>Since everything eventually has to be connected to the same common ground
>somehow (at least in my circuit), how exactly is that done?  I mean, how
>does

There is no short answer to this. You need to analyze what current
circulates between the various chips (computer programs for this really
analyze EACH wire - but they are out of reach for amateurs), and make sure
that this current is 'closed' through a ground (and/or) power line such
that it cannot upset other circuits. This cannot be achieved in practice
(except if you only have two parts or a completely star-wired power grid).

So there is a compromise made between reality and theory, and all the
digital grounds are tied together, and all the analog grounds are tied
together, each in a single ground point, and these two are tied together
at the ground entry point to the circuit. This requirement can place the
ground entry point under a chip (common in A/D and RF applications).
Sometimes designers play more tricks with the grounds (esp. the analog
grounds). These are special cases that invariably lead to lower fiability
(if the analog and the digital gnd on a chip manage to develop some
potential difference - usually 0.6V or less - then the chip will die).

If your project is MP3 or high end audio related you can use the
ground-under-the-chip approach and feed the PSU (or battery) ground
exactly there. Even if it's in the middle of the board ;-)

Peter

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