[SI-LIST] Re: Cable grounding scheme
- From: "Ihsan Erdin" <erdinih@xxxxxxxxx>
- To: "chen_jinhua@xxxxxxx" <chen_jinhua@xxxxxxx>
- Date: Tue, 15 Aug 2006 20:23:17 -0400
Jinhua,
I think it wouldn't be wrong to say that EMI on PCBs is all about the
suppression or rather the management of common-mode currents since the
former is practically impossible. Stitching between digital and chassis
ground is in principle analogous to the decoupling between power and ground
planes. Since we can't short the power planes to ground we use decoupling
caps to reduce the loop areas. In digital-chassis ground stitching, the
incentive is the same: reducing the loop areas against common-mode currents
which flow on the planes whose return path is the shielding enclosure. This
could be done either by caps or by direct connection since we don't have the
dc-shorting concern. In this context, using a single point-grounding against
the common-mode currents is tantamount to using a single decoupling cap
between the power and ground planes for high frequency noise suppression.
Of course the same concerns for power noise decoupling follow us here, too.
Multi-point grounding is not a magic bullet to wipe out all the radiated
emissions from dc to daylight just like discrete decoupling caps lose their
efficiency beyond 300Mhz or so. If you think about it the parasitic
inductances of high frequency discrete decoupling caps invoke resonances at
about 100-150Mhz, which don't exist on blank boards. But that doesn't stop
us from using them. Similarly, stitches may have the same effect compared to
PCBs floating in the shielding enclosures. But their benefits at higher
frequencies usually outweigh these disadvantages and hence my point.
Regards,
Ihsan
On 8/15/06, chen_jinhua@xxxxxxx < chen_jinhua@xxxxxxx> wrote:
>
> Ihsan
>
> If I understand your email correctly, you still use separate chassis and
> logic grounds. But you use many many stitching points to connect them
> together. If you consider the high speed cable application. It will impact
> SI. If the cable does not have separate logic and chassis grounds. Cable
> reference is chassis ground when it connects to the board connector. From
> connector to semiconductor chips, there will be a reference interruption
> because chip references to logic ground. Depends on how bad of the reference
> interruption, the SI impacts will vary. If the signal-point connection is
> used, I would guess the SI impact is huge.
>
> This brings an old question: single-point connection vs. many many points
> of connections, which one we prefer for high speed SI and EMI? Or it depends
> ...
>
> Thanks!
>
> Jinhua
>
> ------------------------------
> *From:* Ihsan Erdin [mailto:erdinih@xxxxxxxxx]
> *Sent:* Tuesday, August 15, 2006 9:40 AM
> *To:* Xilei Liu
> *Cc:* chen, jinhua; si-list@xxxxxxxxxxxxx
> *Subject:* Re: [SI-LIST] Re: Cable grounding scheme
>
>
> Celine,
>
> First of all, I want to express my apologies to Jinhua for -kind of-
> hijacking his topic for a potentially flaring issue.
> I think this issue goes as far back as the debate over single-point
> vs.multi-point connection between chassis and digital grounds. As
> such, it is more of an conducted/radiated emission problem than SI. The
> dilemma is while single-point connection could be justified by the fact that
> it avoids very low frequency common mode (noise) currents from creeping into
> the power line, in order to cut down on the radiated emissions at high
> frequencies, multi-point connection is strictly required between the two
> reference systems. In his "EMC and printed Circuit board design theory and
> layout made simple" book, for example, M. Montrose suggests stitching the
> two reference systems at a distance of lambda/20, with lambda being the
> wavelength of the highest frequency component of the spectrum of the system.
> The book was published in 1999. With today's multi-gigahertz systems, it's
> impossible to achieve such a design goal and it's an overkill at any rate.
> But the necessity of multi-point connection is not a debate any more. Some
> designers try to find a mid-way by connecting the reference systems with
> high frequency caps but the boards are already overly-populated by the same
> type of caps used for decoupling and there's the issue of parasitic
> inductances that defeat the purpose. Today, the commonplace approach, at
> least in the designs that I observe, is to suppress the low frequency CM
> currents with power line filters and directly connect the chassis and
> digital grounds at practically as many points as possible against radiated
> emissions.
