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CMRR formula gives wrong result!

  1. Sep 12, 2013 #1
    Hello,

    If you go to this link:

    http://en.wikipedia.org/wiki/Common-mode_rejection_ratio

    and scroll all the way down to the bottom where they show the:

    "Example: operational amplifiers"

    section.... we have:

    I get +/-222mv ????

    Am I doing the math wrong?
     
  2. jcsd
  3. Sep 12, 2013 #2

    berkeman

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    Staff: Mentor

    I get the 316mV number. Can you show how you are typing the numbers into your calculator?
     
  4. Sep 12, 2013 #3
    90/10
    then the result is multiplied by 10.

    then
    10/the result above

    hence:
    10/((90/20)10) = 0.222???

    and they get 316 micro volts?
     
  5. Sep 12, 2013 #4

    berkeman

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    Staff: Mentor

    I think there are several issues with the way you are trying to solve this. First remember that for voltage ratios, the equation is dB = 20log(ratio).

    So for this problem, you start with 90dB = 20log(10/x).

    To solve that equation, you divide both sides by 20, and then take 10^ for both sides.

    Can you take it from there...?
     
  6. Sep 13, 2013 #5
    Hi berkeman,

    You talking to a very slow guy here!!!
    so I try:

    I went back to my algebra notes of 20 years ago and saw the example:

    2 = log(base10)(100)

    then I tried doing the reverse:

    10^2 = 100

    So then I understood that its 10 to the power of (90/20)...

    So I did the same while following your instructions....

    90dB = 20log(10/x)

    90dB/20 = (20log(10/x))/20

    4.5dB = log(10/x)

    Antilog = 10^4.5 so:

    10^4.5 = 10/x

    which becomes:

    10/31622.8 = x

    316 uV


    Thankyou for your help!
     
    Last edited: Sep 13, 2013
  7. Sep 13, 2013 #6

    berkeman

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    Staff: Mentor

    Woot! Good job! :smile:
     
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