Re: What are ground loops?

A great textbook on the subject is "Noise Reduction Techniques in 
Electronic Systems" by Henry Ott and I recommend it highly as an excellent 
blend of theory and practice for the engineer.

The key is to realize that no wire, ground plane, or power bus is perfect. 
Instead, each has resistance and these resistances serve to couple currents 
from one part of a circuit to another, and that closed circuits can serve 
to pick up induced magnetic fields (e.g., the 60 Hz hum produced by 
electromagnetic equipment)  That is why good noise reduction technique 
requires "single point grounds" (as well as for safety reasons).

Simple example, suppose that the ground returns of circuit A and circuit B 
are connected to each other via perfect resistance-free connections, and 
this node is connected to a ground via a wire with 1 ohm resistance.  The 
ground reference voltage that circuit A sees depends on the current through 
circuit A **as well as** the current through circuit B (the ground 
reference A sees rises by one volt for each ampere in circuit B).  This is 
an example of noise coupling from one circuit into another via grounding.

This would not have happened if each circuit were connectied individually 
to a single, good ground.  (Note the caveat-- GOOD ground).

Steve Czarnecki