[SI-LIST] Re: Split gnd planes - for/against?
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Steve Weir" <weirsi@xxxxxxxxxx>
- Date: Tue, 7 Apr 2009 12:41:51 -0700
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
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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
> >>
> >> Main office
> >> (401) 284-1827 Business
> >> (401) 284-1840 Fax
> >>
> >> Oregon office
> >> (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
> (866) 675-4630 Business
> (707) 780-1951 Fax
>
> Main office
> (401) 284-1827 Business
> (401) 284-1840 Fax
>
> Oregon office
> (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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