[SI-LIST] Re: How to connect Chassis ground to DGND

Thanks for your reply Steve.  You are correct steve in the assumption
 that I have a plane of chassis ground next to a plane of ground.  I
 have an 0603 0ohm resistor connecting the chassis ground to digital
 ground next to the connector that I want to stop radiating.

At low frequencies the chassis ground will not be floating since it
 is connected to Dgnd through a 0 ohm resistor.  However, at higher
 frequencies, the lead and mounting inductance of this resistor
 will cause an impedance between the two planes causing problems.  However,
 the plane to plane capacitance between CHGND and the DGND due to the
 stackup will take over a some higher frequencies effectively shorting out
 the two planes.  Would I be correct in assuming that in the lower
 frequencies and at the higher frequenices that DGND at at the connector
 would be pinned to CHGND thus avoiding radiated noise?  What happens at the
 cross over or does one take over before the other?

From your comments Steve it appears that a simple resistor connection
 between the two is not enough but instead you need lots of vias between
 them.  Do I really want to connect the chassis ground to multiple sites
 over the whole of the DGND plane as it will just look like DGND and I
will have lost the quiet effect of the chassis ground layer.

I am not that wised up on this chassis layer stuff, please be gentle.

Best Regards

Chris





My first quesion is, in this ov


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: 27 February 2004 11:04
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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