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offset voltage error due to power supply voltages

Scot Lester
Expert 5640 points

I am trying to calculate the offset error (referenced to the input) due to common mode voltage on the power rails for a dual rail device.  I know it is simply the common mode voltage divided by the psrr which is stated in the datasheet.  However, what is the common mode voltage?  If the positive voltage is +15V and the negative is -14V is the common mode voltage 15-14=1V or is it 14V or some other value?

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    TI__Genius 13935 points
    Hi Scot,

    Sorry for the late reply. I am going to answer this in case someone does a search and hopefully it will help you as well.

    The baseline voltage for CMRR measurements is the midpoint of the power supplies, so it would be +0.5V in your case. If you are using a ground referenced signal then the common mode "shift" for your scenario is 0.5V.

    The actual common mode voltage is the average of the two input pins, which should be equal to the voltage measured at the non_inverting input.

    Regards,
    Loren