AW: [sdiy] Seperate Grounds

[Ingo Debus]

So, to summarize, we want three things:
1. Star ground for the power supply, so the ground connection from a
module to the power supply doesn't share supply currents of other
modules. This is according to Harry's drain pipe analogy.
2. If a signal is transmitted from one module to another, a direct
ground connection between the two modules that does not share any other
current. Otherwise the voltage drop caused by these "other currents"
will add to the signal.
3. No ground loops.
(I'm using the term "modules" here, these can be modules of a modular
synth or anything else)

Obviously, we can't fulfil all three demands at the same time. If
there's a direct ground connection (this can be the shield of a patch
cord, for instance) between two modules and a star grounding scheme, we
get a ground loop. If ground loops aren't allowed, and the modules are
star grounded, the only ground connection between two modules shares the
supply currents of these modules. If one of the modules involved has
nasty supply current spikes, this can be a killer.
Sometimes this dilemma can be solved by introducing two grounds: a power
supply ground and a signal reference ground. But this often isn't
possible, many ICs have only one ground pin which serves both for power
supply and signal reference.

I think the only thing one can do is find a compromise. There's no
one-and-only solution that fits best for all cases. Sometimes ground
loops aren't that bad. Sometimes bus grounding might work better than
star grounding, because the ground connections between modules become shorter.

There's another demand, by the EMC people: the ground of a jack has to
be connected directly to the chassis where the jack is, no "long" (i.e.
few inches) wire to a ground reference point. Now things really get
complicated ;-)

Ingo