Common-mode rejection ratio

The common mode rejection ratio (CMRR) is the ratio (usually expressed in decibels) between the common mode noise at the input of a power handling device and the transverse mode noise at the output of the device. Figure 7.3 illustrates the distinction between the two modes of noise. Common mode noise is typically due to either coupling of propagated noise from an external source or stray ground potentials, and it affects the line and neutral (or return) wires of a circuit equally. 

FIGURE 7.3 Example of common-mode rejection ratio. 

In principle any voltage difference amplifier, including an ideal operational amplifier, produces an output that is proportional only to the differential voltage K+ — F and is independent of the co/nmon-znof/e vo/ a e [CMV = V+ + IT)]. The extent to which this is true of a real amplifier can be judged by the common mode rejection ratio ... 

Fig. 6.1.10 shows a variation of the current sense approach where the amplifier is connected differentially to the sense capacitors. Compared to the single-ended version in Fig. 6.1.8 b, the differential circuit has improved power supply rejection ratio (PSRR) and ability to reject errors such as substrate noise or coupling from other signals. This is because any error that couples into both inputs of the amplifier equally will be rejected by the amplifier s common-mode rejection ratio (CMRR). 

Common mode rejection ratio (CMRR) CMRR of a differential amplifier is defined as the ratio between the amplitude of a common mode signal and the amplitude of a differential signal that would produce the same output amplitude or as the ratio of the differential gain over the common mode gain CMRR = GD/GCM. Expressed in decibels, the common mode rejection is 20 log 10 CMRR. The common mode rejection is a function of frequency and source impedance unbalance. 

Common-mode rejection ratio greater than 100 dB... 

FIGURE 32.32 Photograph of different electrode configurations, a) Hosmer-Dorrance Myopulse modulation electrodes and controller, note physically separate reference electrode to improve the common mode rejection ratio (CMRR). (6) and (c) Otto Bock electrodes with integrated reference electrodes. 

Note BW bandwidth CMRR common-mode rejection ratio op-amps operational amplifiers. 

Nonzero common-mode gain is caused by the two gains being unequal. If Gx = Gy = G, then Vq = — GVo, and there is no common-mode contribution to the output voltage. The term (Gy — Gx) = Gcm> and (Gx + Gy )/2 is the difference-mode gain of the amplifier. The ratio of the difference-mode gain to the common-mode gain is the common-mode rejection ratio. Thus, CMRR can be expressed as... 

FIGURE 10.284 The concept of common-mode rejection ratio (CMRR) for an active-balanced input circuit (a) differential gain measurement (b) calculating CMRR. 

FIGURE 16.26 Common mode rejection ratio for an active balanced input circuit. 

Common mode rejection ratio (CMRR) A measure of how well a balanced input circuit rejects ground noise it is the ratio of the differential gain and the common mode gain in decibels. 

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