Re: [SI-LIST] : Connecting board grounds to chassis

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Tue Nov 07 2000 - 11:33:10 PST


"Pessotto, Michael" wrote:
>
> In a PCB with audio and video outputs referenced to AGND and digital outputs
> referenced to DGND what is the best way to connect the board grounds to the
> chassis? In our current design we have chassis connected to DGND through the
> digital output connector shells and through standoffs between board and
> chassis. The analog output connector shells are tied to AGND and are
> insulated from chassis. When we went to FCC pre-scan we saw the cables
> connected to AGND radiate with peaks corresponding to the clock frequencies
> used in the digital section. Shorting the analog connector shells to the
> chassis eliminated these peaks but introduced the noise from the digital
> plane onto our analog outputs. I thank you for your help in advance.

Michael,

Regarding the grounding of connector shells -
Pigtailing your cable ground connections will
cause some amazing unintentional radiation to
happen. Always go for 360 degree connections
between shells and cable shields. And always
make as solid a connection with shell ground
to chassis ground. No pigtailing there either.
The pigtail introduces an impedance bump that
causes the chassis and the cable to be arms of
an oddly configured antenna system. Doug Smith
does some terrific live demonstrations with
this effect.

Regarding your grounding attempt -
From what you've said so far, it seems as though
your digital ground is dirty as well. Maybe that's
obvious. But, as has been already said, common
paths should be avoided. Perhaps a real soul
searching analysis of the digital layout may be
needed as well.

- Doug McKean

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