[SI-LIST] Re: AC coupling capacitors (SATA, PCIe)

  • From: Hermann Ruckerbauer <Hermann.Ruckerbauer@xxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Tue, 18 Sep 2012 00:17:00 +0200

Hi Steve,

thanks for the answer (also to the others who responded) !
Good that you have no time machine, otherwise I might be beaten for the
AC/DC based setup/hold requirements of the DRAMs  ... but I guess there
is no danger, as there is enough distance between us   ;-)

Hermann

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Am 17.09.2012 23:23, schrieb steve weir:
> For most current generation SERDES applications many ordinary MLCCs do 
> just fine provided that you optimize the transition, pad, and anti-pad 
> geometries for maximum transparency.  There are differing schools of 
> thought with regard to whether or not one is better off choosing a 
> capacitor that is filled-out resulting in a thin cover layer, or one 
> uses a capacitor that has a relatively thick cover layer.  Arguments for 
> the former concentrate on the electrical superiority of that choice, 
> while arguments for the latter concentrate on the supply source 
> stability of the latter.  Economics dictate that today's thin cover 
> layer capacitor, is tomorrow's thick cover layer capacitor.
>
> If I had a time machine I would go back and beat the engineering 
> ramifications of channel percentage bandwidth into the PC-SIG folks who 
> designed PCI-Express, forcing us into ridiculously large coupling caps, 
> and VCO stability requirements.  The time for that is far behind us, so 
> we have to live with these things.  0201s are now available in large 
> enough capacitances to meet the spec.
>
> Steve.
>
> On 9/17/2012 1:20 PM, Hermann Ruckerbauer wrote:
>> Hello Rohit,
>>
>> thanks for the answer, and sorry for my bad description.
>>
>> I hope to clarify what I ment:
>>   Any kind of Capacitor that is specially designed for AC-Coupling
>> (DC-Blocking) for a specific application (PCIe, SATA, ...) e.g. :
>> - low ESR/ESL caps / high Q caps / controlled ESR caps
>> - Special material
>> - size other than 0402 (e. g. 0201 or reverse geometry)
>> - Special designed to have the impdeance minimum fitting to the
>> application (I have no idea how to fit 10MHz from plot in the link below
>> to e.g. 3GHz of a SATA interface).
>> - ... ????
>>
>> I found a lot of documents dealing with Capacitors for PDN, but not too
>> much for Capacitors used as AC-coupling in serial link applications.
>> e. g.     www.murata.com/products/article/pdf/ta1082.pdf
>>
>> Currently I can optimize my board e. g. in order to avoid the
>> Capacitance of the pads, but I have somehow no idea whether I can use
>> some optimized capacitors as AC-Coupling of a SATA or PCIe interface ..
>>
>> Any feedback welcome ..
>>
>> Hermann
>>
>>
>>
>> Our next Events:
>> ================
>>
>> "Open the Black Box of Memory"
>> Seminar on 08/09. November 2012
>>
>> Check our website or contact us for details
>>
>> EKH - EyeKnowHow
>> Hermann Ruckerbauer
>> www.EyeKnowHow.de
>> Hermann.Ruckerbauer@xxxxxxxxxxxxx
>> Veilchenstrasse 1
>> 94554 Moos
>> Tel.:        +49 (0)9938 / 902 083
>> Mobile:      +49 (0)176  / 787 787 77
>> Fax: +49 (0)3212 / 121 9008
>>
>> Am 13.09.2012 12:38, schrieb Rohit MISHRA:
>>> Hello Hermann,
>>>
>>> I don't know what do you mean by "special AC coupling capacitors" but there 
>>> was a paper from Yaochao Yang & his team in DesignCon 2012, Titled " 
>>> Embedded DC Blocking Capacitors in Connectors - Study of Impacts on PCB 
>>> Design"
>>>
>>> You can have a look, May be you can find what you are looking for.
>>>
>>> Rgds,
>>> Rohit
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
>>> Behalf Of Hermann Ruckerbauer
>>> Sent: Thursday, September 13, 2012 2:34 PM
>>> Cc: SI LIST
>>> Subject: [SI-LIST] AC coupling capacitors (SATA, PCIe)
>>>
>>> Hi,
>>>
>>> i have done a short search on special AC coupling capacitors for serial
>>> interfaces  (PCIe, SATA, .. ) , but found nothing really special.
>>> I have found Steves DesignCon 2012 presentation, but this deals more
>>> with characterization than performance requirements.
>>>
>>> Is there something better available, than a standard C0G/X7R  capacitors ?
>>>
>>> Thanks for any feedback ..
>>>
>>> Hermann
>>>
>>> Our next Events:
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>>> "Open the Black Box of Memory"
>>> Seminar on 08/09. November 2012
>>>
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>>> EKH - EyeKnowHow
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>>> www.EyeKnowHow.de
>>> Hermann.Ruckerbauer@xxxxxxxxxxxxx
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>>> Tel.:       +49 (0)9938 / 902 083
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