Click here to purchase the entire book in PDF format. Common Mode Rejection Ratio (CMRR) In theory, if we send exactly the same signal to both input legs of an op amp and look at the output, we should see a constant level of 0 V. This is because the op amp is subtracting the signal from itself and giving you the result (0 V) multiplied by some gain. In practice, however, you won't see a constant 0 V at the output - you'll see an attenuated version of the signal that you're sending into the amplifier's inputs. The ratio between the level of the input signal and the level of the resulting output is a measurement of how much the op amp is able to reject a common mode signal (in other words, a signal which is common to both input legs). This is particularly useful (as we'll see later) in rejecting noise and other unwanted signals on a transmission line. The higher this number the better the rejection, and you should see values in the area of about 100 dB. Be careful though - op amps have very different CMRR's for different frequencies (lower frequencies reject better) so take a look at what frequency you're talking about when you're looking at CMRR values.