[SI-LIST] Re: power plane spacing

• From: e <evillaf@xxxxxxxx>
• To: si-list@xxxxxxxxxxxxx
• Date: Wed, 11 Jul 2001 23:10:37 -0700
```Steve,

"S. Weir" wrote:

> Ellis,
>
> I guess Dorin should reply, but I don't think his question was about
> vertical spacing,

I didn't think it was,  I did not mean to imply it, and if I did,  I apologize,
I
should be more clear.  I will work on it.

Ellis

> 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.
>

This is my understanding also.

Ellis

>
> 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.
>

I believe this is what I stated, though in more simple terms and with much less
detail.  Nonetheless, the point is the same.  I just imagined the 'noise'
looking
across at the edge of the adjacent plane, and then down at the ground. There is
just so much more 'visible' area to ground, and with typical FR-4 power/ground
spacing of 3-5 mil, the ground is also much closer => much more capacitance =>
much less impedance.  Of course, integration works, too.  No doubt it would have
been beneficial to work the math, but this is a good first step (in the right
direction I hope), and at originally I thought it was enough to convey the idea
without cluterring it with (necessary) details.

I would have liked to get feedback from you on the stackup I proposed.  If there
is anything lacking about it I would like to know what it is (they are) before I
use this stackup in my next project.  I don't remember anything that was ever
proposed in this group that was not criticized -- and I think that is a good
thing.

One thing that was not addressed here is crosstalk.  While shielding may reduce
EMI problems, it does nothing to prevent signal integrity degradation to nearby
signals within the shields,  due to the radiating fields that were causing the
EMI
problems.

Regards,
Ellis

>
> 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
> >
> >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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