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CMRR formula gives wrong result! 
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#1
Sep1213, 04:47 PM

P: 44

Hello,
If you go to this link: http://en.wikipedia.org/wiki/Common...ejection_ratio and scroll all the way down to the bottom where they show the: "Example: operational amplifiers" section.... we have: Am I doing the math wrong? 


#2
Sep1213, 06:23 PM

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P: 40,729




#3
Sep1213, 07:20 PM

P: 44

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? 


#4
Sep1213, 11:41 PM

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P: 40,729

CMRR formula gives wrong result!
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...? 


#5
Sep1313, 10:27 AM

P: 44

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! 


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