[SI-LIST] Re: plane-to-plane decoupling
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: Scott McMorrow <scott@xxxxxxxxxxxxx>
- Date: Tue, 16 Sep 2008 16:43:10 -0700
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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>>>>>
>>>>>
>>>>
>>>>
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>>>
>>>
>>
>>
>
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