[SI-LIST] Re: Split gnd planes - for/against?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Richard Jungert <r_jungert@xxxxxxxxxxx>
- Date: Tue, 07 Apr 2009 16:50:58 -0700
My $0.02 is use moats only where appropriate and then only sparingly,
and always with proper analysis. As Lee has seen again and again, they
have backfired on many an engineer who misapplied them. My first
recommendation is always to try and get the required performance from
placement. Careful placement always pays big dividends and is worth the
time to get right. When placement alone won't do, then moats become an
option. Just make sure the cure is not worse than the disease.
Steve.
Richard Jungert wrote:
> Guys.
>
> I had a very similar case back in 1985 where a sensitive phase lock
> loop ground was between the lower frequency PLL and the power entry to
> the board. It was directly in line where power physically enters the
> board and low F PLL. There were actually 3 PLL's on the same board. It
> took us about 4 months to get this one solved but we ended up cutting
> a moat shape ( like others have mentioned previously ) in the ground
> plane under the sensitive PLL to get the phase jitter to stop. The
> solution worked great and the board ended up in production for a new
> product because of this. This is why I strongly recommend power and
> ground splits to protect sensitive circuits.
>
> Richard Jungert
> 2 patents
> over 160 board designs
> S parameter evaluations
> http://www.rjungert.com
>
>
>
>
>
>
>
>
>
> > From: duane.mattheisen@xxxxxxx
> > To: leeritchey@xxxxxxxxxxxxx; weirsi@xxxxxxxxxx
> > CC: Charles.Grasso@xxxxxxxxxxxx; Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx;
> si-list@xxxxxxxxxxxxx
> > Date: Wed, 8 Apr 2009 00:16:56 +0200
> > Subject: [SI-LIST] Re: Split gnd planes - for/against?
> >
> > Lee,
> > Origanally we had a solid ground plane that was uder all of the
> devices and the current was returnig to the power supply. A gain of
> 100dB is a voltage gain of 100,000 so it does not take much movement
> in the ground to get oscilations.
> >
> > Regards Duane
> > -----Original Message-----
> > From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
> > Sent: 2009 Apr 07 2:29 PM
> > To: Duane Mattheisen; Steve Weir
> > Cc: Charles Grasso; Sol Tatlow; si-list
> > Subject: RE: [SI-LIST] Re: Split gnd planes - for/against?
> >
> > Why were the currents allowed to travel this route?
> >
> >
> > > [Original Message]
> > > From: Duane Mattheisen <duane.mattheisen@xxxxxxx>
> > > To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>; Steve Weir
> <weirsi@xxxxxxxxxx>
> > > Cc: Charles Grasso <Charles.Grasso@xxxxxxxxxxxx>; Sol Tatlow
> > <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>; si-list <si-list@xxxxxxxxxxxxx>
> > > Date: 4/7/2009 5:45:07 PM
> > > Subject: [SI-LIST] Re: Split gnd planes - for/against?
> > >
> > > I had a combiner limiter that had about 100dB of gain, it was used
> in a
> > satilite reciever. There were three aplifier stages to bring up the gain
> > to 100dB. Oscilations would happen because the last output stage's
> current
> > would run through the ground plane and pass by the input of the first
> > stage. The oscilations were stoped by making a trench in the ground
> plane
> > at the middle stage of the amplifiers and moving the ground wire to the
> > bottom of the trench.
> > >
> > >
> > > Regards, Duane Mattheisen
> > >
> > > -----Original Message-----
> > > From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > On Behalf Of Lee Ritchey
> > > Sent: 2009 Apr 07 11:28 AM
> > > To: Steve Weir
> > > Cc: Charles Grasso; Sol Tatlow; si-list
> > > Subject: [SI-LIST] Re: Split gnd planes - for/against?
> > >
> > > Steve,
> > >
> > > You have described a hypothetical case here. I'm looking for a
> real one.
> > >
> > > You didn't reply with "you will have to get a case yourself" as
> some do.
> > > Refusing to supply an example to support claims is tantamount to
> making
> > > things up.
