[SI-LIST] FW: Re: How to connect Chassis ground to DGND
- From: "Dr. Howard Johnson" <howie03@xxxxxxxxxx>
- To: "Si-List@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 27 Feb 2004 12:21:16 -0800
Dear Chris,
I tend to agree with Steve, that if you are going to connect
your chassis and digital grounds, don't do it "partway".
Since you have decided to connect chassis and digital ground
with a zero-ohm resistor, I see no reason why you shouldn't
just make the chassis ground the same as digital ground
everywhere. I assume there is a GOOD connection between the
chassis ground plane in the board and the actual chassis
(like an exposed region of the chassis plane that is bolted
across a broad area to the chassis, or something similar).
I'm going on a trip this afternoon and so unfortunately
can't reply further.
Best regards,
Dr. Howard Johnson, Signal Consulting Inc.,
tel +1 509-997-0505, howie03@xxxxxxxxxx
http:\\sigcon.com -- High-Speed Digital Design seminars,
books, and articles
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Friday, February 27, 2004 3:04 AM
To: cchalmers@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: How to connect Chassis ground to DGND
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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