[SI-LIST] Re: Cable grounding scheme

  • From: chen_jinhua@xxxxxxx
  • To: <erdinih@xxxxxxxxx>, <xileil@xxxxxxxxxxx>
  • Date: Tue, 15 Aug 2006 12:12:33 -0400

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.
        > >
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