Interference Types and Reduction
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Interference has been previously defined in Section 1.3.1 as those signals that affect the measurement system as a consequence of the measurement principle used. Here we are concerned with electronic signal conditioning, and therefore we will cons...
Interference has been previously defined in Section 1.3.1 as those signals that affect the measurement system as a consequence of the measurement principle used. Here we are concerned with electronic signal conditioning, and therefore we will consider as interference any electric signal present at the output of the system or circuit being considered and coming from a source external to it. Interference problems are not exclusive for electronic measurement systems but are also present in other electronic systems having distinct functions. The interested reader should consider reference  for an excellent analysis of interference problems in general, and reference  for interference problems in measurement circuits.
Interference is reduced by applying different techniques that depend on the coupling method for the undesired signals. Depending on whether the coupling method is by means of a common impedar,ce, an electric field or a magnetic field, we will respectively speak of resistive, capacitive, and inductive interference.
Figure 3.42 shows a simple circuit illustrating resistive interference. A signal is measured that is ground referenced at a point far from the reference ground for the amplifier, as indicated by the different symbols used. These reference points may be grounded at the respective locations. Therefore, since the ground is used as a return path for leakage currents from electronic equipment, it happens that there is always a voltage difference between different grounds. In industrial environments, at least I to 2 Y is to be expected.
The use of a differential amplifier connected as shown in Figure 3.43 solves the problem if the total common mode rejection reduces the interfer- ence to an output level below the one desired. We assume that the common mode voltage at the op amp inputs due to Vr does not exceed the m-aximal allowed value. In  the use of instrumentation amplifiers for this purpose is described.
It may happen, however, that either the available CMRR is not high enough or the common mode voltage is too high orjust that in addition to the input amplifier there arc other circuits connected to the same reference. All these situations call for other solutions that will be described in the following sections.
Figure 3.44 shows the general problem of capacitively coupled interfer- ence . Between any pair of conductors there is a finite capacitance. Whenever one conductor is at a certain voltage with respect to a third conductor (the ground plane in Figure 3.44) the second conductor will also increase its voltage with respect to the third conductor. With the terminology of Figure 3.44, across the equivalent input resistor R presented by the circuit encountering the interference there is a drop in
voltage due to Yr, amounting to
If the circuit has a low input resistance, in particular if R < ll[a(Cn Gc)1, then RUMUS
On the other hand, if R > llla(Cn * C26)1, then
That is, if R is low, then the interference increases at increasing frequen- cies, whereas for large R the interference is frequency independent and larger than when R is low. In both cases there is an increased interference for high C12 values. In measurement systems the usual interference sources are the 60 (or 50) Hz power lines, and therefore the situation is better described by (3.76), particularly when it is intended to measure the signal from a low output impedance source.
When the reduction of Cpthat follows from separating both conductors is not enough, then a further reduction of capacitive interference is obtained by shielding conductor 2. It consists of wholly enclosing it by an electrically conductive material connected to a constant voltage. Figure 3.45a shows the situation when the shield is connected to ground and when conductor 2 is not in fact totally enclosed, which is the real situation when there is at least one input and one output.
If R has a much larger impedance than C26 at the frequencies considered, the equivalent circuit is that in Figure 3 .45b . In that case, if the impedance of the shield to ground connection is low enough and Zs ( 1/r,rC15, then we have
where C12 is now much smaller than when no shield is used because it con- cerns only those segments outside of the shield (which is considered as perfect). Then the flnal interference will be greatly reduced. In practice, conductors are enclosed in a wire mesh whose effective shielding or cover- age factor depends on how closely it is woven. In view of the simplifications leading to (3.78) and by considering Figure 3.45b, we can conclude that shielding efficiency depends on the relative value for Z5 as compared to C15.
FIGURE 3.45 (a) Electric shielding of conductor 2 by a shield connected to a constant voltage (ground in this case) and (b) equivalent circuit for its analysis when R is large. (From H. W. Ott, Noise Reduction Techniques in Electronic Systems, O 1988. Reprinted by permission of John Wiley, New York.)
That is, the interference also depends on Cp which is very small. It is very important to recognize that in order for a shield to be effective, it must be connected to a constant voltage. Otherwise, even though Cpwere zero, an interference may result. For the case analyzed, if we take Zs : a, and we suppose, for example, that for the situation where R is large, we have
That is, if Crs is large, the resulting interference may be even larger than the case when there is no shielding. The shield must thus be connected to a constant voltage. We must decide which end of the shield to connect to which voltage. We will answer these questions in the following sections. We say that there is an inductive coupling or a magnetic interference when the magnetic field produced by the current in a circuit induces a volt- age in the signal circuit being considered. The relationship between the current in a circuit and the magnetic flux it produces in another is expressed by means of the mutual inductance M,
In case of a variable magnetic flux, B, the voltage V2 induced in a loop with area S is given by
where B and S are vector quantities. If the loop is static and B changes
sinusoidally at frequency r,r, we have,
where 0 is the angle between B and S.
Therefore, in a way similar to the case of capacitive interference, a current 11 circulating along a conductor induces an interfering voltage V2 in a circuit such as the one in Figure 3.46, as given by (3.84). But in the present case it happens that the interference is always proportional to the frequency (for capacitive coupled interferences there was proportionality only at low frequencies) and is independent ofthe impedance presented by the receiving circuit (capacitive interference increased with increasing circuit impedance).
If a reduction in B is not possible, the usual solution in order to reduce magnetic interferences is by reducing the area S. This is done by twisting leads or by placing the conductor close to the return path, if the return path is not a wire conductor. In some instances it is also possible to reduce the cos 0 term by reorienting the circuit. Note that a conductive shield around 2 does not solve the problem: The shield will be raised to a voltage level V. : jatM6l1, or we will have Vs : 0 if one end is tied to ground, and that is all.
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