[SI-LIST] Re: power plane spacing
- From: "S. Weir" <weirsp@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 11 Jul 2001 21:18:39 -0700
Ellis,
I guess Dorin should reply, but I don't think his question was about
vertical spacing, it was how wide to make the gap between the islands on
the split plane layer. The concern was that a wide gap would lead to an
EMI problem, whereas a small gap would couple noise from one plane to the
other along the edges.
An integration of the capacitance along the edge shows that it is very very
small compared to the plane capacitance, unless the islands are very long
and very narrow, ie traces. Consequently, the plane to ground impedance of
each plane is much lower than the split plane to split plane
capacitance. Let me put it another way, the best coupling we can
typically get on a differential pair is 10-15%. Two adjacent plane islands
will have orders of magnitude weaker coupling.
Regards,
Steve.
At 09:41 PM 7/11/01 -0700, you wrote:
>Dorin,
>
>I suppose if noise from an adjacent power partition reaches the plane split,
>it would see a lower impedance to ground via plane capacitance (assuming
>your power planes are tightly coupled with ground planes) than to the other
>adjacent power plane, and so it would take that path to ground.
>
>One limit I can think of to how small the split should be (with an eye on
>the power-ground spacing) is manufacturability. Sensitive analog supplies
>do require special considerations, but it does not seem like this applies in
>your case.
>
>I can imagine if ground currents were circulating that they could cause
>radiation to 'leak' out where where there is a split, but then I'm no expert
>like most of the people here are (and I'm always imagining things anyway).
>Whether valid or not, consider this stackup:
>
> signal --
> solid ground ----------
> signal --
> signal -- <= be careful routing over split
>
> broken plane ---- -----
> solid ground -----------
> signal --
> signal --
> solid ground ----------
> broken plane ---- ----
> signal -- <= be careful routing over
>split
> signal --
> solid ground ------------
> signal --
>
>The outer ground layers should prevent any 'leaks'.
>
>Ellis
>
>
>
>DORIN OPREA wrote:
>
> > Hi everyone,
> >
> > In high speed design we often use the split planes on a power layer. The
> > spacing between the digital planes is correlated with the distance
> > between the power layer to the ground layer. In general we have 50 mil
> > spacing. It just happen to run into a different opinion which recommends
> > a very small spacing due to the slot antenna effect (the gap between the
> > planes) that allows the eddy currents to radiate. A small spacing may
> > couple noise into the low power supply (1.5V or 1.8V).
> > How critical is this radiation effect versus noise coupling between the
> > two power distribution systems ?
> > What is the practice out there ?
> >
> > Thanks very much for you guys always being there,
> > Dorin
> >
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