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Maximum common mode voltages

  1. Jun 12, 2016 #1
    • Member advised to use the homework template for posts in the homework sections of PF.
    Hi,
    I have this question.

    An instrumentation measuring system uses a differential amplifier having a CMRR of 120 dB and differential gain of 105. The maximum differential input signal is 60 μV. If the amplified noise voltage is specified to be not more than 1% of the maximum output voltage, calculate the maximum common mode voltage that can be present in the input to the amplifier.

    Just need to know if this is correct or I am on the right track.

    I have the formula

    CMRR=20log10(Adiff/Acm)

    Then

    120=20log10(10^5/Acm)

    Then

    120/20=log10(10^5/Acm)

    Then

    6=log10(10^5/Acm)

    Then taking anti logs

    1000000=10^5/Acm

    Transpose this into

    Acm=10^5/1000000=0.1volt

    Is this correct? as when I plug the 0.1 back into the original equation I do not get the original 120dB answer.

    Thanks.
     
  2. jcsd
  3. Jun 12, 2016 #2

    Tom.G

    User Avatar
    Science Advisor

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