Re: [SI-LIST] : Earth Ground

Jerry Johnson (jjohnson@minn.teradyne.com)
Tue, 17 Feb 1998 09:13:56 -0600

Melinda,

I really wish the industry would stop using the word "ground"
for so many different things. If its use were restricted to
safety ground only, I believe things could be less confusing.
I know about force, force return, sense, sense return, guard,
and reference paths. They can co-exist (the force return for
one supply can share force return with another), but are still
essentially different. If these are on a PCB plane, for example,
the same plane might be used for a reference voltage of 0 Volts,
and that *might* work, if plane drops due to other return
currents are low enough.

However, if one is led to *think* in a way that labels lots of
different paths as ground just because they are all at (or near)
a zero volt potential, that way of thinking can lead to all sorts
of problems. Far better to call them what they are, and deal with
them appropriately. Questions of how to handle "Digital Ground" vs.
"Analog Ground" are less confusing to most people if you think
in terms of force/return paths. For critical reference voltages,
some of which are at zero volts, call them (and treat them) as
just another reference voltage. Don't lump the reference voltage
with force returns, just because they are both "assumed" to be
at or near zero volts.

Just the opinion of someone who has to deal with these concepts
frequently.

Jerry Johnson

> From mjp@hedgehog.Colorado.EDU Mon Feb 16 19:20:18 1998
> From: Melinda Piket-May <mjp@hedgehog.Colorado.EDU>
> Subject: [SI-LIST] : Earth Ground
> To: si-list@silab.Eng.Sun.COM
> Date: Mon, 16 Feb 1998 18:03:27 -0700 (MST)
>
> I am teaching a class out of "High Speed Digital Design" to first year
> grad students. We recently discussed grounding issues and there was a
> lot of confusion about "digital" ground vs "analog" ground vs chassis
> / earth ground. Grounding in a cellular system .. etc..
>
> If anyone has clever ways to explain these concepts intuitively I (and
> my students) would appreciate the help. How do you view these issues
> from a practical everyday point of view?
>
> Thanks
>
> --
>
> Melinda
>
> ______________________________________________
> Melinda Piket-May
> Assistant Professor
> University of Colorado at Boulder
> ECE Department , Campus Box 425
> Boulder, CO 80309-0425
>
> (303) 492-7448 Fax: (303) 492-2758
> E-mail: mjp@boulder.colorado.edu
> Office: Engineering Center OT4-14