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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