[SI-LIST] Re: PDN related

Hi, Steve and Rajesh,

Enough caps of one size will work, until your signal content goes too 
far past the resonance frequency.
And yes, different packages have different parasitics and different 
resonance freqs.

Using a variety of (carefully selected) caps covers a wider frequency 
range (several decades).
At the same time, this reduces the total number of caps needed, and 
quite possibly total cost.
These days, there are EDA tools to make this easier.

And I agree wholeheartedly with Lee that there are plenty of good books 
out there.  Istvan Novak's
home page lists several of them.  http://www.electrical-integrity.com/

Lynne


steve weir wrote:
> And yet there are any number of boards and systems that have been 
> built using mostly one capacitor value such as 0.1uF that work fine.
>
> Steve.
>
> Scooby Doo wrote:
>> Hi Lynne,
>>  
>> This is what exactly what is wanted to ask. If the slew rate is 
>> 1v/ns, then my 0.1uF capacitor value (designers blindly using value) 
>> will not support due to existance inductive nature at that frequency.
>>  
>>  
>> So kindly give some reference / notes so that choosing right value.
>>  
>>  
>>  
>>
>> --- On Mon, 3/15/10, Lynne D. Green <lgreen22@xxxxxxxxxxxxxx> wrote:
>>
>>
>> From: Lynne D. Green <lgreen22@xxxxxxxxxxxxxx>
>> Subject: [SI-LIST] Re: PDN related
>> To: "Scooby Doo" <si.scooby@xxxxxxxxx>
>> Cc: "SI LIST" <si-list@xxxxxxxxxxxxx>
>> Date: Monday, March 15, 2010, 5:20 PM
>>
>>
>> Depends on the frequency range where you need PDN clean.
>> If it is a wide range (generally true) then one would use
>> more than one value of capacitor.  Remember, capacitor
>> impedance increases at high frequency due to inductive
>> package effects.
>>
>> Lynne
>>
>>
>> "IBIS training when you need it, where you need it."
>>
>> Dr. Lynne Green
>> Green Streak Programs
>> http://www.greenstreakprograms.com
>> 425-788-0412
>> lgreen22@xxxxxxxxxxxxxx
>>
>>
>> Scooby Doo wrote:
>>  
>>> Hi SI Experts,
>>>     I have a question reg PDN.
>>>   Suppose an FPGA has 100 I/O pins with 10 VCCO/Gnd pairs. So an 
>>> average of 10 I/O pins for 1 VCCO/Gnd pair. Assume each I/O pin 
>>> driving 10pf load.
>>>   My qn is, to make the clean PDN, is it enough to provide 100pf of 
>>> capacitor to each Vcc/Gnd pair?
>>>   I am not convinced with blindly provinding 0.1uF cap to Vcco pins.
>>>   kindly clarify.
>>>   Thanks in advance for your valuable feedback.
>>>   Rajesh
>>>
>>>        
>>
>>
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