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operational amplifier

akhilesh sati
Intellectual 950 points

Other Parts Discussed in Thread: OPA140

I am having a doubt regarding CMRR of Op-amp i.e. When we see the data sheet of any op-amp its CMRR is given. Since CMRR is Ad/Ac . Ad i.e. differential gain(which depends on closed loop gain)) would be keep on changing,  consequently CMRR would also be  change. so how  manufacturer  decides the the value of CMRR to write in datasheet.

  • 0
    TI__Genius 12375 points

    Hello,

    The CMRR value given in the datasheet is generally the maximum value of CMRR which occurs at low-frequencies. You are correct that CMRR is frequency dependent which is why most manufacturers include a CMRR vs frequency curve in their datasheets like this one which I pulled from the OPA140 datasheet:

  • 0 in reply to John Caldwell
    Intellectual 950 points

    Dear sir

                   CMRR also depands on differential mode gain. then how it differential mode gain plays role in deciding CMRR apart from frequency ?

  • 0
    TI__Guru**** 174746 points

    Akhilesh,

    CMRR is op-amp input referenced.

    For example if op-amp CMRR is 80dB and gain is set to 20dB then the input common mode has a gain of -80dB + 20dB = -60dB.
    So the output only has a 60dB rejection on a op-amp that has 80dB CMRR rating.

    However, The differential gain is 20dB per application. So the 80dB CMRR still apllies to signal to 'noise' ratio. Where 'noise' is the unwanted common mode signal. Desired signal is +20dB and unwanted signal is -60dB so S/N ratio is 80dB

    Regards,
    Ron Michallick