CMRR stands for common mode rejection ratio. It is a
measure of the ability of a test instrument to reject
interference that is common to both of its measurement
input terminals. It is expressed in decibels and it is the
ratio of the actual or common signal level appearing on the
two input terminals together to the measured level.

CMRR whichis defined as the ability to reject the common
gain.it is also defined as the ratio between the
differential mode gain to the common mode gain.

The common-mode rejection ratio (CMRR) of a differential
amplifier measures the tendency of the device to reject
input signals common to both input leads. CMRR is often
important in reducing noise on transmission lines

CMMR is abbreviated as common mode rejection ratio.
it means cmmr of an amplifier tells the ablility to
amplify the differance signal more than the common signal
at the input of the two terminal of the amplifier.
so high value of cmmr is required ideally infinity. it
means the reduction of noise at the input.

CMMR=[AD/AC]
AD= differential gain of the amplifire.
AC= common mode gain of the amplifire.
IN decibel
CMMR(log)=20log[base10](ad/ac)

ideally the two input terminals must be identical but
practically it is often not possible but effort is put to
make them as identical as possible, that's where cmrr
comes into picture. its ratio of
differential gain of the amplifier
and
common mode gain of the amplifier.
if input are almost identical then common mode gain must be
almost zero, in other words cmrr is infinity....

CMRR means common mode rejection ratio is the ratio of
diffrential gain to the common mode gain.
CMRR = Ad/Acm
where Ad = diffrential gain
Acm= common mode gain
its ideal value is infinite.
For high gain of opamp it should be infinite.
CMRR is represent in decibel.
decibel = 20log(Ad/Acm)

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Sir, I hav applied for the post of sec. engg in RRB in
telecom-sigal domain. Can u suggest hows the question
pattern. Do I hav 2 follow technical books or general
aptitude books ?