[SI-LIST] Re: Split gnd planes - for/against?
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Steve Weir" <weirsi@xxxxxxxxxx>
- Date: Tue, 7 Apr 2009 14:25:09 -0700
Steve,
I'm interested in people claiming that ground plane splits are a good
answer, to show us.
> [Original Message]
> From: steve weir <weirsi@xxxxxxxxxx>
> To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
> Cc: Charles Grasso <Charles.Grasso@xxxxxxxxxxxx>; Sol Tatlow
<Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>; si-list <si-list@xxxxxxxxxxxxx>
> Date: 4/7/2009 1:27:08 PM
> Subject: [SI-LIST] Re: Split gnd planes - for/against?
>
> Lee, if someone has a real case with A/B characterizations that's
> great. If you are just interested in proving the principles the simple
> card I suggested will do.
>
> Best Regards,
>
>
> Steve.
> Lee Ritchey wrote:
> > Steve,
> >
> > Several people responded with claims of real cases. Those are the ones
> > that should be advanced to support the claims made.
> >
> >
> >
> >> [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 11:59:58 AM
> >> Subject: [SI-LIST] Re: Split gnd planes - for/against?
> >>
> >> Lee, the physics holds up. I'm not in the practice of designing
boards
> >> for my customers that don't work, so I don't have a before and after
of
> >> a physical assembly to show you. If you question the numbers I
offered,
> >> we can build up a hypothetical case in EDA tools if you would like to
> >> see it.
> >>
> >> But I can give you a Radio Shack experiment to demonstrate it. In the
> >> Radio Shack experiment, we need a power supply and a very simple
circuit
> >> board.
> >>
> >> Card 1: Two sided Card 2" x 2" 1oz Cu bottom surface solid plane, top
> >> surface signal test points only.
> >> Four test vias from the plane to the top surface
> >> TPA 0.25, 0.25
> >> TPB 0.25, 1.75
> >> TPC 1.75, 0.25
> >> TPD 1.75, 1.75
> >>
> >> Card 2: Same as Card 1, but with 0.025" moat at 1.00, 0.00 to 1.00,
2.00
> >>
> >> Apply current limited supply set to 1A. between 1.5", 0.00 and 1.5",
2.00
> >> Measure the voltage from TPA to TPB and TPC to TPD
> >>
> >> Card 1, voltage difference from TPC to TPD will be about 380uV,
voltage
> >> difference from TPA to TPB will be a little less.
> >> Card 2, voltage difference from TPC to TPD will be about 750uV,
voltage
> >> difference from TPA to TPB will be virtually zero.
> >>
> >> Now for HF signals a couple of things will happen that are a little
> >> different: First Lenz will confine the crosstalk, so the voltage
ratio
> >> from the signal source to the monitor points will be far less than the
> >> DC case. Second, there will be slight capacitive coupling across the
> >>
> > split.
> >
> >> So we repeat the experiment, except that this time, we use a VNA to
> >> inject at TPC with a terminator at TPD, and monitor in successive
> >> experiments the insertion loss to TPA and TPB. The difference is the
> >> noise signal. For the split plane case the insertion loss will be
much
> >> higher than for the single plane case.
> >>
> >> Best Regards,
> >>
> >>
> >> Steve.
> >>
> >> Lee Ritchey wrote:
> >>
> >>> 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
> >>>>>> (866) 675-4630 Business
> >>>>>> (707) 780-1951 Fax
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> >>>>>> This e-mail contains proprietary and confidential intellectual
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> >
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> >>>>>
> >>>> --
> >>>> Steve Weir
> >>>> Teraspeed Consulting Group LLC
> >>>> 121 North River Drive
> >>>> Narragansett, RI 02882
> >>>>
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> >>> of Teraspeed Consulting Group LLC
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> >>>
> >> --
> >> Steve Weir
> >> Teraspeed Consulting Group LLC
> >> 121 North River Drive
> >> Narragansett, RI 02882
> >>
> >> California office
> >> (866) 675-4630 Business
> >> (707) 780-1951 Fax
> >>
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> >> (401) 284-1827 Business
> >> (401) 284-1840 Fax
> >>
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> >> (503) 430-1065 Business
> >> (503) 430-1285 Fax
> >>
> >> http://www.teraspeed.com
> >> This e-mail contains proprietary and confidential intellectual property
> >>
> > of Teraspeed Consulting Group LLC
> >
> >
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> --
> Steve Weir
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
>
> California office
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> (707) 780-1951 Fax
>
> Main office
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