[SI-LIST] Re: Chassis ground

All,

Some observations :-

> Greetings all,
> Perhaps I can help with the chassis ground/EMI aspects.  Grounding
> schemes for tying the digital ground to chassis ground vary, but there
> are a few rules of thumb for interfaces.  Ethernet for example should
> have a small localized and fully moated ground plane tied well (2-3
> connections) to chassis.  The common mode choke should be
> placed "over"
> the moat.  Also, treat any LED leads with SMT reistors or chokes and
> located these above the moat as well (you have to put
> resistors on them
> anyway, so just locate them over the moat).  This isolates the exiting
> UTP wires from the high speed noise on the board and prevents
> re-coupling of said noise after the common-mode choke.
>

This follows the recommendations I have read for most Ethernet Phys
and related transformers. I have just looked at this in relation to PCI
based Ethernet controllers and a PC motherboard. The PCI Ethernet controller
implements an isolated ground scheme, connected to the PCI bracket to form a
chassis ground. The PC motherboard has a number of plated through holes
conncted to the 0V line that are then screwed into the chassis. So once you
plug your PCI card with isolated ground plane into the  motherboard, screw
then
screw both boards into the chassis you have shorted your nice isolated
ground area
back to the motherboard 0V line. As far as I can tell this completely
negates the
effect of the isolated grouunding schemes implemented in such cards.
Personally I like to strap every thing to one Ground potential at as many
places as
feasible to try and get as low an impedance return path as possible no
matter
what path the noise takes.

Best Regards

John

> Multiple grounding schemes to chassis can backfire because of the
> possibility for large loop size eddy currents or alternate return
> currents for HF signals (again, large loop size).  These can generate
> high levels of RF energy which have a nasty habit of finding a way out
> of the box.  The chassis is very low impedance for RF (unless
> you have a
> conductively painted plastic box, then who knows).  For single point
> grounds, I would use a short standoff or other method with a low
> inductive reactance at HF.  Wire is probably not a good idea,
> particularly if this wire exits the box.  Multipoint grounds
> can also be
> used but care must be taken to ensure that the HF return paths have a
> sufficiently low impedance to stay near the signal trace.
>
> Best Regards,
> Dave Heald
>
>


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