[SI-LIST] Re: Split gnd planes - for/against?
- From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- To: <leeritchey@xxxxxxxxxxxxx>, "Steve Weir" <weirsi@xxxxxxxxxx>, "Sol Tatlow" <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 7 Apr 2009 12:07:47 -0600
Digi guys worry about signals in the mV range - EMI guys worry about
signals in the 100uV range - R.F guys (at least the ones here) worry
about spurious signals in the 1uV range or less.
When you have in-band noise that affects the functionality of a
receiving circuit then splitting planes is a very powerful technique for
mitigating
spurs.
And no - you'll have to get this data yourself!
Chas
-----Original Message-----
From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
Sent: Tuesday, April 07, 2009 11:32 AM
To: Grasso, Charles; Steve Weir; Sol Tatlow
Cc: si-list
Subject: RE: [SI-LIST] Re: Split gnd planes - for/against?
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
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