[SI-LIST] Re: Cable grounding scheme
- From: "Salkow, Steven" <steven.salkow@xxxxxxxx>
- To: erdinih@xxxxxxxxx
- Date: Tue, 15 Aug 2006 13:12:26 -0700
Seems a lot of rubbish is floating around about grounding.
Grounding considerations deal with electrical, safety standards, and
EMI.
Chassis grounds and signal ground do not have to be tied together. Many
standards allow a single point bonding of the two GROUNDING SYSTEMS
usually on the backplane or chassis. With respect to cables running
between two boxes some remote distance from each other, the single point
bonding of the two GROUNDING SYSTEMS may be:
1. Not joined at either end
2. Joined at one end but not the other.
3. Joined at each end
Mandates for safety in the United States may require one of the above
options depending on the class and service of the equipment (VDE is
different)
1. Class two, low Volt-Amps (VA), low voltage
2. Class one, over typically 50 VA, 50 Volts
EMI
A continuous low impedance envelope surrounding emissive devices such as
cables help contain radiated energy. How much radiated energy depends on
DI/DT, impedance, effectiveness of the shield(ing), and if the signal is
differential or single ended. Slower edges, differential, low impedance,
well shielded lines emit less than their counterparts.
Noise
In grounding cable systems, common mode noise associated with a current
flowing from one box to another either from induced or conducted current
has less effect on differential signals unless the common mode voltage
exceeds the acceptable common mode voltage range of the
receiver/transmitter circuits. A typical offender may be a CRT display
that due to safety regulations required the bonding of the chassis and
signal ground. Non conforming CRT display may leak considerable ground
current. When that display is plugged into a chassis not well grounded,
a ground potential will develop on the chassis' ground equal to the sum
of L*Di/DT and the IR developed.
Radiated noise, from parallel signals, travel in the same bundle over
the same distance. Coupling again is dependent on Mutual Inductance,
cable impedance, Di/Dt, length of coupling (in this case a lot), and the
effectiveness of the shielding between them.
There are many books on the subject that go into this in great detail.
Those that do not have a good one should look for one that is practical
and easy to read.
Steven Salkow
Lockheed IS&S
3130 Zanker Rd, San Jose
Ca. 94588
(408) 473-4058
steven.salkow@xxxxxxxx
salkow@xxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of chen_jinhua@xxxxxxx
Sent: Tuesday, August 15, 2006 12:33 PM
To: leeritchey@xxxxxxxxxxxxx; erdinih@xxxxxxxxx; xileil@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Cable grounding scheme
What I referred to 'Chassis ground' was the piece of metal in board,
which directly connected to metal cage. This cage mechanically hold the
boards, and also acted as EMI shields. That metal cage connected to the
green wire of AC cord as well. Are there any other definitions for
'chassis ground'?
Thanks!
Jinhua
-----Original Message-----
From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]=20
Sent: Tuesday, August 15, 2006 1:10 PM
To: chen, jinhua; erdinih@xxxxxxxxx; xileil@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Cable grounding scheme
What is this ting called "Chassis Ground" that everyone keeps referring
to?
Last I remember it was just the green wire on the AC cord that UL
requires.
How does that play into discussion of EMI?
> [Original Message]
> From: <chen_jinhua@xxxxxxx>
> To: <erdinih@xxxxxxxxx>; <xileil@xxxxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 8/15/2006 9:16:30 AM
> Subject: [SI-LIST] Re: Cable grounding scheme
>
> Ihsan
> =20
> 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.
> =20
> 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 ...
> =20
> Thanks!
> =20
> Jinhua
> ________________________________
>
> From: Ihsan Erdin [mailto:erdinih@xxxxxxxxx]=20
> 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,
> =20
> First of all, I want to express my apologies to Jinhua for -kind of-
> hijacking his topic for a potentially flaring issue.=20
> 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. =20
> 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.
> =20
> Regards,
> =20
> Ihsan =20
>
> =20
> On 8/15/06, Xilei Liu <xileil@xxxxxxxxxxx> wrote:=20
>
> Hey, Ihsan,
> =09
> 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=20
> 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=20
> 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 ;)=20
> =09
> Regards,
> Celine
> =09
> =09
> >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=20
> >
> >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=20
> >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=20
> > > ground to chassis ground in boards? What is the pros and
> cons for SI
> > > and/or EMI?=3D20
> > >
> > > Scheme 2: Cable keeps separate logic ground and chassis
> ground.
> > > System/boards also keep the separate logic and chassis
> ground. Cable=20
> > > 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?=20
> > >
> > > 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!=20
> > >
> > > Jinhua Chen
> > > SI of Hardware Engineering
> > > EMC Corp.
> > >
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