Ground loops are a hard-to-solve problem usually first encountered in practical situations rather than in a book or classroom. Ground loops are sometimes attributed to a difference in "ground potential" between two different circuits, but this "explanation" contradicts the concept that ground is a "unique" potential. Trial and error methods may make a ground loop problem go away temporarily, but it might reappear later at a most inconvenient time. The trial and error method might consist of installing isolators, optocouplers, differential amplifiers, or special cables. Another trial and error method, called "ground lift", involves disconnecting cable shields and hoping this does not affect functionality. These trial and error methods involve expenditure and complication and may degrade signal quality by introducing new forms of interference, by limiting bandwidth, or by making the system more vulnerable to faults.
A better approach than trial and error is diagnosing what is causing the problem and then removing the cause. Diagnosis enables elimination of a problem without expenditures on unnecessary treatments, without introducing complexity, and without degrading signal quality. However, diagnosis requires understanding physics principles that are beyond the scope of electronics. This tutorial surveys these physics principles and then discusses how they can be used to undertake a systematic diagnosis of ground loops.
Some basic ideas in electronics and physics will be examined first in order to categorize the various types of spurious signals that can contaminate an electronic system. Here "electronic system" means two or more spatially separated electronic devices connected by conductors. Attention will be given to issues resulting from the physical layout of these conductors, their mutual interaction, and their being affected by externally produced electric and magnetic fields. The tutorial should be read in the order presented.
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No claim is made that the information herein is free of errors.

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Troubleshoot ground loops with ease
without disconnecting anything!
US Patent 7,791,353 B2