[SI-LIST] Re: How to connect Chassis ground to DGND
- From: steve weir <weirsp@xxxxxxxxxx>
- To: cchalmers@xxxxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 27 Feb 2004 03:03:35 -0800
Fun with grounds. If I read you correctly, you have put a plane into your
board that you call chassis ground that is for all intent and purposes a
big piece of floating metal as far as DGND is concerned. If so, this
should give you no end of grief. Either stitch that plane to DGND making
it just another DGND layer, or get rid of it.
What you want to get rid of is common mode on your signals relative to the
potential on the chassis near the exit point of the signals. There are
several steps you can take to go about this:
1) Provide good bonding between the DGND and the chassis ground in the area
surrounding each connector.
2) Stitch DGND and the chassis itself over the area of the board.
3) Insert CM rejection filters in series with signals leaving the
box. Some people do this with discretes, others use selective moating (
don't try if you don't fully understand it ) with or without other CM
suppression. The idea of the moat and the drawbridge is to put the signals
traversing the drawbridge and the island on the other side at the same CM
potential and to anchor that potential as the chassis at the exit point.
Steve.
At 10:30 AM 2/27/2004 +0000, Chris Chalmers wrote:
>Folks,
> Sorry to bring this old chestnut back to life. I have a question in
>relation
> to Dr Howard Johnson's note on radiated ground noise (on sigcon website).
>
> I have a board that has a graphics DVI connector on it. I have connected
>the grounds on the
> connector to DGND on my board. I have also put a chassis ground layer in
>my stackup
> next to a solid ground layer. From Dr J's note, I have connected DGND on
>my board
> to the chassis layer at the DVI connector. This is to pin the DGND to the
>same potential as the
> chassis ground (which should not be bobbing about like the DGND) at this
>point so that
> noise on my DGND is not radiated on to the graphics cable. Currently I
>have made the
> connection between CHGND and DGND with an 0603 resistor. Is this OK or
>should
> I make multiple connections with vias instead of the resistor?
>
> I have a second question as well if I may. I have a connector at the
>other end of the
> board which I want to implement a similiar strategy. Can I do exactly the
>same at this
> other connector or will I start to have problems with ground loops between
>the two.
>
>Thanks in advance
>
>Chris
>
>
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