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IN322 Gain Setting

Joe57656
Prodigy 40 points
Other Parts Discussed in Thread: INA322, TINA-TI

I am using an INA322 Instrumentation Amplifier set up for gain of 5, (R2 short and R1 open). Sounds simple, and the amp works fine, except the gain is 10, not 5. Can't figure out why. I have it set up with single 5V supply, REF pin is not used. Sound familiar to anyone?

  • 0
    TI__Guru 62935 points

    Hi Joe,

    Is it possible that you are driving the INA322 with a differential signal between the inputs?  If so, then you would see an output amplitude that is (2 * (INA322Gain)) larger than either of the individual input amplitudes.

    Also, the REF pin needs to be connected to a low-impedance source to set the "Zero-Value" for the output.

    Please see attached schematics and .PDF file in the .ZIP file that show the difference between using a differential and single-ended input.  The schematics can be opened and run using TI's "TINA-TI" SPICE simulator that is available on the TI website.

    Thank you,
    Collin Wells
    Precision Analog
    Linear Applications

    INA322 Differential vs Single-Ended Input.zip
  • 0 in reply to Collin Wells
    Prodigy 40 points

    Collin,

    Thanks for the response. I am using it differentially, so that is the cause. Not a problem, just need to scale the input differently. Regarding the REF pin, if no zero adjustment is required can it be left floating? I read the data sheet as using the REF input being optional. Maybe I am not clear on this.

  • 0 in reply to Joe57656
    TI__Guru 62935 points

    Hi Joe,

    In the “Reference” section on page 10 of the datasheet it states that the “REF” pin needs to be connected to a low-impedance source.  If you leave it floating, the first stage amplifier will work as a voltage follower (Gain = 1V/V) opposed to having a non-inverting gain of 1.25V/V (1.94dB).  The second stage will maintain its non-inverting gain of 4V/V.  This will cause the total gain of the INA322 to change from 5V/V (1.25 * 4) to 4V/V (1 * 4).  You can see this behavior easily if you use the INA322 model and run simulations with and without the reference connection.

    Regards,
    Collin Wells
    Precision Analog
    Linear Applications