[SI-LIST] Re: ferrites article on PCD&F and other stories

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Fri, 01 Apr 2011 18:20:15 -0700

Siming, you can read the paper available on the ipblox website:  
www.ipblox.com/papers.html.  It directly addresses your issues.

A ferrite bead unless properly compenstated introduces simple L-C 
resonances.  Breaking the board up into smaller sections shifts the 
modal frequencies, and increases their Q.

Combining caps and loads does not reduce the worst-case noise.  It 
improves the average case, provided that by combining resonance issues 
were not created that did not exist before.  The resonance issue cuts 
both ways which means care must be taken to make sure comparisons are 
apples to apples.

Steve.

On 4/1/2011 3:14 PM, Siming Pan wrote:
> Hi all,
>     I have a question here: why could board resonance be the price of using
> ferrite beads? I agree that small isolated board would cause severe board
> resonances. However, the first modal resonance is always at several MHz. For
> board-level PDN design, we usually only care about the inductive curve up to
> 100 MHz. Chip with proper on-die decap and package will never see those
> board resonances.
>     I think one advantage of using shared power plane for multiple chips is
> that all decaps on this power plane can be shared to low the impedance.
> However, the total loop inductance is consisted by two portions. One is the
> inductance caused by power vias connecting with decaps and the other portion
> is inductance due to IC power pin. Although by combing the power planes the
> inductance of decap via can be reduced, the mutual effects caused by mutual
> inductance of the IC power pins may worse the situations, even when the IC
> noise current of multiple chips is at the similar level. So I always use
> ferrite beads to isolate chips and control the PDN impedance for each chip.
> Although to use ferrite beads might not be the best solution to optimize
> decaps, it is always safe to prevent interferences.
> Regards,
> Siming Pan
> Cisco Systems
>
> On Fri, Apr 1, 2011 at 1:54 PM, Eric Bogatin<eric@xxxxxxxxxxxxxxx>  wrote:
>
>> Hi folks-
>>
>>
>> I thought you might be interested in the piece I wrote for PCD&F magazine
>> based on Steve Weir's DesignCon talk on ferrites. Check it out here:
>>
>>
>>
>>
>> http://pcdandf.com/cms/component/content/article/171-current-issue/7893-sign
>> al-integrity-insights.
>>
>>
>>
>> You can add comments to the article on the PCD&F site, if you are so
>> inclined.
>>
>>
>>
>> I've also been writing a blog for EE Times on SI topics, based on
>> interviews
>> with experts.
>>
>>
>>
>> http://www.eetimes.com/electronics-blogs/4207357/BeTheSignal/
>>
>>
>>
>> If you have a suggestion for a topic or someone for me to interview, drop
>> me
>> a note- offline, please!
>>
>>
>>
>> Additional info on other SI topics is on my web site, www.beTheSignal.com
>> <http://www.bethesignal.com/>  .
>>
>>
>>
>> --eric
>>
>>
>>
>> *******************************************************
>> Dr. Eric Bogatin, Signal Integrity Evangelist
>> Bogatin Enterprises
>> Setting the Standard for Signal Integrity Training
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>> EE Times Blog:
>> <http://www.eetimes.com/electronics-blogs/4207357/BeTheSignal>
>> http://www.eetimes.com/electronics-blogs/4207357/BeTheSignal
>> beTheSignal Blog:<http://www.bethesignal.com/blog>
>> www.beTheSignal.com/blog
>> 26235 W 110th Terr
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-- 
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV  89501
www.ipblox.com

(775) 299-4236 Business
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