From: Keith R. Williams
Subject: Re: Grounding advice
Date: Mon, 14 Oct 2002 23:51:25 -0400
References: <3D9DFA4D.firstname.lastname@example.org> <3DA726DB.email@example.com> <firstname.lastname@example.org>
X-Newsreader: MicroPlanet Gravity v2.60
In article ,
> On Fri, 11 Oct 2002 23:19:50 -0400, Keith R. Williams
> >In article <email@example.com>,
> >jjlarkin@highSNIPlandTHIStechPLEASEnology.com says...
> >> On Fri, 11 Oct 2002 19:30:47 GMT, Ryan Gammon
> >> wrote:
> >> >I happened to come across the following in AoE (pp853):
> >> >
> >> >"... we suggest as many redundant ground connections as possible...
> >> >... Don't worry about about "ground loops" in a digital or RF circuit;
> >> >that's a microvolt audio circuit issue"
> >> >
> >> >
> >> Now *that* is one of the few sensible statements ever made on the
> >> subject.
> >I took a course (professional, rather than academic) in EMI
> >abatement, measurement, and standards a couple of years ago (I'm
> >now an official EMI Guru (tm) ;-). One of the points was that
> >ground loops are meaningless above audio frequencies. At RF
> >frequencies the induced currents will cause loops even if
> >connections don't. The theory then was to maximize ground
> >connections, even in metal parts (unconnected gaps in metal,
> >a.k.a. slots are antennas as much as wires are).
> > Keith
> I may have to hire you to say that in front of one of my customers.
Can't happen. I still work for the same company that sent me for
a week to Texas in February to take the course (even Texas beats
Vermont in February ;-).
> They're building a pulsed-power widget, and I'm doing the timing box,
> and their Pointy-Haired-Boss (tm) insists that we ground nothing
> because 'it would create ground loops'. He thinks cable shields should
> be grounded at the source end only!
- Why does he need you? He knows everything.
- What are you still doing there?!
BTW, low level audio cables are usually grounded at the
termination end. Turntables aren't grounded at the turntable,
rather at the (pre)amp. Perhaps he uses Monster Cable on his
RadioShack surround system?
> By deftly (mis)managing a couple
> of design reviews, I managed to ground *everything* to my metal case
> and present a fait accompli (French for 'fooled you, Jack!')
> prototype, which we don't have time to change.
I've been here (not quite *this* bad though) and attacked the PH
problem a little differently. The position you were in is no
win. You must work a "no win" into at lest a "may win" or better
a "can win".
In a case like this I'd show them their design, then make make it
work properly (perhaps the same hardware ;-). You're now a hero
(program saved, ta da! in the nick of time) rather than perhaps
an insubordinate chump (works, but not as instructed and then
only with incredible luck).
> Vision: a couple of dozen metal boxes, all isolated from the nice
> solid-metal baseplate, connected by a spider's nest of crisscross
> cables carrying power, logic, and low-level analog signals, all
> ringing like the Bells of Saint Mary's every laser shot... nightmare!
Nice vision. You forgot the cables grounded at only one end. Of
course all the power connections were filtered and all boxes
closed and sealed to better then 1/20 wavelength of the fastest
edge in the box.
No, I've never been quite in your position. I've always been
able to at least talk it through to the PHWIC (well other than
the time an indirect manager though microprocessors were a "flash
in the pan" and instructed us not to use them).
To be fair, in most cases the boss only wanted to know what was
going on with the design because he was bored with the management
stuff. When schedules slipped (when don't they) things sometimes
got "interesting", but that's why we get the big bux, eh? ;-)