Common-mode voltage

The dc level of the logarithmic converter can be changed with the reference current (/ref) or with the common-mode voltage (Fcm). The bulk-chip temperature sensor can be used to compensate for the temperature dependence of the logarithmic converter. The performance of the logarithmic converter is shown in Fig. 5.6. 

Five calibrated resistors (1% accuracy) with values of 1 kQ, 10 kQ, 100 kQ, 1MQ, and 10 MQ were connected to the input of the logarithmic converter (i.e., the input, to which the metal-oxide-covered electrodes are connected), and the output voltage (A Veb) of the logarithmic converter was measured. The common-mode voltage was 1V, the reference current was 0.1 pA, and the ambient temperature was kept at 25 °C. The offset voltage between the emitter-base voltages was less than 2 mV. The resistance values were estimated from the measurements at 2% resolution. 

Bearing Currents Caused by Common Mode Voltage. 329... 

Figure 17.10 Bearing currents caused by common mode voltage...

The differential solution adds some additional circuit complexity in the form of auxiliary feedback loops to set the common-mode voltage at the amplifier input and output [17, 18]. 

The transformer-based device shown in Figure 9-5 exploits the fact that common mode voltages cannot cause transformer action — because transformer action requires that a differential voltage be applied, so as to produce current in the windings, and thereby causes the flux to swing within the core. Unlike the LISN MATE, in this case both CM and DM noise components are outputted. This device used to be available from AEMC in France, at www.aemc.fr. 

In many modern biopotential amplifiers, the reference electrode is not grounded. Instead, it is connected to the output of an amplifier for the common mode voltage, op-amp 3 in Figure 9.10, which works as an inverting amplifier. The inverted common mode voltage is fed back to the reference electrode. This... 

Cross-talk is by no means the only instrumentation problem experienced in HIT. Rosell and Riu (1992) discussed the significant common-mode voltage sensed by the potential... 

The indifferent electrode wire can be coupled to the reference wire of the preamplifier to minimize the common mode voltage and noise. 

Figure 8.18 Driven right leg (RL) circuit for reduced common mode voltage. Differential preamplifier. Input equipped with three-wire connections for two PU electrodes and one RL drive wire. The purpose is to equipotentialize human body and instrument and to reduce the common voltage between electrodes. Note power line ground symbol. See text.

Figure 8.20 illustrates how the common-mode voltages are controlled in an often used configuration, by grounding one side of the signal somce (Grimnes and Martinsen, 2007). In medical instrumentation terms, this means that the instrument is a class B device (Section 10.16.6). Even so, a differential amplifier is used. 

At the same time, the motor cable length will have an impact on common-mode voltage, and the simulations for Im, 20m and 100m were made using Mat-lab. The simulation circuit is shown in Figure 9. 

Electric interference in fhe environment induces current into the body, which is then picked up by the biopotential amplifier as a common-mode voltage [27]. The CMRR property of the amplifier is essenfial for reduction of the common-mode voltage [25]. Finally, the driven right leg design [27], described earlier, can be optionally used to reduce further the common-mode voltage and the effective interference. 

The input voltages Vi and Vi are assumed to be of reasonable amplitude and frequency, but otherwise they are arbitrary. These arbitrary signals can be expressed in terms of common-mode and difference-mode excitations. The arithmetic average of the two excitations is the common-mode voltage. That is... 

FIGtJRE 7.95 Difference amplifier with excitations expressed as difference-mode and common-mode voltages. 

Two basic types of noise can appear on AC power, audio, video, and computer data lines within a facility normal mode and common mode. Each type has a particular effect on sensitive load equipment. The normal-mode voltage is the potential difference that exists between pairs of power (or signal) conductors. This voltage also is referred to as the transverse-mode voltage. The common-mode voltage is a potential difference (usually noise) that appears between the power or signal conductors and the local ground reference. The differences between normal-mode and common-mode noise are illustrated in Fig. 10.278. 

The common-mode noise voltage will change, depending upon what is used as the ground reference point. It is often possible to select a ground reference that has a minimum common-mode voltage with respect to the circuit of interest, particularly if the reference point and the load equipment are connected by a short conductor. Common-mode noise can be caused by electrostatic or electromagnetic induction. 

An ideal differential measurement system responds only to the potential djj erence between its two terminals, the (-I-) and (—) inputs. Any voltage measured with respect to the instrumentation amplifier ground present at both amplifier inputs is referred to as a common-mode voltage. The term common-mode voltage range measures the ability of a DAQ board in differential mode to reject the common-mode voltage signal. 

The preferable input mode for a grounded signal is differential or NRSE mode. With either of these configurations, any potential difference between references of the source and the measuring device appears as common-mode voltage to the measurement system and is subtracted from the measured signal. 

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. 

It is the ratio of the differential voltage gain to the common-mode voltage gain... 

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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