tiny inductor with every bypass cap?
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From: Talal Itani on 21 Jun 2008 12:06 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.
From: Dirk Bruere at NeoPax on 21 Jun 2008 12:41 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
From: donald on 21 Jun 2008 13:42 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
From: Tim Wescott on 21 Jun 2008 14:24 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
From: John Larkin on 21 Jun 2008 14:45
On Sat, 21 Jun 2008 16:06:02 GMT, "Talal Itani" <titani(a)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 |