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

Chris, you are setting up a cavity that will resonate at a frequency that 
depends on the size of your board.  I think the problem here is the 
perception that any large area of metal has a constant AC 
potential.  Anything with currents running through it does not.  However, 
to make an antenna, there has to be a potential difference with something 
else, like the nearby earth.  If we stick a chassis in the middle of a room 
and ground it to earth with long piece of lamp cord that chassis will 
radiate.  The chassis is quiet when it has good connections to earth as in 
metal equipment rack.  In the ideal case for EMI, everything in the box 
would be shorted dead to chassis.  That's not too handy for signals, so we 
don't short them to chassis.

What this means is that we want the DGND to look the same as chassis at any 
place that DGND is exposed.  DGND is directly present on top of all of the 
signals in your cable.  The potential difference is going to be the 
potential of the chassis at the shield bonding versus whatever is on DGND 
at the signal sources, unless you place a series CM impedance, or shunt to 
chassis.

The two end connections of that floating plane to the DGND plane does 
little that helps and actually will create problems.  If you don't need an 
extra DGND layer, your best bet is to remove that floating plane altogether.


Steve.

At 04:42 PM 2/27/2004 +0000, Chris Chalmers wrote:
>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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