Question

What is a CMRR (Common-Mode Rejection Ratio) in instrumentation amplifiers?

Answer 1

In instrumentation amplifiers, the Common-Mode Rejection Ratio (CMRR) is a measure of how well the amplifier can reject unwanted common-mode signals. Common-mode signals are signals that appear with the same voltage level and phase on both input terminals of the amplifier. These signals are typically noise, interference, or other undesirable common-mode voltages that may be present in the measurement system.

CMRR is a critical parameter for instrumentation amplifiers, as it indicates the amplifier's ability to amplify the differential signal (the difference between the two input terminals) while rejecting the common-mode signal.

The CMRR is usually expressed in decibels (dB) and is calculated as follows:

CMRR (dB) = 20 * log10(Adm / Acm)

where:

Adm is the differential-mode gain (amplification of the desired differential signal).
Acm is the common-mode gain (amplification of the unwanted common-mode signal).

Ideally, an instrumentation amplifier should have a very high CMRR, meaning that it effectively rejects common-mode signals while amplifying the differential signals accurately. A high CMRR is crucial in applications where the common-mode noise can significantly affect the measurement accuracy.

It's essential to consider the CMRR when selecting or designing an instrumentation amplifier for applications in noisy environments or when dealing with signals with significant common-mode interference. Manufacturers usually specify the CMRR value in the datasheets of instrumentation amplifiers.