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

  • From: "Michael Rose" <mrose@xxxxxxxxxxxx>
  • To: <leeritchey@xxxxxxxxxxxxx>, "Scott McMorrow" <scott@xxxxxxxxxxxxx>
  • Date: Mon, 15 Sep 2008 16:49:06 -0400

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 <mailto:scott@xxxxxxxxxxxxx>  

        To: leeritchey@xxxxxxxxxxxxx 

        Cc: Vinu Arumugham <mailto:vinu@xxxxxxxxx> ; Michael Rose
<mailto:mrose@xxxxxxxxxxxx> ; si-list <mailto:si-list@xxxxxxxxxxxxx> 

        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(r) 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 <mailto:vinu@xxxxxxxxx>  

                To: Scott McMorrow <mailto:scott@xxxxxxxxxxxxx>  

                Cc: Lee Ritchey <mailto:leeritchey@xxxxxxxxxxxxx> ;
Michael Rose <mailto:mrose@xxxxxxxxxxxx> ; si-list
<mailto:si-list@xxxxxxxxxxxxx> 

                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(r) 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>
<mailto:mrose@xxxxxxxxxxxx> 
                                To: SI-List <si-list@xxxxxxxxxxxxx>
<mailto: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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