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MILHDBK419A
Ideally, the CMR of an amplifier should be infinite, or as large as possible. Under the worse case conditions,
CMR = 1. As it is defined, the CMR conveys a measure of how well the amplifier can reject a commonmode
noise signal at its input. Typical values for a good differential amplifier with balanced input impedances are in
the vicinity of CMR = 1000. Often this is expressed in decibels which, in this case, would be CMR = 60 dB.
The CMR for the amplifier in Figure 614 is easily derived from Equation 621 to be
CMR =
(625)
6.3.2 Differential Amplifier. A differential amplifier is designed to make
large compared to
and
large compared to
Since
are normally functions of frequency, it can be seen that the CMR
will also be a function of frequency. Typically
are resistors shunted by capacitors. Thus, it can be
seen that the CMR will inevitably decrease with increasing frequency when the capacitive reactance becomes
smaller than the resistor. Consequently, a high CMR is difficult to achieve at high frequencies.
6.4 MINIMIZATION TECHNIQUES. Signal interaction, i.e., interference, can be minimized by reducing the
coupling between the signal systems by modifying the signal systems in such a manner that interaction between
the systems does not produce interference in either one, by eliminating the source of the interference, and by
filtering the interference out of the susceptible signal system.
6.4.1 Reduction of Coupling. The techniques for reducing coupling include minimizing the impedance of the
reference plane, increasing the spatial separation between the signal systems, shielding the systems from each
other, reducing the loop area of each signal system, and balancing the signal lines in each system.
6.4.1.1 Reference Plane Impedance Minimization. Minimizing the impedance of the signal reference plane
lowers the potential difference between any two points in the reference plane, thereby reducing the conductive
coupling of interference in susceptible circuits referenced to these points. The impedance of the signal
reference plane is reduced by minimizing both the resistance (R) and the series reactance (X) of the conductors
forming the reference plane. The resistance decreases with a decrease in either the length of the conductors or
the signal frequency (because of skin effect  see Section 5.2.2.1) and with an increase in conductor cross
sectional area. The reactance also decreases with a decrease in the signal frequency and with a decrease in the
inductance of the conductors; the inductance is a function of both the conductor length and crosssectional
area. The impedance of the signal reference plane can be reduced by making the reference plane conductors as
short as possible and by using conductors with crosssectional areas as large as practical. The overall
impedance of the signal reference plane also depends upon the establishment of low impedance bonds between
ground conductors. (The various aspects of bonding and bond resistance are discussed in Chapter 7.)
623


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