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

  • From: Vinu Arumugham <vinu@xxxxxxxxx>
  • To: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • Date: Tue, 16 Sep 2008 17:39:37 -0700

Scott,

To keep the via stub small, signals are commonly routed on layers near 
the bottom of the board and connect through vias to the AC coupling 
capacitor mounted on the top side of the board. On dense boards where 
there is no space for ground return vias, the AC coupling capacitor 
seems to be operating with the same poorly-referenced loop as the bypass 
capacitor.

Thanks,
Vinu

Scott McMorrow wrote:
> Vinu
>
> Forget about "inductance" and follow the field patterns in 3D and you 
> will quickly see the difference.  A bypass capacitor operates in a 
> poorly-referenced loop.  When designed correctly, an AC coupling 
> capacitor is operating as one half of a transmission line, where the 
> PCB plane serves as the other half of the line. The differences are 
> significant.  As a result, a very reasonable 50 ohm match for a 
> coupling capacitor can be engineered, with good return loss out to 10 
> GHz.
>
>
> 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:
>> Scott,
>>
>> On many high density 3Gbps layouts,  the AC coupling capacitor has 
>> more loop inductance than the  0.5nH  Steve cited for a decoupling 
>> capacitor. So it is not clear to me how the AC coupling capacitor can 
>> outperform the decoupling capacitor.
>>
>> Thanks,
>> Vinu
>>
>> Scott McMorrow wrote:
>>> Vinu
>>>
>>> Your statement:
>>> '"A single capacitor will still impose a substantial bump in the 
>>> impedance for a single signal.", yes, but the same applies if that 
>>> capacitor were in an AC coupling configuration. '
>>>
>>> Is just not true.  There are significant electromagnetic differences 
>>> between a capacitor used for AC coupling in a through configuration 
>>> and one used for power plane shunt applications.
>>>
>>> Hint: follow the magnetic fields
>>>
>>>
>>> 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:
>>>> Steve,
>>>>
>>>> "A single capacitor will still impose a substantial bump in the 
>>>> impedance for a single signal.", yes, but the same applies if that 
>>>> capacitor were in an AC coupling configuration.
>>>> It certainly has an impact but I don't see it being "unusable" as 
>>>> Lee put it.
>>>>
>>>> "A typical plane cavity will impose a much smaller bump.", yes, but 
>>>> if the 1 sq. in. cavity has to support several links, then the 
>>>> difference between (1) and (2) is not as big.
>>>>
>>>> Thanks,
>>>> Vinu
>>>>
>>>> steve weir wrote:
>>>>> Vinu, I think it is more mind set than anything else.  Let's put 
>>>>> some additional numbers to this:
>>>>>
>>>>> A really well-mounted 0402 capacitor is going to exhibit 0.5nH or 
>>>>> more mounted inductance.  At 1.5GHz that's 5Ohms.  Ignoring 
>>>>> resonances, a typical 3 mil cavity even 1" on a side is going to 
>>>>> exhibit impedance in the 100's of milliOhms.  So:
>>>>>
>>>>> 1) A single capacitor will still impose a substantial bump in the 
>>>>> impedance for a single signal.
>>>>> 2) A typical plane cavity will impose a much smaller bump.
>>>>>
>>>>> Since we are talking differential signaling, the even-mode signal 
>>>>> components should shrink at all frequencies below Fknee, so we 
>>>>> don't need tons of charge storage. Cavity is going to be more 
>>>>> effective.  (But I would far prefer a contiguous return in the 
>>>>> first place.)
>>>>>
>>>>> Steve
>>>>> Vinu Arumugham wrote:
>>>>>> 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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