Engineers know they must provide a complete circuit in measurement systems because current flow requires a return path. People who work with electronic devices may forget, though, that a ground-return circuit has resistance and a voltage can develop between ground points in electronic equipment. The resulting voltage can "ride" on signals you want to measure and throw off test results. An example shows how currents can arise" in a "ground" loop. Consider a function generator connected to an oscilloscope through a piece of coaxial cable. The cable provides a signal path and a ground, or return path. Both instruments reference their input or output to the ground in their building's electrical supply, which can create a long loop or independent circuit. Voltages of from several tens of millivolts to hundreds of millivolts, usually at the power-line frequency (Figure 1), can occur between the grounds in the two instruments. This sort of ground loop will add unwanted signals to measurement results.
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