Common mode rejection ratio CMRR

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. 

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. 

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. 

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

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. 

The voltage is said to be a common-mode voltage (signal). It is common to both input voltages and is subtracted out by the difference amplifier. The degree to which a difference amplifier is insensitive to a common-mode voltage is called the common-mode rejection ratio (CMRR). If V is increased by a factor of 10,000 before a factor-of-one change in is noted, then the CMRR is 10,000 1. 

OTA has critical effects on the overall performance of the amplifier such as input equivalent noise, offset voltages, and common-mode rejection ratio (CMRR). A current-mirror OTA with cascade output stage [13] is commonly used in conventional designs. 

So that the insulation/isolation of the measuring circuits to ground of one meter can be compared with those of another meter, a convention has been established. This concerns a hypothetical situation represented by Fig. 22, which provides for the evaluation of the common mode rejection ratio (CMRR), which equals 20 logio( om/- e) dB, where E m is the common mode voltage and the error introduced in the reading by 001 when the unbalance resistor is 1 k 2. Because there is a capacitive component in the insulation impedance, the ac CMRR should be specified at a particular frequency and will be less than the dc value. 

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