Re: [SI-LIST] : Single versus multiple point grounding ...

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From: D. C. Sessions (dc.sessions@vlsi.com)
Date: Wed Oct 27 1999 - 17:15:50 PDT


Douglas McKean wrote:
>
> From: Denomme, Paul S. [mailto:Paul.Denomme@viasystems.com]
> >
> > Regarding comment 2, he was stating that grounding in
> > multiple locations is a bad idea. This can create some
> > ground loops within the chassis. There will be some type
> > of potential difference from one chassis connection
> > point to another and this will create some current
> > in the chassis. This is very bad from an EMI perspective.
> > He believes one Solid connection to the ground plane is sufficient.
> >
> > I would like to hear a take from an EMI person.
>
> Well, personally I'd like to see some measured
> results from testing that prove this opinion.
> I've used multiple point grounding in real system
> design with great success for not only EMI but
> for ESD robustness. The fact that I instituted
> multiple point grounding from a former design
> single point grounding that had sufficient problems
> to warrant a redesign and that such redesign worked
> was proof enough for me.

In many ESD situations multipoint grounding is a Good Thing
because it minimizes the net local change in potential for
strikes far from the groundpoint. OTOH it can cause problems
in extreme cases if the chassis is relatively high impedance.

For EMI the same tradeoff applies. A multipoint connection
to the chassis will be good if the chassis itself is low
impedance and the contacts are relatively high impedance.
A single-point ground *can* be a problem if the enclosure
impedance is high enough that internal fields can induce
potential differences across it; in that case shunting the
induced currents through internal conductors may be better
than leaving the skin to flap around.

-- 
D. C. Sessions
dc.sessions@vlsi.com

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