Datasheet MAX6675 (Maxim) - 4

MAX6675 Cold-Junction-Compensated K-Thermocouple- to-Digital Converter (0°C to +1024°C) Where:
Pin Description
V
PIN NAME FUNCTION
OUT is the thermocouple output voltage (µV). 1 GND Ground TR is the temperature of the remote thermocouple junc- tion (°C). 2 T- Alumel Lead of Type-K Thermocouple. Should be connected to ground externally. TAMB is the ambient temperature (°C). 3 T+ Chromel Lead of Type-K Thermocouple
Cold-Junction Compensation
Positive Supply. Bypass with a 0.1µF The function of the thermocouple is to sense a differ- 4 VCC capacitor to GND. ence in temperature between two ends of the thermo- 5 SCK Serial Clock Input couple wires. The thermocouple’s hot junction can be read from 0°C to +1023.75°C. The cold end (ambi- 6 CS Chip Select. Set CS low to enable the serial interface. ent temperature of the board on which the MAX6675 is mounted) can only range from -20°C to +85°C. 7 SO Serial Data Output While the temperature at the cold end fluctuates, the 8 N.C. No Connection MAX6675 continues to accurately sense the tempera- ture difference at the opposite end.
Detailed Description
The MAX6675 senses and corrects for the changes in the ambient temperature with cold-junction compen- The MAX6675 is a sophisticated thermocouple-to-digi- sation. The device converts the ambient temperature tal converter with a built-in 12-bit analog-to-digital con- reading into a voltage using a temperature-sensing verter (ADC). The MAX6675 also contains cold-junction diode. To make the actual thermocouple temperature compensation sensing and correction, a digital con- measurement, the MAX6675 measures the voltage from troller, an SPI-compatible interface, and associated the thermocouple’s output and from the sensing diode. control logic. The device’s internal circuitry passes the diode’s volt- The MAX6675 is designed to work in conjunction with an age (sensing ambient temperature) and thermocouple external microcontroller (µC) or other intelligence in ther- voltage (sensing remote temperature minus ambient mostatic, process-control, or monitoring applications. temperature) to the conversion function stored in the
Temperature Conversion
ADC to calculate the thermocouple’s hot-junction tem- perature. The MAX6675 includes signal-conditioning hardware to convert the thermocouple’s signal into a voltage compat- Optimal performance from the MAX6675 is achieved ible with the input channels of the ADC. The T+and T- when the thermocouple cold junction and the MAX6675 inputs connect to internal circuitry that reduces the intro- are at the same temperature. Avoid placing heat-gen- duction of noise errors from the thermocouple wires. erating devices or components near the MAX6675 because this may produce cold-junction-related errors. Before converting the thermoelectric voltages into equivalent temperature values, it is necessary to com-
Digitization
pensate for the difference between the thermocouple The ADC adds the cold-junction diode measurement cold-junction side (MAX6675 ambient temperature) and with the amplified thermocouple voltage and reads out a 0°C virtual reference. For a type-K thermocouple, the the 12-bit result onto the SO pin. A sequence of all voltage changes by 41µV/°C, which approximates the zeros means the thermocouple reading is 0°C. A thermocouple characteristic with the following linear sequence of all ones means the thermocouple reading equation: is +1023.75°C. VOUT = (41µV / °C) 5 (TR - TAMB) www.maximintegrated.com Maxim Integrated │ 4