Re: tiny inductor with every bypass cap?

Talal Itani wrote:
> The DSP is a TI F2808.  The schematics I was referring to are here 
> http://www.ti.com/litv/zip/sprr098.
 It is a zip file.  Once you unzip
the 
> file, 2 pdf files appear.  The larger file has the schematics I am
referring 
> to.  The inductors are at the top-left corner of the screen.
> 

To be honest 50uH and 0.1uF is a recipe for disaster. At the most a 
ferrite SMT-bead should be used but usually I don't even do that. A nice 
full ground plane and a nice full VCC plane is usually best. Problem 
with DSP like this is that you need an additional lower voltage supply 
so now you are up to three supply planes, meaning you won't get away 
with less than a 6-layer board.

If it's super critical you could have the analog supplies come from a 
separate regulator but often converters on a chip with fast digital 
processing going on are quite disappointing. A bond wire affords only so 
much in RF conductivity.

Hint: Carefully read up on power supply sequencing. Best case wriobng 
sequencing leads to a locked up DSP, worst case to a dead DSP.

Oh, and please don't top post.

> 
> 
> 
> 
>>
>>> 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?
>> Are you sure that these are ordinary inductors or just a wire through
>> a ferrite bead?
>>
>> While the ferrite will increase the inductance, a suitable ferrite
>> material is also quite lossy at higher frequencies, reducing the risk
>> for unwanted resonances with the capacitors.
>>
>> Paul
>>
> 
> 


-- 
Regards, Joerg

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