What is the CMRR definition exactly?

[ilbs]

Hello,

First and foremost I would like to introduce myself... I am ilbs, and I am new to this forum!

I have seen many definitions in many articles throughout the Internet... here are some of the definitions I found:

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 ability of a differential amplifier to not pass (reject) the portion of the signal common to both the + and - inputs.

and so on....

But my question is, for example, for a given differential op amp circuit, if we have a CMMR of:

CMRR = Ad./Acm
CMRR = 2/1

Can we say that in this case the CMRR:

"is the ability of rejecting 1 volt of common voltage for every 2 volts of differential voltage"

I am looking for a definition of CMRR that involves a reference to its numbers in its ratio!

Thanks all
 

[mrflibble]

From the tutorial from AD:

"The op amp common-mode rejection ratio (CMRR) is the ratio of the common-mode gain to differential-mode gain."

Hopefully the tutorial clarifies things...

[tszaboo]

I think your confusion comes from the two different definition of CMRR. If you speak from a system with two inputs (ie. Instrumentation amplifier) the definition is clear.
If you speek of an opamp common mode rejection, the common mode voltage is (as it has only 1 input) the input voltage (which is referenced to ground) divided by two. So the number "two" comes from that.

[ilbs]

Thank you both for your help!