Transmitter Precision Thermocouple Amplifier Circuit

This circuit is shows how a low drift preamplifier can be added to improve the measurement resolution of a thermocouple. The preamp is powered from the reference regulator, and bridge feedback is used to bias the preamp input within its common mode range. Cold junction compensation is provided with the offset voltage set into A1, it being directly proportional to absolute temperature. 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. Here is a schematic drawing:

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. In this application, the preamp is run at a lower voltage than standard parts are tested with, and the maximum supply current specified is high. However, there should be no problem with the voltage; and a lower, maximum supply current can be expected at the lower voltage. Even so, some testing may be in order.