Re: [SI-LIST] : Regarding plane splits

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From: Itzhak Hirshtal (hirshtal@is.elta.co.il)
Date: Wed Dec 06 2000 - 11:50:07 PST


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 > > > > ------------------------------------------------------------------------ > > > > Itzhak Hirshtal <hirshtal@is.elta.co.il> > > Eng > > Elta > > > > Itzhak Hirshtal > > Eng <hirshtal@is.elta.co.il> > > Elta > > Netscape Conference Address > > Netscape Conference DLS Server > > That's myself > > Additional Information: > > Last Name Hirshtal > > First Name Itzhak > > Version 2.1


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