Question

Define common-mode rejection ratio (CMRR) in op-amps.

Answer 1

Common-Mode Rejection Ratio (CMRR) is a crucial parameter in operational amplifiers (op-amps) and other differential amplifier circuits. It quantifies the ability of an op-amp to reject or ignore common-mode signals that are present on both its inverting and non-inverting inputs. Common-mode signals are those that appear in phase and with the same amplitude on both inputs.

CMRR is typically expressed in decibels (dB) and is calculated using the following formula:





=
20
⋅
log
⁡
10
(





)
CMRR=20⋅log
10
    ​

(
A
cm
    ​

A
d
    ​

    ​

)

Where:



A
d
    ​

 is the differential gain, which is the amplification of the difference between the inverting and non-inverting inputs.



A
cm
    ​

 is the common-mode gain, which is the amplification of the common-mode signal that appears on both inputs.

A high CMRR value indicates that the op-amp is effective at amplifying only the desired differential input signal while rejecting common-mode interference. In practical terms, a high CMRR is desirable because it means that the op-amp's output is mainly influenced by the intended signal and not by unwanted noise or interference that may be present on both inputs.

Op-amps designed with better CMRR are suitable for applications where noise or interference rejection is critical, such as instrumentation amplifiers used in measurement systems, medical equipment, and communication systems.