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. 

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?

Seems overkill.
ADI reference designs don't use anything like that, although they follow 
PSU chips with an inductor.

-- 
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
Remote Viewing classes in London

Talal Itani wrote:
> Hello,
> 
> I was looking at the schematics for a DSP-based board, running at 100 MHz.

Great, which DSP ?

> They have a tiny inductor with every bypass cap around the DSP.  Do you 
> think this is necessary?  

Depends, which DSP ??

This DSP has analog stuff built-in.  If we do not
> need analog, can the inductors be eliminated?
> 
> Thanks,
> T.I. 

It must be a secret, what does the manufacture of the DSP say ??


donald

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.

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.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Sat, 21 Jun 2008 16:06:02 GMT, "Talal Itani" <t...@verizon.net>
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. 
>

Inductors don't generally help digital chips, and may actually reduce
timing margins. We do use ferrite bead+capacitor filters on the supply
rails of some fast opamps and adc's, to keep switcher noise and
other-channel crosstalk from sneaking in.

The best way to power big digital chips is with solid power planes,
reasonably bypassed. That will present lower rail impedances than you
could get by isolating the bypass caps on a per-pin basis.

John

On Jun 21, 12:06 pm, "Talal Itani" <tit...@verizon.net> 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.

Is it possible the inductors represent the inductance of the layout,
rather than actual parts on the BOM?

On Sat, 21 Jun 2008 16:06:02 GMT, "Talal Itani" <t...@verizon.net>
wrote:


>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
 

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.





>
>
>>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
> 

I cannot tell.   I do not have BOM.  The schematics are here, a zip file 
that has 2 PDF files.  The large PDF file has the schematics.  The inductors 
are at the top-left corner of the page.  Thanks.



> Is it possible the inductors represent the inductance of the layout,
> rather than actual parts on the BOM? 

On Jun 21, 4:14 pm, "Talal Itani" <tit...@verizon.net> wrote:
> I cannot tell.   I do not have BOM.  The schematics are here, a zip file
> that has 2 PDF files.  The large PDF file has the schematics.  The inductors
> are at the top-left corner of the page.  Thanks.
>
> > Is it possible the inductors represent the inductance of the layout,
> > rather than actual parts on the BOM?

That's interesting, the resonance frequency for 100nF with 50uH is
about 21KHz. I wonder what the resonance-decoupling-multi-value caps
crowd has to say about this design?