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Homework Help: Determining CMRR for an op amp with two electrode inputs

  1. Sep 25, 2007 #1
    1. The problem statement, all variables and given/known data
    I have an op amp with two electrode inputs. Each input has 5V of common 60 Hz noise. The size of the signal I want to measure is 9 mV. Signal to noise ratio needs to be 10 to 1.

    2. Relevant equations

    I know that CMRR = 20 log (DMG/CMG), where DMG is differential mode gain and CMG is common mode gain.

    SNR = range of signal/range of noise

    3. The attempt at a solution

    I know that CMG is equal to the ratio of Vout to Vcommon. Is Vcommon 10 V since I have two 5V inputs? Or would it be the average (5V)? How do I get Vout???? Where does the frequency come into play?

    Also, SNR = 10 and range/size of signal is 9 mV = .009 V, so range of noise = .0009 V.

    What is the equation for DMG?

    Can anyone direct me to a good internet reference that explains these ideas? The class I am in does not use a textbook.

  2. jcsd
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