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INA121: Amplifiers forum

Joseph Kitching
Prodigy 70 points
Part Number: INA121

Hello:

I have a +15V/-15V power supply. I have read several threads indicating that an OpAmp would only see the supply voltage as delta 30V across the Supply+ and Supply-.  It would not be aware of a negative supply per se. However my signal voltage will indeed go negative.

Do I have to connect the Power Supply's (third) common wire in someway to the OpAmp in my circuit? I guessed maybe to the Ref pin of the OpAmp but I need to put a bias voltage on the Ref pin to get the Voutput of the Amp to be positive for the ADC I plan to use.

I am clearly confused on how to wire this all up.

  • +1
    Guru 140200 points

    Hi Joseph,

    there's no need to connect signal ground to the INA121, as with any other standard OPAmp. But the REF pin of INA121 must be connected to a suited potential. This can be 0V, if you want to reference the output signal to 0V, or (almost) any other potential you like.

    An example: If REF is connected to 1.5V and your differential input signal is 0V, then the output of INA121 is emitting 0V + 1.5V = 1.5V. All output signals are shifted by this 1.5V REF potential then. That's how a differential amplifier (A3 of INA121) is behaving.

    Kai

  • 0 in reply to Joseph Kitching
    TI__Genius 10850 points

    Kai is correct.

    Hi Joseph, Glad you got this resolved. We do have training collateral to help you understand IA theory & application. You can find the first video here. There are currently 5 videos and I'm working on releasing the 6th video in about a week. 

  • +1 in reply to Tamara M Alani
    Prodigy 70 points

    Great I will check these out!