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INA2332: Output does not match Vref when differential input is 0

Michael Chiasson
Prodigy 20 points
Part Number: INA2332
Other Parts Discussed in Thread: INA332

Hello,

I'm seeing a strange thing when trying to use one circuit of an INA2332 to measure the differential voltage generated by a thermocouple.  If I replace the thermocouple with a short wire to simulate a differential voltage of 0V, I do not see my reference voltage at the output of the INA.  Instead, I see about 157mV lower than expected (I expect 0.5V, I see 0.343V).

Here is my full circuit:

You can see that I bias the negative terminal of my thermocouple using the same voltage as my reference voltage (0.5V). 

Here is the circuit that supplies my reference voltage, also known in my schematic as Vbias:

Can you see anything wrong with my circuit?  Is my input CMV or Vref set improperly?  I used TI's provided "Vcm vs Vout for Instrumentation Amplifiers" software to determine theses values (albeit for the INA332, not the INA2332).

Thanks for any help you can provide.

Michael

  • 0
    Prodigy 20 points

    I am realizing through research that this is likely the result of the input offset voltage of this particular part, which is considerably higher than the part I started the design with. 

    It sounds like I will need to either adjust my reference voltage on a per unit basis or choose a part with far less offset.

     

  • 0 in reply to Michael Chiasson
    TI__Prodigy 645 points

    Hi Michael,

    Thank you for reaching out. Input offset voltage in this range will noticeably affect your output, and you are right about considering a part with lower offset as this will definitely improve your accuracy. You are using a dual channel instrumentation amplifier so you can consider the INA2331 (see comparison table below) which offers much improved CMR, input offset voltage, and gain error. 

    Depending on your design flexibility there may be other options. Consider the INA333 (micropower, low offset, high CMR, single channel instrumentation amplifier) as well.

    To satisfy your accuracy requirements, it is important that you consider all other potential error sources. Here are some great resources to learn more:

    How do Instrumentation Amplifiers (INAs) fit into my design?

    Comprehensive Error Calculation for Instrumentation Amplifiers

    I hope that this helps answer your question.

     

    Best,

    Daniel