Re: [SI-LIST] : Regarding plane splits

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From: Itzhak Hirshtal (hirshtal@is.elta.co.il)
Date: Sun Dec 31 2000 - 22:53:01 PST


Jan-1-01

Hello, again

After a long wait I haven't get any response from you regarding this issue, I re-iteate my
question.
Does anyone else know if my description below is correct?

Thanks again and have a good new year

Itzhak Hirshtal wrote:

> Dec-6-00
>
> Lee,
>
> So what you say is that if my 2 split voltage planes are well decoupled to GND by being
> adjacent to a GND plane, then I've got essentially a continuous return path for the AC
> current. Its path will be from the 1st sub-plane to the GND underneath and then back to
> the 2nd sub-plane. Do I understand it correctly?
>
> How can one estimate how much decoupling one needs, and whether a certain amount of plane
> capacitance is enough? Is there a calculator or simulator for this kind of calculation?
>
> Thanks
>
> --
> Itzhak Hirshtal
> Elta Electronics
> Ashdod
> POB 330
> Israel
> Tel: 972-8-8572841
> Fax: 972-8-8572978
> email: hirshtal@is.elta.co.il
>
> Ritchey Lee wrote:
>
> > Properly done decoupling produces a very low impedance between Vcc and ground at all
> > of the frequencies involved in switching. Since most of the edges are a nanosecond
> > or less, this means a good plane capacitor. Very few, if any, application notes even
> > acknowledge that this is needed. Therefore, most engineers don't get this right and
> > have lots of high frequency noise on Vcc.
> >
> > Lee
> >
> > Itzhak Hirshtal wrote:
> >
> > > Hello Mr. Lee,
> > >
> > > What do you mean by saying "the decoupling is not done well"? Can you detail what
> > > is the good way to do it?
> > >
> > > Thanks
> > >
> > > Ritchey Lee wrote:
> > >
> > > > That is a DC view of an AC problem. When the decoupling is done well enough
> > > > to provide the switching currents required to create the fast edges, the two
> > > > sides of the split are at the same AC potential, namely that of the underlying
> > > > plane. That's because you shorted them to this plane with the capacitors.
> > > >
> > > > Clearly, if the decoupling is not done well, this is not true. Also, if the
> > > > decoupling is not done well, there will be excessive noise on both VCC
> > > > planes. This will show up in many ways, one of them being potentially high
> > > > EMI.
> > > >
> > > > Lee
> > > >
> > > > Chan, Michael wrote:
> > > >
> > > > > What happen if the split is for two different voltage planes?
> > > > >
> > > > > MChan
> > > > >
> > > > > -----Original Message-----
> > > > > From: Ritchey Lee [mailto:leeritchey@earthlink.net]
> > > > > Sent: Wednesday, November 22, 2000 11:39 AM
> > > > > To: Zabinski, Patrick J.
> > > > > Cc: si-list@silab.eng.sun.com
> > > > > Subject: Re: [SI-LIST] : Regarding plane splits
> > > > >
> > > > > All of these discussions fail to take into account the fact that the
> > > > > ground plane and the two power planes msut be well decoupled in order to
> > > > > create a low impedance source for the switching currents that are involved
> > > > > in those same swithcing edges. If this had been done well, the power
> > > > > planes will, of necessity, be shorted to the ground plane and that will be
> > > > > the path around the split.
> > > > >
> > > > > In my experience, people who see the effects of a split have failed to do
> > > > > a good job of power plane decoupling. Learn how to do this well and
> > > > > splits won't bother you.
> > > > >
> > > > > Lee
> > > > >
> > > > > Zabinski, Patrick J. wrote:
> > > > >
> > > > > > Aloke,
> > > > > >
> > > > > > As I mentioned in a recent posting, there are conditions in which
> > > > > > routing stripline in a configuration you describe sees no effects
> > > > > > from the split. More specifically, if your trace layer is "closer"
> > > > > > to the solid ground plane than the split power planes, then the
> > > > > > solid plane has "more" of an influence on the trace. As such, you
> > > > > > will "less" of a discontinuity from the split. I've tested this
> > > > > > in the lab under several conditions, and I believe this to be true.
> > > > > >
> > > > > > However, the reason I used "'s in the above statements is that I
> > > > > > haven't taken my experiments far along enough to be able to provide
> > > > > > any guidance as to how "close" is "close enough" to reduce the
> > > > > > discontinuity effects to the point where your system can tolerate
> > > > > > them.
> > > > > >
> > > > > > In what I call a 50/50 case where the stripline layer is centered
> > > > > > vertically in the stackup such that the distance to the solid ground
> > > > > > plane is the same as the distance to the split plane, you will notice
> > > > > > the discontinuities.
> > > > > >
> > > > > > Pat
> > > > > >
> > > > > > >
> > > > > > > Hello all,
> > > > > > > I had a doubt regarding plane splits:
> > > > > > >
> > > > > > > In the stackup, if there is a power plane on one side of the signal
> > > > > > > layer and a ground plane on the other side of the signal
> > > > > > > layer(symmetric
> > > > > > > stripline config), and if the ground plane is a solid ground plane
> > > > > > > having no discontinuities, then can I have splits in the
> > > > > > > power plane and
> > > > > > > run traces over the splits? Is the ground plane alone not
> > > > > > > sufficient to
> > > > > > > provide paths for return currents?
> > > > > > >
> > > > > > > With regards,
> > > > > > > Aloke--

Itzhak Hirshtal
Elta Electronics
Ashdod
POB 330
Israel
Tel: 972-8-8572841
Fax: 972-8-8572978
email: hirshtal@is.elta.co.il



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