ends. The thermocouple’s 6 CS Chip Select. Set CS low to enable the serial interface. hot junction can be read from 0°C to +127.875°C. The 7 S0 Serial Data Output cold end (ambient temperature of the board on which the MAX6674 is mounted) can only range from -20°C to 8 N.C. No Connection +85°C. While the temperature at the cold end fluctuates, the MAX6674 continues to accurately sense the tempera-
Detailed Description
ture difference at the opposite end. The MAX6674 is a sophisticated thermocouple-to-digi- The MAX6674 senses and corrects for the changes in tal converter with a built-in 10-bit analog-to-digital con- the ambient temperature with cold-junction compensa- verter (ADC). The device also contains cold-junction tion. The device converts the ambient temperature compensation sensing and correction, a digital con- reading into a voltage using a temperature-sensing troller, an SPI-compatible interface, and associated diode. To make the actual thermocouple temperature control logic. measurement, the MAX6674 measures the voltage from the thermocouple’s output and from the sensing diode. The MAX6674 is designed to work in conjunction with The device’s internal circuitry passes the diode’s volt- an external microcontroller (µC) or other intelligence age (sensing ambient temperature) and thermocouple in thermostatic, process-control, or monitoring applica- voltage (sensing remote temperature minus ambient tions. The µC is typically a power-management or key- temperature) to the conversion function stored in the board controller, generating SPI serial commands by ADC to calculate the thermocouple’s hot-junction tem- “bit-banging” general-purpose input-output (GPIO) pins perature. or through a dedicated SPI interface block. Optimal performance from the MAX6674 is achieved
Temperature Conversion
when the thermocouple cold junction and the device The MAX6674 includes signal conditioning hardware to are at the same temperature. Avoid placing heat-gener- convert the thermocouple’s signal into a voltage that is ating devices or components near the MAX6674 compatible with the input channels of the ADC. The T+ because this may produce cold-junction-related errors. and T-inputs connect to internal circuitry that reduces the introduction of noise errors from the thermocouple wires.
Digitization
The ADC adds the cold-junction diode measurement Before converting the thermoelectric voltages into with the amplified thermocouple voltage and reads out equivalent temperature values, it is necessary to com- the 10-bit sequence onto the S0 pin. A sequence of pensate for the difference between the thermocouple all zeros means the thermocouple reading is 0°C. A cold-junction side (MAX6674 ambient temperature) and sequence of all ones means the thermocouple reading is a 0°C virtual reference. +127.875°C. www.maximintegrated.com Maxim Integrated │ 4