> > >
> > > If a claimer can't show a case where a rule works, the claim
> should not be
> > > made.
> > >
> > > Lee
> > >
> > >
> > > > [Original Message]
> > > > From: steve weir <weirsi@xxxxxxxxxx>
> > > > To: <leeritchey@xxxxxxxxxxxxx>
> > > > Cc: Charles Grasso <Charles.Grasso@xxxxxxxxxxxx>; Sol Tatlow
> > > <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>; si-list <si-list@xxxxxxxxxxxxx>
> > > > Date: 4/7/2009 11:13:14 AM
> > > > Subject: Re: [SI-LIST] Re: Split gnd planes - for/against?
> > > >
> > > > Lee and this will be true in the majority of cases, and almost
> without
> > > > exception where a board is digital only. For an example of where a
> > > > split works, and it must be done properly or all bets are off:
> > > >
> > > > Circuit region 1, noise sensitivity is in the uV's. Let's assume
> the
> > > > power / ground separation is 4 mils. The spreading inductance is
> > > > roughly 128pH / square. Now assume that there is an adjacent
> circuit
> > > > region that has a 64 bit memory bus on it. Let's assume very
> > pedestrian
> > > > 200ps rise times on 18mA swing. That bus is switching just over
> > > > 5E9A/s. If circuit region 1 has a noise limit of 10uV across its
> > length
> > > > and a square aspect ratio, then we need over 90dB isolation. We
> aren't
> > > > going to get there with placement and bypass caps alone. But we can
> > get
> > > > there by including well designed moating
> > > >
> > > > You are absolutely correct that many people put moats in they don't
> > need
> > > > and worse do so in a way that creates other more serious problems.
> > > > Moats are sort of like ferrite beads: they are tools that have
> > > > particular value is specific circumstances. They always come with a
> > > > price. Their must be justified as needed and appropriate, and once
> > used
> > > > engineered correctly.
> > > >
> > > > Best Regards,
> > > >
> > > >
> > > > Steve.
> > > > Lee Ritchey wrote:
> > > > > I've taught my high speed class to more than 7000 engineers and
> > > designers
> > > > > over the years. In each class, I ask for examples where
> splitting a
> > > ground
> > > > > plane actually made a circuit work better with the promise to
> add the
> > > > > example to my class.
> > > > >
> > > > > To date, there have been no examples provided. There has been
> a bit
> > of
> > > > > hand waving on the topic but no clear examples that can be
> defended.
> > > The
> > > > > usual reason is "we've done it this way for years and it has
> worked."
> >
> > > To
> > > > > me, that sounds an awful lot like the man who jumped off the
> 20 story
> > > > > building and reported as he passed the 10th floor, "so far, so
> good".
> > > > > Splitting the ground plane just hasn't shown up as a problem, not
> > that
> > > is
> > > > > actually fixed anything.
> > > > >
> > > > > I have fixed EMI problems several times by removing splits in
> ground
> > > > > planes. Some of see this as easy money!
> > > > >
> > > > > I'll make the same offer to this group. Show me an example where
> > > splitting
> > > > > ground planes helps and I'll make it a part of my course.
> > > > >
> > > > > Lee Ritchey
> > > > >
> > > > >
> > > > >
> > > > >> [Original Message]
> > > > >> From: Grasso, Charles <Charles.Grasso@xxxxxxxxxxxx>
> > > > >> To: steve weir <weirsi@xxxxxxxxxx>; Sol Tatlow
> > > > >>
> > > > > <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>
> > > > >
> > > > >> Cc: si-list <si-list@xxxxxxxxxxxxx>
> > > > >> Date: 4/7/2009 9:57:52 AM
> > > > >> Subject: [SI-LIST] Re: Split gnd planes - for/against?
> > > > >>
> > > > >> We use split planes all the time.
> > > > >> When you have circuits of *vastl8 different noise floors
> co-existing
> > > > >> on one board - it's the only way to go.