> If you want to see some numbers and charts to support these ideas, in "EMI
> and Troubleshooting Techniques" book, M. Mardiguian gives a very good
> example that compares the two grounding strategies.
>
> Regards,
>
> Ihsan
>
>
> On 8/15/06, Xilei Liu <xileil@xxxxxxxxxxx> wrote:
> >
> > Hey, Ihsan,
> >
> > I've ever seen notes that published in the past in 2002, saying that"we
> > learned from NRAO engineers that it is both feasible and advisable to
> > physically separate digital circuits from analog systems, and to
> > interpose a
> > minimum of two levels of Faraday shielding acting in series." From my
> > point
> > of view, it should be easier to employ different EMI solutions for power
> >
> > line and signal line separately when the digital/analog grounds are
> > separated and connected somehow at a single-point. What will be the
> > problems
> > in terms of SI? Welcome your 'fight back' so that I can learn more ;)
> >
> > Regards,
> > Celine
> >
> >
> > >From: "Ihsan Erdin" <erdinih@xxxxxxxxx>
> > >Reply-To: erdinih@xxxxxxxxx
> > >To: chen_jinhua@xxxxxxx
> > >CC: si-list@xxxxxxxxxxxxx
> > >Subject: [SI-LIST] Re: Cable grounding scheme
> > >Date: Tue, 15 Aug 2006 06:29:32 -0400
> > >
> > >This question makes me wonder if there're any designers left who still
> > >separate logic ground from the chassis ground in high-speed digital
> > circuit
> > >design -and on what basis? I thought this whole issue of chassis vs.
> > logic
> > >ground was something of the past.
> > >Regards,
> > >
> > >Ihsan
> > >
> > >On 8/14/06, chen_jinhua@xxxxxxx <chen_jinhua@xxxxxxx > wrote:
> > > >
> > > > Hi,
> > > >
> > > > I have a few general questions about the high speed cable grounding
> > > > scheme. It could impact both SI and EMI. I would like to have your
> > > > inputs about this issue.
> > > >
> > > > Scheme 1: cable does not separate logic ground and chassis ground.
> > But
> > > > when it connects the system/boards, the system/boards have separate
> > > > logic ground and chassis ground. How do you separate/connect the
> > logic
> > > > ground to chassis ground in boards? What is the pros and cons for SI
> > > > and/or EMI?=20
> > > >
> > > > Scheme 2: Cable keeps separate logic ground and chassis ground.
> > > > System/boards also keep the separate logic and chassis ground. Cable
> >
> > > > logic ground and board logic ground connects, and chassis connects
> > the
> > > > chassis ground. How do you separate/connect the logic ground to
> > chassis
> > > > ground in boards? What is the pros and cons for SI and/or EMI?
> > > >
> > > > Do you prefer scheme 1 or scheme 2? What is the pros and cons of
> > scheme
> > > > 1 vs. scheme 2 for SI and/or EMI? Does SI and EMI have conflict
> > > > requirements?
> > > >
> > > > Thanks!
> > > >
> > > > Jinhua Chen
> > > > SI of Hardware Engineering
> > > > EMC Corp.
> > > > ------------------------------------------------------------------
> > > > 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 FAQ wiki page is located at:
> > > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> > > >
> > > > List technical documents are available at:
> > > > http://www.si-list.org
> > > >
> > > > List archives are viewable at:
> > > > //www.freelists.org/archives/si-list
> > > > or at our remote archives:
> > > > http://groups.yahoo.com/group/si-list/messages
> > > > 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 FAQ wiki page is located at:
> > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> > >
> > >List technical documents are available at:
> > > http://www.si-list.org
> > >
> > >List archives are viewable at:
> > > //www.freelists.org/archives/si-list
> > >or at our remote archives:
> > > http://groups.yahoo.com/group/si-list/messages
> > >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 FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
