[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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>
>
>
>   



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