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INA2321 OP AMP Formula

Other Parts Discussed in Thread: INA321, INA2321

This datasheet for this part can also be found using INA321.

My problem is the following:

The formula for the datasheet basically says that the Op-Amp is NOT V(ref) dependent.  However, my results say otherwise.  We are using this op-amp to amplify the signal of a dual-bridge rotary sensor.  The bridges are out of phase.  According to the datasheet provided by our sensor, the peak input should be something like 40mV and we are measuring an the Op-Amp output of 3.2 with a Voltage Reference of 2.048V.  This is a gain of about 80.  However, this is where the discrepancies come in to play.  Our R2 is equal to 210K and our R1 is equal to 47K.  Based on the datasheet formula, our Gain should be something like 27.  Also, because it is a dual bridge and dual op amp, we are using two different voltage references.  The first being the already mentioned 2.048 and the second one being 1.8.  Now, if the device was not V(ref) dependent, which the datasheet suggests, should not the V(out) of both be the same.  In our case, the peaks and troughs are not the same.  The output of the lower Vref is slightly lower than that of the higher Vref.  This would then tell me that this is indeed Vref dependent.  Can someone please look at the datasheet and confirm the equations or perhaps let me know whether or not my gain equations are right or wrong.  Thank you.

  • Hi Jeff,

    Sorry you're having trouble with the INA2321.  Would you please share a schematic of your circuit so we can properly diagnose your system?  I'm not completely sure about your comments that the output voltage is not reference dependant.  The reference voltage will set the "Zero" output of the INA.  So if VREF = 2.048V, when (Vin+ - Vin) = 0V, the output will be 2.048V.  This is described in the "Reference" section of the datasheet that I've copied below.  The gain of the OPA will not change with the VREF voltage, but the voltage the output is referenced to will change. 

    It is possible to saturate the output of the internal amplifiers which you could be doing that would result in an incorrect output, once we see your schematic we should be able to determine if this is the effect or not.

    Collin Wells
    Precision Linear Applications

  • Based on the structure of the Op-Amp in the datasheet, the following formula it was I was able to derive:

    Vout  =  [5*Vin(+)]*[(R2/R1) + 1] + Vref


    Vout  =  [-5*Vin(-)]*[(R2/R1) + 1] + Vref

    Can anyone please confirm this?  Also, perhaps explain how the given formula in the datasheet is equal to this.  I must have missed something somewhere.

  • Yes, below is schematic the Op-Amp is being used in.

  • Hi Jeff

    I'm not sure why the image didn't come through that I tried to attach, but it was just the "Reference" section on page 11 of the datasheet. 

    Regarding your equations, there is only one equation for Vout based on the differential voltage between IN+ and IN-.  There are not two individual equations for IN+ and IN-.  The applications section and other areas specify the output voltage based on the gain of the INA321 as:

    Vo = (Vin+ - Vin-) * G

    Where G is defined to be:  G = 5 + 5*(Rf / Ri) 

    Vout is so expanding yields:  Vo = 5*(Vin+ - Vin-) + 5*(Rf / Ri)*(Vin+ - Vin-)

    When the reference voltage is used, it simply adds to this equation yielding the final equation of:

     Vo = (Vin+ - Vin-)*[5+ 5*(Rf / Ri)] + Vref

    Let me know if we can be of further assistance.  Your schematic looks appropriate. 

    Collin Wells
    Precision Linear Applications

  • Ah, right, sorry.  I had put into my equations the assumption that either Vin(+) or Vin(-) were 0.  But it looks like you were able to confirm.  And I went and read the reference part on page 11.  I had read that before, but I guess I didn't quite understand it.  Anyways, thank you for you help.  Greatly appreciated.