I have a question regarding your comment:
>Since all grounds are (usually) at the same nominal potential (zero volts)
>they are usually tied together. But if tied at more than one point, there is
>a possibility for a (current)(ground)(noise) LOOP to exist, causing (you
>guessed it) noise. So they are tied together at ONE point --- to prevent loops.
I have heard in the industry of people who believe very strongly that tieing
the grounds together to prevent loops means tie ALL ground together at a single
point and furthermore, make that point back at the power supply. This includes
the chassis ground as well. However, if you have parts/traces radiating energy
onto (for instance) a chassis ground, don't you want to minimize the return
path for those currents to minimize EMI.
In other words, tieing grounds at a single point, but not ALL at the same
point, and tieing them together as soon as possible. (This is actually the
question - when is comes to implementation, we find we have several different
grounds, what is the theory and practive behind tieing them together?)
Any other si-list who cares to add their theories or experience in this area
please do so.