[SI-LIST] Re: plane-to-plane decoupling
- From: "Tony Luan" <luant@xxxxxxxxxxx>
- To: "Michael Rose" <mrose@xxxxxxxxxxxx>, "SI-List" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 15 Sep 2008 10:35:04 -0700
Hi Mike,
Adding decaps between two power islands may not have smaller
inductance path. Also it increases the noise coupling between two power
nets, which in most of cases is what we want to avoid.
The better way is to swap layer 5 and layer 6 if it is possible. Or
add decaps between two power islands to gnd.
Thanks
Tony
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Michael Rose
Sent: Monday, September 15, 2008 9:57 AM
To: SI-List
Subject: [SI-LIST] plane-to-plane decoupling
I looking for some suggestions regarding decoupling between co-planar
plane splits. I'm working on a backplane with a number of 3.125Gbps diff
pairs. I've specified a dual stripline stackup assigned as follows:
1 - P
2 - G
3 - S
4 - S
5 - P
6 - G
and so on
Some diff pairs on L4 will cross power plane splits (actual different
power sources and loads) and I wanted to provide an effective AC path
for any common-mode return currents. I was thinking about placing some
nearby decoupling caps from plane-to-plane across the split. Do you
think it would be better to decouple from plane-to-ground on both sides
to steer the current through the L6 ground layer? L5 and L6 are already
coupled through the inter-plane capacitance (they're about 4mils apart).
Which will provide a lower inductance path?
Mike
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