CMRR formula gives wrong result!


by simpComp
Tags: cmrr, formula, result
simpComp
simpComp is offline
#1
Sep12-13, 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:

So for example, an op-amp with 90dB CMRR operating with 10V of common-mode will have an output error of 316uV.
I get +/-222mv ????

Am I doing the math wrong?
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berkeman
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#2
Sep12-13, 06:23 PM
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Quote Quote by simpComp View Post
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:



I get +/-222mv ????

Am I doing the math wrong?
I get the 316mV number. Can you show how you are typing the numbers into your calculator?
simpComp
simpComp is offline
#3
Sep12-13, 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?

berkeman
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#4
Sep12-13, 11:41 PM
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CMRR formula gives wrong result!


Quote Quote by simpComp View Post
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?
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...?
simpComp
simpComp is offline
#5
Sep13-13, 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!
berkeman
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#6
Sep13-13, 11:07 AM
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Woot! Good job!


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