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This simple interface circuit that demonstrates the practicality of direct thermocouple connection to the LTC2400 using low output voltage thermocouples (a Type S thermocouple, as shown, produces a full-scale output voltage of 18mV). This circuit uses the LT1025, a micropower thermocouple cold junction compensator, to sense the temperature of the cold junction and introduce an offset voltage. This offset voltage is equal in magnitude, but opposite in polarity, to the voltage generated by the thermocouple cable/PC board termination.
This circuit can be easily adapted for use with other thermocouple types—simply replace the thermocouple (note the polarity of the wires!) and connect to the most appropriate output pin on the LT1025. To minimize any additional error into the measurement, the LT1025 must be mounted at the cold junction and the connections made to the LT1025, the thermocouple, and the LTC2400 must be isothermal.
Because of the LTC2400’s noise floor, this circuit is capable of resolving temperatures to within 0.25°C without averaging. Since the LTC2400 does not exhibit any easily discernible quantization effects, averaging multiple readings can significantly extend the resolution for slow varying processes.