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INA326

Richard Barthel
Intellectual 935 points
Other Parts Discussed in Thread: INA326

Hello all,

I'm working with a customer that is using the INA326 instrumentation amplifier and is simulating in TI-TINA. After working out the calculations in the datasheet, my customer tries those values in TINA and they do not match the expected result.

I'd appreciate if someone could take a look to see if I missed anything obvious.

According to the datasheet and Mathcad, these resistor values should give a gain of unity and an offset of 1.5V. However, with an input of -1.5V the output is 412.66mV (when it should be 0V). An input of +1.5V will yield only 2.57V (when it should give 3V). What is wrong with this picture?

Thanks!

-Richard

  • 0
    TI__Guru 62935 points

    Hi Richard,

    Without doing too much analysis, it looks like the signal on the "IN-" pin of the op-amp is at 3.79V.  The Power supply to the INA is 3.3V.  This will not work.

    The INA has a rail-to-rail input that extends +0.1V above the V+ supply.  In this application the input is about +0.4V higher than the supply, change the resistor divider ratio to produce smaller voltages and try again.  If this doesn't fix the issue you're experiencing try to include a few formulas or descriptions of what you're desired transfer function is and I will look deeper into the problem. 

    Regards,
    Collin Wells
    Precision Analog

  • 0 in reply to Collin Wells
    TI__Intellectual 935 points

    Collin,

    Thanks for the pro tip. I see how that could be a problem. However, even after lowering the voltage to below the input rail I get the following:

    The goal here is to have a gain of 1 and an offset of 1.5V. From page 13 of the datasheet (SBOS222D), I find that the gain for a voltage offset setup is G = 2 (R2 || R2`) / R1. I worked the math in Mathcad:

     I guess I'm just not using this device correctly. Why else is this deviating from the expected output?

    Thanks for your time Collin.

    -Richard

  • 0 in reply to Richard Barthel
    Genius 5430 points

    Richard;

    I may be wrong since I just glanced at your circuit, but with a voltage of +1.5V in the - input and 0V on the + input, aren't you asking the INA326 output to swing below ground?

  • 0 in reply to Neil Albaugh
    TI__Guru 62935 points

    Hi Richard,

    Neil is correct, you have your inputs reversed.

    .

    6712.Offset+Gain.TSC

     

    Regards,
    Collin Wells
    Precision Analog

  • 0 in reply to Collin Wells
    TI__Intellectual 935 points

    Thank you both for responding so quickly.

    So if I have a +1.5V voltage offset and I input -1.5V, I won't get 0V on the output?

    Does this equation not apply: Vout == (V+ - V-)*Aol + Vos

    Thanks for clearing this up for me.

    EDIT: Nevermind, we don't have a negative voltage to subtract from the offset. I see now -- thanks to both of you again.

    -Richard