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

  • From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
  • To: "Vinu Arumugham" <vinu@xxxxxxxxx>
  • Date: Mon, 15 Sep 2008 16:47:20 -0700

Vinum,

You are forgetting the series inductance of the capacitor.  It doesn't hurt
much when used as an AC coupling capacitor becasuse it is part of a
transmission line that has substantial inductance along its length.  When
you are trying to build a PDS that same inductance renders the capacitors
unusable at frequencies much avove 150 Mhz. 


> [Original Message]
> From: Vinu Arumugham <vinu@xxxxxxxxx>
> To: <leeritchey@xxxxxxxxxxxxx>
> Cc: Scott McMorrow <scott@xxxxxxxxxxxxx>; Michael Rose
<mrose@xxxxxxxxxxxx>; si-list <si-list@xxxxxxxxxxxxx>
> Date: 9/15/2008 4:02:18 PM
> Subject: [SI-LIST] Re: plane-to-plane decoupling
>
> Lee, Scott,
> I don't see the difference whether you want to look at the capacitor as 
> a series or shunt element. A decoupling capacitor may look like a shunt 
> element when it is part of a PDN but that PDN could serve as a series 
> element in the signal return path. So, if a capacitor is acceptable in 
> an AC coupling role in the signal path, the same capacitor should be 
> acceptable as part of a PDN that is a return path for that signal. In 
> other words, think of it as an AC coupling capacitor for the return path 
> instead of the signal path (US Patent 7262974).
>
> For this application, the capacitor only needs to support ~10mA of 
> switching current at 1.5GHz, and a few tens of mV drop across its 
> impedance would be acceptable.
>
> Thanks,
> Vinu
>
> Lee Ritchey wrote:
> > Scott,
> >
> > I suspect you are right, but the thread was about decoupling power
planes.
> >
> >
> >   
> >> [Original Message]
> >> From: Scott McMorrow <scott@xxxxxxxxxxxxx>
> >> To: <leeritchey@xxxxxxxxxxxxx>
> >> Cc: Vinu Arumugham <vinu@xxxxxxxxx>; Michael Rose <mrose@xxxxxxxxxxxx>;
> >>     
> > si-list <si-list@xxxxxxxxxxxxx>
> >   
> >> Date: 9/15/2008 12:53:27 PM
> >> Subject: [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 <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® 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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