Amplifier common mode rejection

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 ... 

The conventional TCD is configured with the filaments being connected to form a Wheatstone bridge. A property of the Wheatstone bridge is common mode rejection of the noise which is primarily due to the electronics (l.e. power supply stability and the amplifier circuit). The TCD noise spectrum resembles white (shot) noise rather than the 1/f (flicker) noise of ionization detectors. Modulation techniques for noise rejection of white noise is no better than a simple Wheatstone bridge. 

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. 

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... 

The common mode rejection ratio is important for comparators and other difference amplifiers. If the output voltage changes hy 10 V for an input difference voltage t i of 300 pV and by 1.0 V for a common mode input voltage of 500 mV. what is the common mode rejection ratio of the amplifier ... 

A variation on this principle would be to connect the differential cable to a reversed polarity output, so that the subtraction would, in addition to removing the transients, double the signal amplitude. Common mode rejection is likely to come into its own when the preamphfier-to-amplifier distance is large and/or the environment is very noisy. 

Figure 12.5 Common mode rejection device using the differential input of an amplifier...

A circuit which avoids the need for a differential amplifier with high common-mode rejection is shown in Figure 2.14 (Schwan and Ferris, 1968). The null detector is a single-ended grounded amplifier which acts as a bridge null detector. Different parts of the total sample impedance are balanced against other parts of the sample in series with the variable RC network. Let the subscripts b and c represent the impedance values of the RC network which apply for the different balance equations when the detector is con-... 

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. 

I he largei ilic C MRR of a diffcience amplifier, the belter it is at rejecting common mode. signals, that is. signals that are applied to both inputs simulianeou.sly. 

The output of a real biopotential amplifier will always consist of the desired output component owing a differential biosignal, an undesired component because of incomplete rejection of common mode... 

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