What Is the Common Mode Rejection Ratio?

In order to illustrate the ability of differential amplifier circuits to suppress common mode signals and amplify differential mode signals, common mode rejection ratio is commonly used as a technical indicator to measure. It is defined as the amplifier's voltage amplification factor Aud to the differential mode signal and the voltage to the common mode signal The ratio of the magnification Auc is called the common mode rejection ratio, and the English name is Common Mode Rejection Ratio, so it is generally expressed by the abbreviation CMRR, the symbol is Kcmr, and the unit is decibel db.

The common mode rejection ratio CMRR is defined as the absolute value of the ratio of the voltage gain of the amplifier circuit to the differential mode signal to the voltage gain of the common mode signal. Because we want to suppress zero drift, the smaller the common-mode voltage gain, the better, and the larger the differential-mode voltage gain, the better. Therefore, it is hoped that the larger the KCMR, the better, and the larger the KCMR, the better the performance of the amplifier circuit.

Circuit symmetryThe symmetry of the circuit determines the amplitude of the residual common mode interference of the amplified signal. The worse the circuit symmetry, the smaller the common mode rejection ratio, and the ability to suppress the common mode signal (interference). Worse.

The linear operating range of the circuit itselfThe linear range of the actual circuit is not infinite. When the differential mode signal exceeds the linear range of the circuit, even normal signals cannot be amplified normally, let alone common-mode rejection. The linear operating range of the actual circuit is smaller than its operating voltage, which is why the front-end circuit of the device that requires high common mode rejection also uses a higher operating voltage. ^[1]

Common mode rejection ratio formula for common mode interference:

1. Common mode rejection ratio CMRR is the ratio of the absolute value of the differential mode voltage amplification factor Aud to the common mode amplification factor Auc

2. CMRR = Aud / Auc or CMR = 20lgAud / Auc (dB)

3. Common mode interference: refers to the same amplitude of the interference voltage on the signal line and its return line (commonly referred to as the signal ground line). The voltage here is based on any nearby object (earth, metal chassis, reference ground board, etc.) as The reference potential and the interference current loop flow in the loop formed by the wire and the reference object;

4. Common mode rejection: In order to illustrate the ability of the differential amplifier circuit to suppress common mode signals and amplify differential mode signals, common mode rejection ratio is commonly used as a technical indicator to measure. It is defined as the amplifier's voltage amplification factor of the differential mode signal The ratio of the voltage amplification factor of the analog signal Auc is called the common mode rejection ratio. The English name is Common Mode Rejection Ratio, so it is generally expressed by the abbreviation CMRR, the symbol is Kcmr, and the unit is decibel db.