Precision Current Loop Thermocouple Transmitter

Current loop analog data transmission is used in industrial environment because of its robustness (good noise immunity), while providing wiring fault detection (short and broken wiring) with a simple mechanism by detecting out range current level. In 4-20mA current loop standard, the valid signal should be in the range of 4 to 20mA. Zero ampere current would indicate broken wiring, and higher than 20 mA indicate shorts.

Thermocouple is one of the popular temperature sensor in industrial application, a great choice for durability and stability. Excerpted from the application note:

The maximum drift specification for the preamp is 0.2 μV/°C. For this particular circuit, an equal drift component would result for 0.004%/°C on the reference, 0.001%/°C mismatch on the bridged-feedback resistors (R2–R4) or 3 μV/°C on the op amp offset voltage. The op amp drift might be desensitized by raising the preamp gain (lowering R7–R9), but this would require raising the output voltage of the reference regulator and the minimum terminal voltage.

[Circuit’s schematic diagram source: National Semiconductor Application Note]