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INA131 output problem

Dean Gouramanis
Prodigy 20 points
Other Parts Discussed in Thread: INA131, OPA333, OPA378, INA333

I am experimenting with an INA131 amp for use with a thermocouple.

 I have it set up on a breadboard as shown in the datasheet with with both inputs tied to ground through 100k resistors. I also tried a voltage divider circuit to set the input to 150mv.

It appears that regardless of the input the output remains at 650mv. I thought it might have been a faulty IC so i swapped it with a new one but nothing changed. Does anyone know what the problem might be? Thank you.

V+ = 5.7V

  • 0
    TI__Expert 7185 points

    Hi Dean,

    The issues that you are experiencing are due to the common mode limitations of the INA131 from connecting the resistors to ground.  See the following from the data sheet specification table:

    Notice that the minimum guaranteed compliance from a +/-15V rail is +/-11V, meaning that the common mode voltage needs 4V from either rail to function linearly.  You can fix this issue either by adding a negative supply that is equivalent to your positive supply or by raising your positive supply from 5.7V to 10V and pulling the resistors up to V+/2 = 5V.  This will keep the common mode voltage > 4V off ground and >4V from the positive supply.

    I hope this helps.

    Matt

  • 0 in reply to Matthew Hann
    Prodigy 20 points

    Can you recommend an amplifier that will operate on five volts? The maximum difference in input voltage will be 45mv. Is this possible or is there something im missing? I was planning on tieing the negative of the thermocouple to ground (along with Vin-) and the positive to Vin+.

  • 0 in reply to Dean Gouramanis
    Guru 21975 points

    Dean,

    You have made reference to the INA131. If you need an instrumentation amplifier, consider the INA333. I'm not sure of your exact configuration but if you are connecting one input to ground, perhaps you can use an op amp. Consider the OPA333 or OPA378.

    Regards, Bruce.