Hello, I am designing a differential thermocouple (no need for reference T) and I have decided to use two AD8421 amps in order to get a 0-5V output for my DAQ system. The differential input signal is expected to be around 41u-415u using a set of K-type thermocouples.

I have imported my circuit into TINA, using the SPICE model of the amplifiers, and the calculated gain is approximately 10% higher than the expected value, as calculated from the datasheet and the diamond plot tool (41.5mV for the first amp and 4.15V for the second, for a differential input of 415uV).

Is this result reasonable since the SPICE model includes the gain error, voltage offset error and bias current error or is there a more fundamental issue? This is my first time designing an electronic circuit so please forgive the naivety of my question. I have also attached a picture of the circuit diagram in TINA, also showing the output voltages from each amp for a differential input of 415uV .

Hi Nick,

I've checked the spice model of the AD8421 and its offset voltage is modeled to have 60 uV (max offset for A grade). This is the main contributor of the error that you are seeing: (410u + 60 uV) x (G = 100) = 47 mV.

As a side note, for computing the gain error, we usually do the two point system to eliminate the other error sources like input offset, output offset, etc.

It is kinda complicated to design for a signal conditioning for the thermocouple sensor that's why I would want you to check and read this very useful article that could help and guide you on your designing process. There are also some circuits available that you could adapt and implement on your applications

Here is the link: Two Ways to Measure Temperature: Analog Dialogue: Analog Devices

Let me know if you need anything else.

Best regards,

Emman