l Relatively small electromagnetic energy: This could come from AC current on a nearby
power cable, or RF energy transmitting through the air, and can cause electrical noise that
either corrupts an analog signal or disrupts digital communications.
l Larger electromagnetic energy: The antenna loop scenario can have a more damaging
effect when a large current is discharged nearby. The creation of an electromagnetic pulse
can induce a surge that damages attached electronic devices.
Another way ground loops affect a system is by allowing ground current to flow between devices.
This can be either a DC or AC effect. For various reasons, the voltage potential between two
different points on the surface of the earth is not always 0 V. Therefore, when two electrical
devices are both connected to a local earth ground, there may exist a voltage difference between
the two devices. When a cable is connected between the two devices at different voltages,
physics dictates than an electrical current must flow between the two points through the cable.
See FIGURE 9-2 (p. 144).
FIGURE 9-2. Leakage current (AC or DC) from nearby load
l One effect of this DC ground current-flow is a voltage offset error in analog measurements.
Errors of this sort are usually not obvious but can have meaningful effects on
measurements.
l For digital communications, an offset in the ground voltage reduces the dynamic range of
the digital signals. This makes them more susceptible to noise corruption. If the ground
voltage changes by one volt or more, the digital communications could stop working
because the signals no longer reach the thresholds for determining the state of each bit.
l If the ground voltage differences reach several volts, damaging effects may occur at the
terminals of the electronics devices. Damage occurs when the maximum allowable voltage
on the internal components is exceeded.
9. Tips and troubleshooting 144
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