[SI-LIST] Re: power plane spacing
- From: e <evillaf@xxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 11 Jul 2001 21:41:31 -0700
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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