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# Common Mode Rejection

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#### D.M.

##### New Member
my understanding for CMRR is 20*log_10(Vcom/Vdif)

Vcom is just my common mode input voltage. I understand we choose a Vdif that will output the same number that the Vcom output. for example, if i had an amplifier with a gain of 10 and i apply a common mode V_in of 5 volts and the output was say 1mV... then Vdif would be the value of the differential input needed to achieve a 1mV output.. so if my gain was 10, Vdif would be 0.1 mV.. now my cmrr would be

20*log_10(5V/.0001V) = 93.97dB

now my questions are:
1)does frequency change my CMRR?
2) does input voltage at the common mode change my CMRR?

if my description is too confusing let me know, i been working on homework and labs for 12 hours now and am kind of tired..

can anyone help?

1) Yes
2) Normally not much until you reach the extremes of the CM range.

so in a report or something i should specify Vcom input and the frequency in which i tested it.

thanks sir

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