Common Mode Rejection

[D.M.]

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?

 

[crutschow]

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

 

[D.M.]

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

thanks sir