[SI-LIST] Re: Chassis GND and Digital GND shorting on daughterboard
- From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- To: 'Ignas Mikulevicius' <mikulevi@xxxxxxxxx>, "'o. laney'" <olaney@xxxxxxxx>
- Date: Wed, 30 Jun 2010 14:50:02 -0600
Ignas - I think I see the problem.
You first assumption (that: "The way I understand it, for ESD, we ideally want
to connect chassis ground and digital ground with a large resistor(say, 1
MOhm), to allow any ESD charge to be bled off to chassis without
causing any excessive currents and correspondingly dangerous voltages across
components.) a little off target. The addition of high resistance values
to "bleed off" charge applies to people (in manufacturing) not to
product design. There is no conflict between shorting chassis/digi gnd
for ESD or emissions. It works fine for both.
Best Regards
Charles Grasso
Desk 303-706-5467
Cell: 303-204-2974
Chamber: 303-706-5144
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Ignas Mikulevicius
Sent: Wednesday, June 30, 2010 10:58 AM
To: o. laney
Cc: shuklas@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Chassis GND and Digital GND shorting on daughterboard
Thanks for the reading recommendations, everyone. I will be sure to spend
some more time looking through Doug Smith's website and Henry Ott's book and
see if I can come up with a solution.
On Tue, Jun 29, 2010 at 12:45 AM, o. laney <olaney@xxxxxxxx> wrote:
> The reason there are so many seemingly conflicting answers is, as always,
> "it depends". If you are hoping for rules of thumb or accepted "best
> practices" you will be disappointed. The only adequate way through the
> EMI/ESD thicket is a good grasp of the fundamentals. For instance, there
> are LF tricks that don't work at HF and vice versa and you need to
> understand why. Ground partitioning and current steering are often not
> well suited to cookbook solutions. Really, it is impossible to ask
> general questions and obtain compact, one size fits all working answers.
> You'll have to do the homework on learning the physical reasoning.
> Another ad naseum discussion cannot substitute for that because it's a
> fool's errand. Try the books on noise control by Henry Ott, and as
> previously suggested troll the archives until you understand the
> reasoning (both good and bad) behind various recommendations. Good luck,
> let us know how you're doing.
>
> Orin Laney
>
>
> On Tue, 29 Jun 2010 12:39:44 +0530 "Shailendra Shukla"
> <shuklas@xxxxxxxxxx> writes:
> > facing problem with hyperlynx as attached
> > >>> Ignas Mikulevicius <mikulevi@xxxxxxxxx> 6/29/2010 7:20 AM >>>
> > Hello everyone,
> > Yes, the topic's back. I know this has been discussed ad nauseum in
> > various
> > previous posts, but I felt none of them conclusively established a
> > solution.
> > First of all, from previous discussions it seems to me like there
> > are some
> > trade-offs between ESD and EMI protection in terms of chassis GND -
> > digital
> > GND connections. The way I understand it, for ESD, we ideally want
> > to
> > connect chassis ground and digital ground with a large resistor
> > (say, 1
> > MOhm), to allow any ESD charge to be bled off to chassis without
> > causing any
> > excessive currents and correspondingly dangerous voltages across
> > components.
> >
> > On the other hand, for EMI, I understand it may be advantageous to
> > short
> > digital ground and chassis ground at multiple locations in the
> > vicinity of
> > the I/O connector to divert any problematic currents to chassis
> > ground
> > before they can radiate out on the cable. So is there any way to
> > reconcile
> > the two and get the best of both worlds?
> > And how should these recommendations be applied when designing a
> > standard
> > PCI or PCI Express add-in card?
> > The specific scenario I am interested in is this:
> > Say I have a PCI Express daughterboard (add-in card) that plugs into
> > a
> > motherboard in a standard PC (let's call this System A). The PCB
> > bracket is
> > screwed on to the I/O connector of the PCB and, of course, also
> > contacts the
> > computer chassis. The board sends differential signals across a
> > long,
> > shielded cable to another system (say, System B).
> > First of all, in the case of differential signals inside a shielded
> > cable,
> > how much potential for EMI can there be? I would assume the
> > differential
> > nature of the signals would greatly minimize radiation.
> > The cable shield is shorted to chassis GND. Each differential pair
> > in the
> > cable has its own ground wire, and, finally, there is a wire that is
> > called
> > "Frame Ground", tied to chassis ground at system B. On the
> > daughterboard,
> > there is a "chassis GND" island underneath the connector in the
> > layer below.
> > The connector posts as well as the "Frame GND" signal are shorted
> > here. So
> > basically we have the chassis GND of system A and system B grounded
> > and this
> > also grounds the cable shield. Now clearly I can't leave this
> > floating as it
> > would seem that would create an EMI nightmare.
> > What would be the ideal way to connect this island and digital GND?
> > A
> > capacitor, dead shorting, a resistor?
> > It seems whatever method I choose I can find at least one article to
> > endorse
> > it.
> > Any thoughts or insight would be greatly appreciated. I am not
> > expecting
> > someone to remedy my dillemma for me, but would really like to
> > understand
> > the theory and thinking behind this once and for all. If someone
> > could
> > either elucidate me or point me to some definitive, conclusive
> > articles or
> > books on this subject I would be very grateful.
> > Thank you very much,
> > Ignas M.
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
