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INA322: Considerations when operating at 1000x gain

Matt Calvo
Genius 14425 points
Part Number: INA322

Hey Amplifier team,

I have a customer evaluating the INA322 for a new project development and we are hoping you could help clear up a couple of things.

We expect to run the device at 1000x gain. Are there any considerations that should be taken into account when running at the max gain?

On page-9 of the INA322 datasheet there is this diagram:

There are no values shown for configuring 1000x gain. Is there a limit to how small the value of R1 can be? Must it be a minimum of 10k? Or could we lower it to say 2k? If it must be a minimum of 10k, then R2 would need to be 2M for 1000x gain, but if it’s OK to lower R1 to 2k, then R2 could be 402K, for example. If you can provide guidance about any limitations or requirements for the acceptable range of values for R1 & R2 that would be helpful, the datasheet does not seem to discuss this.

Best regards,

Matt Calvo

  • +1
    TI__Guru 64680 points

    Matt,

    I do not see a reason why you couldn't use R1 of 2k for G=1000 BUT if you use it R2 needs to be 398k and not 402k;  G=5+5*(398k/2k) = 5+5*199=1000

    In case you use R1 of 10k, R2 needs to be 1.99M; G=5+5*(1.99M/10k) = 5+5*199 = 1000.

    Having said that the INA322 maximum input voltage offset, Vos, is +/-10mV (see below), which would require unit-to-unit precision calibration to remove it. Otherwise, Vos would get gained up by 1,000 to 10V resulting in the output collapsing on one of the rails even in the case of the differential input voltage of zero. Additionally, with its 60 nV/rt-Hz input noise spectral density, the total INA322 integrated output voltage noise would about 15mVpp.

    Also, please keep in mind that bandwidth is a function of gain.  Thus, in G=1000, the bandwidth of INA322 is only about 1kHz - see below.

  • 0
    Guru 140200 points

    Hi Matt,

    the INA322 is a micropower instrumentation amplifier being optimized not only for very low supply current but also for very low signal current. In order to reduce the signal current the internal feedback resistances are with 40k and 160k rather high. This results in a much higher noise level compared to instrumentation amplifiers drawing a much higher supply current. Marek mentioned an output noise voltage of INA322 of 15mVpp at a gain of 1000V/V. Because of this it's a bit contradictory to use a micropower instrumentation amplifier in a high gain application.

    If very low noise is a key requirement in your application, you may want to choose an instrumentation amplifier offering lower noise.

    Kai

  • 0 in reply to kai klaas69
    TI__Genius 14425 points

    Thank you both Kai and Marek for your extremely helpful feedback and advice! :)

    This will be very useful for my customer and I when evaluating the tradeoffs and ultimately selecting the right part for the application.

    Best regards,

    Matt Calvo

  • 0 in reply to Matt Calvo
    TI__Guru 64680 points

    No problem. Good luck!