[SI-LIST] Re: plane-to-plane decoupling
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Vinu Arumugham <vinu@xxxxxxxxx>
- Date: Mon, 15 Sep 2008 14:45:19 -0400
Vinu
Not quite. As long as there is a ground plane underneath, and close to,
the capacitor, some return path energy will get across. But, there is a
mismatch in impedance between the capacitor and plane, and here is still
an inductive loop for the return energy to get to the capacitor.
Because of this, quite a bit of the common mode energy will be reflected
back into the near end power/ground plane cavity.
regards,
Scott
Scott McMorrow
Teraspeed Consulting Group LLC
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Vinu Arumugham wrote:
> Lee,
> The capacitor used for AC coupling on the signal path, should be able
> work just as good if it were placed on the return path as a decoupling
> capacitor for that signal.
> Each signal trace will of course need a dedicated capacitor to avoid
> additional crosstalk.
>
> Thanks,
> Vinu
>
> Lee Ritchey wrote:
>
>> Michael,
>>
>> There are no capacitors that work at 3.125 Gb/S for decoupling. The way to
>> provide this path is by placing the planes close to each other. I use 3
>> mils all of the time for this purpose. Works greast!
>>
>> Lee Ritchey
>> Speeding Edge
>>
>>
>>
>>
>>> [Original Message]
>>> From: Michael Rose <mrose@xxxxxxxxxxxx>
>>> To: SI-List <si-list@xxxxxxxxxxxxx>
>>> Date: 9/15/2008 10:01:17 AM
>>> 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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>
>
>
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