0195136047.pdf

650 SIGNAL PROCESSING

layer of special magnetic shielding material may become necessary sometimes in extreme cases

of magnetic-coupling interference. The grounding terminals, the equipment cases, and the shields

are generally tied together at a single system ground point so as to prevent ground-loop current-

coupling interference.

The transducer in some cases may have a local ground that cannot be disconnected. In such

a case, a separateground strap(braided-wire straps used because of their low inductance) is used

to connect the local ground and the system ground point. The shield is also disconnected from

the amplifier so as to prevent ground-loop current through the shield. Because the ground strap

has nonzero resistance, any stray current through the strap will cause an interference voltagevcm

known ascommon-mode voltagesince it appears at both the transducer and the shield terminals.

vcmis generally quite small; however, when the information-bearing signal voltage itself is rather

small, the common-mode voltage may pose a problem, which can be eliminated by using the

differential amplifier, as shown in Figure 14.3.1. The analysis with the virtual-short model of the

op-amp reveals that

vout=K(v 2 −v 1 ) (14.3.1)

amplifying the difference voltagev 2 −v 1. With reasonable assumptions thatRs<<R 1 andRt<<

R 2 , we havev 1 ∼=vcmandv 2 ∼=x+vcm, so that

vout=K[(x+vcm)−vcm]=Kx (14.3.2)

in whichvcmhas been eliminated as desired. Such an op-amp circuit is also known as an

instrumentationortransducer amplifier.

Any interference at frequencies outside the signal band can be eliminated by appropriate

filtering. However, in order to combat interference within the signal band (after proper shielding

and grounding), anotch filteris sometimes used to avoid the bothersome interference at a single

frequencyf 0. Figure 14.3.2 illustrates the point: Part (a) shows the composite amplitude spectrum

including the single-frequency interference; part (b) depicts the amplitude ratio of the notch filter.

The notch-filtering technique does, of course, introduce some inevitable signal distortion, and

such filtering should precede amplification to prevent possible saturation of the amplifier due to

the interference.

+

+

Shield

Transducer

Differential

amplifier

vcm

vout

v 1

v (^2) R
2
R 1
RF = KR 1
R 3 = KR 2
Rs
x(t) Rt
Model of a transducer
with common-mode voltage

• −
  Figure 14.3.1Differential amplifier to eliminate the common-mode voltage.