From: John Larkin
Subject: Re: Ground loops?
Date: Sat, 14 Sep 2002 20:39:35 -0700
Organization: Posted via Supernews, http://www.supernews.com
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On Sun, 15 Sep 2002 01:23:46 GMT, "Brill Pappin"
>> >This has got me thinking about noise as well... in my system , the boards
>> >are all connected in series... I had planned to have a single ground line
>> If the power grounds and the signal grounds are one and the same, then
>> the last board in the string will see some of the signals from boards in
>> between. It's similar to the "ground bounce" that you can get on a single
>> board. Whether the extra signals will be trouble or not remains to be
>> IMHO, a series connected string of boards is just asking for problems.
>Yes I could see this being a problem, however the series boards are really
>not much of an option for me in this design... I may be able to reduce the
>number... and I may be able to put some of them into that "star" topology
>mentioned in another post, but doing that will increase the length of the
>So, the problem remains... and I need to think of ways to reduce noise in
>the system. As I see it there are a few things I can try:
>- Separate the power, digital and analog grounds into separate line (makes
>for larger cables, but doable I think).
>- Add filters to the power ground. (can they be added to the analog ground?,
>not to digital I would expect).
>- Ensure that all analog and digital grounds on a particular device contain
>their own ground planes etc, and only feed to power ground once properly
>filtered (Is that even right? or should it be multiple points?).
>- Use buffers/inverters between boards... mostly this would not be a problem
>I would welcome any suggestions anyone has on this... there will be times
>when I just can't do anything about it, but when I can, I should.
>- Brill Pappin
Grounding is more religion than science. So...
Use only one ground plane on each board. Connect it to other boards as
many places and ways as possible, and as directly as possible. If you
have a metal chassis, bolt all the boards - and their ground planes -
solidly to the chassis, using lots of fat spacers and screws. The idea
is to make all the grounds on all the boards as equipotential as
possible. Now, if you have really low-level analog signals, or
picosecond logic signals, handle them differentially or some other way
so they are not affected by millivolt differences in grounds.
What this will do is make sure you have minimal EMI problems and
minimal digital logic noise.
You haven't said exactly what you're trying to do, so this suggestion
might be tweaked in unusual circumstances. But the conventional wisdom
of star grounding, and isolating analog and digital grounds, is just a
recipe for trouble in most cases. All it does is ensure that various
parts of the system have grounds that are all over the place
electrically at high frequencies, and flailing around relative to the
enclosure, and who wants that?