The schematic diagram below show signal conditioning circuit for remote current loop temperature transmitter. This thermocouple temperature transmitter is loop powered, means it doesn’t need its own power supply since the supply is provided by the current loop receiver. Thermocouple is basically a junction of two different metallic material that measures the temperature difference.
This thermocouple signal conditioner circuit is designed for chromel-alumel thermocouple type. Thermocouple measures actually the temperature difference between hot and cold junction, not the absolute one. To be precisely indicate the absolute temperature, the cold junction of a thermocouple is traditionally stabilized using an ice bath. To avoid the complexity of this traditional cold junction stabilization method, an electronic cold junction compensation is included in this sophisticated thermocouple signal conditioner. A temperature sensor LM134 is used to sense the cold junction, producing current that is proportional to its temperature. This current flows through R2, and the value of R2 is selected to give voltage drift that is equal to the thermocouple voltage drift caused by cold junction temperature shift. [Circuit’s schematic diagram source: National Semiconductor Application Note]