> > > > >>
> > > > >> Chas
> > > > >>
> > > > >> -----Original Message-----
> > > > >> From: si-list-bounce@xxxxxxxxxxxxx
> > > [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > > > >> On Behalf Of steve weir
> > > > >> Sent: Tuesday, April 07, 2009 10:11 AM
> > > > >> To: Sol Tatlow
> > > > >> Cc: si-list
> > > > >> Subject: [SI-LIST] Re: Split gnd planes - for/against?
> > > > >>
> > > > >> Sol, unfortunately there is not a single answer. In most cases
> > > moating
> > > > >> is a bad idea, particularly if one does not understand the
> caveats
> > and
> > > > >> how to deal with them. It's not just the moats: It's the
> > placement,
> > > > >> clearances, stitching, and routing that all need to be
> considered.
> > > > >>
> > > > >> Steve
> > > > >>
> > > > >> Sol Tatlow wrote:
> > > > >>
> > > > >>> I know this subject has been raised before, countless times
> in one
> > > > >>> guise or another. I have also googled plenty. I'm not
> looking for
> > > > >>> theoretical opinions, either, about whether or not, or when,
> they
> > > > >>> should be used (specifically not, "it depends", unless
> you've got
> > > > >>> REAL-LIFE examples, for and against!!!).
> > > > >>>
> > > > >>> This subject raised its head for me in this case due to using
> > > > >>> 2 A/Ds as well as 2 D/As, both from Analog Devices, where one
> > > > >>> specifies a split plane, the other specifies no split. Now, I am
> > > > >>> all too wary of relying simply on evaluation boards, where, in
> > > > >>> general, one layout is done, and if it works, that's how
> everyone
> > > > >>> should do it (_without_ comparing 2 different approaches).
> > > > >>>
> > > > >>> I personally have 3 concrete cases where split gnds had no
> positive
> > > > >>> effect on SI, but significantly worsened EMC results (despite
> > > > >>> sticking to all the usual guidelines, like no tracks over the
> > > > >>> splits, etc.), but I have no concrete case FOR split ground
> planes.
> > > > >>>
> > > > >>> So, what I'm interested in is: does anyone have CONCRETE
> examples
> > > > >>> which they would like to share for/against split planes? The
> kind
> > > > >>> of thing I mean would be like in one of the cases I had, where I
> > > > >>> wanted to go against the suggested approach of using a split
> gnd,
> > > > >>> and persuaded my customer to pay for 2 variants of the same
> board
> > > > >>> on the same manufacturing panel, one with split ground, one with
> > > > >>> solid ground. Both variants were assembled and tested, with
> regards
> > > > >>> to both SI as well as EMC: both were functionally
> satisfactory; at
> > > > >>> EMC testing, however, the split-plane bombed out big time, while
> > > > >>> the non-split sailed through. I like to think that it wasn't due
> > > > >>> to any screw-ups on my side, that the split ground failed - I am
> > > > >>> not a newbie to PCB layouts, and, while for sure no professional
> > > > >>> expert on all areas of SI, I believe I avoided the typical
> blunders
> > > > >>> often present in split ground layouts.
> > > > >>>
> > > > >>> Anyway, my customer was more than happy, but not everyone
> has the
> > > > >>> money/time/desire to do as I suggested. So, any 'war stories' to
> > > > >>> support one or the other approach would be appreciated to help
> > > > >>> expand my knowledge and understanding of this subject -
> obviously,
> > > > >>> we all respect confidentiality, so I'm not looking for circuits,
> > > > >>> layouts and so on, but I figure many of you must have
> stories that
> > > > >>> can be related regarding this subject. Or perhaps some good
> links
> > > > >>> to non-confidential 'real-life' examples/studies?
> > > > >>>
> > > > >>> Regards,
> > > > >>> Sol
> > > > >>>
> > > > >>>
> > > > >>>
> > > > >> --
> > > > >> Steve Weir
> > > > >> Teraspeed Consulting Group LLC
> > > > >> 121 North River Drive
> > > > >> Narragansett, RI 02882
> > > > >>
> > > > >> California office
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> > > >
> > > >
> > > > --
> > > > Steve Weir
> > > > Teraspeed Consulting Group LLC
> > > > 121 North River Drive
> > > > Narragansett, RI 02882
> > > >
> > > > California office
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