From: John Larkin
Subject: Re: Ground loops?
Date: Sun, 15 Sep 2002 12:04:53 -0700
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On Sun, 15 Sep 2002 14:20:27 GMT, James Meyer
>On Sat, 14 Sep 2002 20:39:35 -0700, John Larkin wrote:
>>Grounding is more religion than science. So...
> Anything one doesn't understand appears to be more religion than
But star grounded systems are far more difficult to understand than
hard-grounded ones. If you were to excite the common plane of each
board in a star system with a swept RF signal and look at the
resulting AC potentials to ground (whatever that is! probably the
enclosure) you'd have a nightmare of complex resonances, and they
would change every time you moved a cable or plugged in an i/o
connector. Such a system is impervious to analysis, and difficult to
analyze experimentally. So all you need is a nearby security guard
with a walkie-talkie, or maybe a cell phone, and you've got RF all
over the place; and if there's low-level analog stuff, every opamp
becomes a crystal radio.
And don't even get me started about the ESD implications of
One of my customers has a guy (a manager, hardly an engineer) who is
telling us that we have to float our subsystem so that high-frequency
'ground loop noise' from a laser firing won't mess up our circuits. I
told him that we couldn't meet specs if our PCB was at different AC
potentials from its metal box. So he told us to add lots of bypass
caps from our PCB ground plane to the enclosure! Think about that for
two or three seconds!