Electromagnetic interference
(EMI) is a double-acting problem. Especially in military usage, equipments must
not radiate interference beyond levels allowed by specifications nor be
affected by defined levels of external interference. These levels are defined
for both radiated and conducted levels.
Radiated interference into and
out of equipment can usually be thwarted by good grounding techniques and
shielding of critical circuits. Interference conducted by supply, control, or
load wiring may require the installation of low-pass traps such as shown in
Fig. 5.4. A “T” section of this sort will reduce conducted noise in both
directions, into and out of the equipment.
Local EMI can cause problems with
instrumentation. Shunts are often used to measure DC current levels and
occasionally for AC. Isolation amplifiers are used to amplify the shunt output
voltage that is usually in the 50 to 100 mV range.
Difficulties may arise if care is
not taken with the lead dress to the shunt, especially if the power circuit has
a high ripple level. Figure 5.5 shows the nature of the problem.
The lead dress at left provides a
large loop that magnetic fields can penetrate and induce spurious voltages. At
right, the leads have been twisted to minimize the loop. Even with twisted
leads, there is a less obvious loop caused by the thickness of the shunt
itself.
The shunt should be made such
that the terminals are symmetric about the geometric center of the shunt, and
this requires an even number of leaves and location of the terminals on
opposite sides of the shunt body and opposite ends of the leaves. Figure 5.6
shows a recommended shunt construction that several vendors are willing to make
at a small premium in price.
Even with these precautions, it
is not always possible to eliminate all stray pickup in circuits with AC or
high-ripple currents. However, contaminating signals can be neutralized with an
air core pickup coil of a few turns connected in series with the shunt output.
The location and orientation of the pickup coil must be determined
experimentally.
The precautions on shunt metering
apply also to oscilloscope measurements of low-level signals in the presence of
interference. The ground lead of the scope should be looped back and forth
along the probe to minimize the loop area and the resultant induced voltages.
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