Common mode rejection

Main system parameters are shown in table 1. The fine field resolution is important. When operating in an unshielded space, however, at least such important is the high common mode rejection and the gradient rejection in order to suppress parasitic magnetic fields. 

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. 

The thermopile depends on basic physical principles with the inherent ability to measure temperature differences directly with high common-mode rejection [13]. Furthermore the compatibility with silicon technology enables size reduction and direct immobilization of the biochemical sensing part onto the chip. 

The du/dt of motor voltage can be reduced by an output filter. Shaft current (ground current I3) and circular current are thereby reduced. Good common-mode rejection is important here. 

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 ... 

A major drawback has been the unwillingness of the public to accept an uncommon technique, resulting in fewer developments. The common mode rejection (detection is non specific), non discriminative, compared to spectroscopic (wherein specific wavelength is used) and electrochemical (wherein specific potential is used) for detection. In addition the technique is not very sensitive. The theoretical sensitivity is 0.1 pM using the GOX/catalase system. 

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. 

Solutions are shielding, active opamp circuit clamping the body to ground (Chapter 8), increased distance to the noise source, increased common mode rejection circuitry (Chapter 8), averaging over several heart beats, and wireless telemetry of electrode pick-up signals. 

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. 

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