[SI-LIST] Re: plane-to-plane decoupling

  • From: Vinu Arumugham <vinu@xxxxxxxxx>
  • To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
  • Date: Mon, 15 Sep 2008 11:29:24 -0700

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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