Re: tiny inductor with every bypass cap?

Terry Given wrote:
> Tim Wescott wrote:
>> Joerg wrote:
>>
>>> Tim Wescott wrote:
>>>
>>>> Talal Itani wrote:
>>>>
>>>>> Hello,
>>>>>
>>>>> I was looking at the schematics for a DSP-based board, running at 
>>>>> 100 MHz. They have a tiny inductor with every bypass cap around the 
>>>>> DSP.  Do you think this is necessary?  This DSP has analog stuff 
>>>>> built-in.  If we do not need analog, can the inductors be
eliminated?
>>>>>
>>>>> Thanks,
>>>>> T.I.
>>>>>
>>>> Search for newsgroup postings with "Jeorg" and "ground" or 
>>>> "grounding" in them.
>>>>
>>>> You'll get a load of (AFAIK) good opinions.
>>>>
>>>
>>> Thanks for the kudos. It would have to be "Joerg" though. Sometimes I 
>>> wish I had an easier name.
>>>
>> 'O' before 'E' unless I'm at sea?
>>
>> Dunno why I can't keep it straight.
>>
>>>
>>>> Inductors in series with the caps would tend to isolate the power 
>>>> supply from noise in the DSP, but it would also create a bunch of 
>>>> odd resonances.  It's not how I'd want to isolate a power supply 
>>>> from a chip.
>>>>
>>>
>>> It will become really interesting when the DSP exhibits a somewhat 
>>> burst-like load behavior. On the scope it'll look like Dolphins 
>>> frolicking in the ocean.
>>>
>> That's kinda what I thought.  Plus I see no reason to do each power 
>> line individually, and some good reasons not to (Different versions of 
>> VDD at different points in the circuit, oh boy!).
>>
> 
> they are also probably ferrite beads, rather than inductors per se. but 
> I have corrected a few designs where the "engineers" really did use 
> inductors. nasty little bobbin core things. yuk!
> 

It never fails to impress clients when changes consist of lots of parts 
being replaced with a snippet of wire.


> I've seen quite a few app notes with FBs liberally sprinkled everywhere.

> I presume this is because its easier than thinking. that being said, I 
> have a design thats soon to undergo EMC testing where I have exactly 
> followed the manufacturers recommendations for the FPGA & HY ****ps, but 
> I plan on muntzing most of the FBs during a day at the EMC lab. odds on 
> I can leave ALL of them off...
> 

You had Muntz TV sets in NZ???

-- 
Regards, Joerg

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