Re: [SI-LIST] : Earth Ground

Fred Townsend (fred@luxtron.com)
Mon, 16 Feb 1998 18:03:37 -0800

I commend your attention to this critical design detail that is usually
missed. Grounding was never addressed in any of my under grad classes
where I think it belongs. In fact my senior project advisor was totally
clue less about grounding.

In schematics I use closed downward pointing triangles with either A1,
A2, A3, etc. or D1, D2, etc. inside the triangle to denote the various
grounds. The more traditional ground symbols such as the familiar
stacked tapered bars or the pitchfork are reserved for chassis grounds.
These symbols are defined by things like CE Mark and their use should be
avoided unless they are actually part of the circuit.

The numbers are used because I have used up to 4 analog and two digital
grounds in one design. Wherever there is a "dirty signal" (meaning very
high di/dt) separate grounds are a candidate.

Incidentally I usually define grounds as voltage planes to my CAD system
since a few of the older CAD systems out there can't handle multiple
grounds.

Once all the ground circuits have been defined, the critical point
becomes determining where they will be common to each other and whether
they will be DC common or AC (capacitor coupled) common. Normally there
will be only one common point (radial common) to break up current
loops. This is not always possible where there are a lot of A to D
converters that have both analog and digital grounds.

Does this help?

Fred Townsend
DC to Light Consulting Services
Silicon Valley

Melinda Piket-May wrote:
>
> 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