|MadSci Network: Engineering|
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
Try the links in the MadSci Library for more information on Engineering.