[SI-LIST] Re: Multiple power planes coupling to ground
- From: istvan Novak <Istvan.Novak@xxxxxxx>
- To: Joseph Pankow <jhpankow@xxxxxxxxx>
- Date: Mon, 22 Sep 2008 08:19:18 -0400
Joseph,
As always, it depends... This stackup works fine as long as we make
sure that the
power planes are used primarily for low-frequency power distribution,
where the
capacitors attached to the planes will decouple the power planes from
each other.
At high frequencies VCC2 primarily couples to VCC1 and VCC3. This is
reciprocal,
so noise on VCC3 is pushed on VCC1 and VCC3 and vice versa. Also chances
are that these plane layers are split, and in that case chances are that
the splits do
not line up vertically in the three power planes (if they did, why would
we bother
using separate planes?). The plane shapes in any of the three power planes
over splits will create bridges between the power domains also horizontally.
So the structure at high frequencies may have a large number of cross
couplings,
making it more challenging to ensure that each power rail is properly
bypassed.
Regards,
Istvan Novak
SUN Microsystems
Joseph Pankow wrote:
> Hello all,
> If you have multiple power planes sandwiched between 2 ground planes -
> somewhere in your stackup - next to each other like so:
>
> |
> GND
> |
> VCC1
> |
> VCC2
> |
> VCC3
> |
> GND
> |
>
> From a high-frequency noise filtering/decoupling standpoint, VCC1 and VCC3
> have relatively good capacitive coupling to ground, but what about VCC2?
>
> I understand that there will be capacitive coupling between VCC2 and the
> other planes, but does this still give me a reasonably good filtering scheme
> for VCC2 at high frequency? Or does it just couple VCC2 noise onto my other
> power planes?
>
> I'm trying to visualize the loop currents to ground when devices draw power
> from VCC2. Will the VCC2 power noise will couple directly through the other
> power planes to ground, or will the other power planes will act like shields
> so that VCC2 no longer has good low-inductance filtering for high frequency
> noise?
>
> Any insight would be appreciated.
>
> Thanks,
> Joseph
>
>
>
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