[sdiy] Dirty Ground & Bypassing

[Magnus Danielson]

From: "Jay Schwichtenberg" <schwich at qwest.net>
Subject: RE: [sdiy] Dirty Ground & Bypassing
Date: Tue, 25 Sep 2001 08:37:40 -0700

> Antti,

Antti, Jay,

> It sounds like you have the right idea on doing the star ground. But!
> Anything hooked on the digital side of things should go to the digital
> ground.

Indeed. Trouble arises as soon as digital meets analog.

To which ground should one hook up?

Whenever you change state on any of those digital lines, you will push
current on that line but pull the same amount from either the ground
or Vcc lines. To keep emissions low for instance, one has to ensure
that the full loop of the current is small. If you have a signal
passing over a ground floor (say digital) and then jump over to
another ground floor (say analog), then the return current will pass
over the most suitable point, and this is most probably quite far
away. The bigger loop means bigger inductance, and this means more
ground noise most of the time.

One of the tricks being used is to build a narrow bridge between the
grounds and one point. Over this bridge is the signal passed in the
narrow path. They will "feel" a continous ground, but the quite or
analogue ground will not feel the passage of many other stray
currents.

Another trick is to put small caps near the signal so that the
returnpath is through the caps.

All this assumes that one has done the propper bypass cap work. Here
one should really spend some though to snap at the peaks in the right
way.

Inductance is really your enemy here. Well, most of the time at least.

There are many good books to read on these matters.

Cheers,
Magnus