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CMMR simulation in Pspice for Opamp ICs

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themaccabee

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CMMR Defintion-Please help

Hi,

May be this is a stupid question,.. but i ve got a doubt with CMRR of an amplifier..
I came to read an equation saying CMRR is defined for an amplifier as
CMRR(dB)=20log((Gain*Vcm)/Vout)
gain = (differential) gain of amplifier ;VCM = common mode voltage present at the input
VOUT = output voltage resulting from the presence of common mode voltage at the input.

I was thinking about the CMRR as CMRR(dB)=20log(Ad/Ac)

Ad -Differential Mode gain
Ac-Common mode gain

Can somebody tell me how the first equation comes??

Thanks
 
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Are the VCM and VOUT in the first equation are ac signal or dc. If it's ac, the first one is equal to the next one.
 

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