[SI-LIST] Re: Chassis ground

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.  

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


Alex Horvath wrote:
> 
> I know most SI engineers work at the board level but I am often asked if
> digital signal ground should be connected to the chassis ground at the board
> (normally through standoffs). It seems to me that the chassis would be a
> very high impedance signal path so in effect only DC currents would flow
> thus becoming part of the power supply return.  Is there any advantage to
> keeping the chassis ground isolated from digital ground and connecting them
> at a single point with the green AC wire? I'm asking from the point of view
> of reducing EMI.
> 
> I'm talking about your typical high speed digital systems with multiple
> boards in a large chassis. Also, assume no conductive connections to the
> outside world (optical only, or unshielded Ethernet).
> 
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