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

Plane to plane capacitance is indeed a series element.  If it has been sized 
correctly on each power rail, its impedance in the frequencies you are 
interested in will be a few milliohms as compared to many milliohms for the 
discrete capacitors due to their series inductance.

----- Original Message ----- 
From: Michael Rose 
To: leeritchey@xxxxxxxxxxxxx;Scott McMorrow
Cc: Vinu Arumugham; si-list
Sent: 9/15/2008 1:50:22 PM 
Subject: RE: [SI-LIST] Re: plane-to-plane decoupling


Lee,
 
Actually, I pictured the plane-to-plane caps as a series pass element ? one 
between two distinct power planes (really 2 independent power domains). This is 
why I imagined P-P caps would be more effective than shunt decoupling each 
plane to ground.
 
Mike 
 
 



From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx] 
Sent: Monday, September 15, 2008 4:46 PM
To: Scott McMorrow
Cc: Vinu Arumugham; Michael Rose; si-list
Subject: Re: [SI-LIST] Re: plane-to-plane decoupling
 
Ah!   That's a different conversation than the one that started this thread!
 
Lee
----- Original Message ----- 
From: Scott McMorrow 
To: leeritchey@xxxxxxxxxxxxx 
Cc: Vinu Arumugham; Michael Rose; si-list
Sent: 9/15/2008 12:53:27 PM 
Subject: Re: [SI-LIST] Re: plane-to-plane decoupling
 
Lee

I believe  that Vinu is speaking of using a capacitor as a series pass element, 
rather than as a shunt element. 

Scott



Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
 
http://www.teraspeed.com
 
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC


Lee Ritchey wrote: 
Vinu,
 
Can you show me a capacitor that works at 3.125 Gb/S for decoupling?
 
Lee
 
 
----- Original Message ----- 
From: Vinu Arumugham 
To: Scott McMorrow 
Cc: Lee Ritchey; Michael Rose; si-list
Sent: 9/15/2008 12:24:44 PM 
Subject: Re: [SI-LIST] Re: plane-to-plane decoupling
 
Scott,

I was not suggesting that capacitors connecting split planes were a "clean" 
solution. I just wanted to point out that Lee's statement, "There are no 
capacitors that work at 3.125 Gb/S for decoupling.", is not entirely true.

Thanks,
Vinu


Scott McMorrow wrote: 
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
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
 
http://www.teraspeed.com
 
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
  


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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