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

Chris - You might find the paper "Effect of an Image Plane on Printed
Circuit Board Radiation by German Ott and Paul" very instructive. In it the
connection of the signal return path to a reference plane (aka chassis) is
examined and test data and analysis. The net effect is a reduction in
emissions. You can download this from
http://www.hottconsultants.com/papers.html


Might I suggest that anyone interested in the mitigation of emissions find
their nearest EMC chapter and attend some *free* EMC Society meetings.? 

Best Regards
Charles Grasso
Senior Compliance Engineer
Echostar Communications Corp.
Tel:  303-706-5467
Fax: 303-799-6222
Cell: 303-204-2974
Email: charles.grasso@xxxxxxxxxxxx;  
Email Alternate: chasgrasso@xxxxxxxx
 

-----Original Message-----
From: Chris Cheng [mailto:Chris.Cheng@xxxxxxxxxxxx] 
Sent: Friday, February 27, 2004 2:56 PM
To: 'weirsp@xxxxxxxxxx'; cchalmers@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: How to connect Chassis ground to DGND

Steve,

This is an interesting area I am always curious. If you look at the GBIC 
spec, it requires DC isolation at the connector point between chassis
and logic gnd. FCAL on the other hand is exactly opposite and requires
dead short between chassis and logic gnd. I never quite figure out 
why two huge groups of very smart people can come out with specs that
are completely opposite to each other. It certainly makes my grounding
scheme looks funny.

To make things worst, a lot of add on cards like PCI cards seems to
have a grounding scheme based on the phases of moon. And most of
the disk drivers vendors like to tie their chassis to logic gnd 
internally just to make your integration a little more complicated.

EMI engineers always told us to DC isolate the entire chassis with
a single shorting point between chassis and logic ground. The rest
of the system have decoupling caps to AC connect the chassis to 
logic ground. I never quite understand the logic and I have 
experimented with DC shorting the entire chassis with logic ground
at every point. The EMI scan shows no difference. 

-----Original Message-----
From: steve weir [mailto:weirsp@xxxxxxxxxx]
Sent: Friday, February 27, 2004 11:57 AM
To: cchalmers@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [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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