Loading ...
Sorry, an error occurred while loading the content.
 

Re: [beam] Noise suppression caps question

Expand Messages
  • Dave Lag
    Wilf covered why distribute the capacitance. Another isssue- While capacitance is the dominant charateristic it is not the only property of a cap. ESR and
    Message 1 of 5 , Mar 31, 2006
      Wilf covered why distribute the capacitance.
      Another isssue-
      While capacitance is the dominant charateristic it is not the
      only property of a cap. ESR and inductance of the leads can come into play.
      The value may not be uniform across "frequencies" or among materials
      used in the manufacture.
      Bypassing a big cap with one decades smaller is common practice,
      trying for the best of both worlds.
      one link: http://download.micron.com/pdf/technotes/TN0006.pdf
      For an audiophile slant use Walt Jung's article, its one of the
      classics, was at capacitor.com?...

      HTH
      Dave

      wilf wrote:

      > Main reason is distributed inductance of the circuit . Current
      > transients are rapid changes in current which can generate
      > voltage transients across the inductance of the conductor through
      > which the current flows. Since the power supply leads are widely
      > distributed these voltage transients can appear in many places in the
      > circuit. It is these voltage transients that appear on power supply
      > leads of logic chips that can cause logic input levels (which use the
      > local powersupply voltage for reference) to change. In a digital
      > system, voltage transient induced logic changes causes havoc. Proper
      > layout of power conductors is the first line of defence by
      > isolating conductors that carry high current from the rest of the
      > circuit. Using distributed small capacitors will provide many
      > localized low impedance paths for high
      > frequency current thereby reducing "ground bounce" from being
      > misinterpreted by the circuit as logic changes.
      >
      > wilf
      >
      > ----- Original Message -----
      > *From:* Joseph Charles <mailto:jodicalhon@...>
      > *To:* beam@yahoogroups.com <mailto:beam@yahoogroups.com>
      > *Sent:* Friday, March 31, 2006 4:57 AM
      > *Subject:* [beam] Noise suppression caps question
      >
      > Hi all,
      >
      > I've been thinking (never a good thing!) and realise I'm uncertain
      > about what
      > is going on with noise supression caps across supply lines.
      >
      > Across the supply lines of a noisy circuit you might see a 100uF
      > cap, a 10uF
      > cap, and a 0.1uF cap, perhaps a smaller one as well. Presumably
      > these are to
      > deal with voltage transients of differing speed, or width.
      >
      > If they are there to provide a low impedance path to earth for
      > high frequency
      > spikes why not just use the biggest cap and leave off the others?
      > The larger
      > cap will pass a greater range of frequencies to earth than the
      > others, which
      > would seem redundant.
      >
      > I'm missing something simple, I know!
      >
      > Cheers
      >
      > Jo
      >
      > (I have my hand poised to slap myself on the forehead while
      > muttering "Of
      > course...." )
      >
      >
      >
      >
      >
      >
      > ------------------------------------------------------------------------
      > YAHOO! GROUPS LINKS
      >
      > * Visit your group "beam
      > <http://groups.yahoo.com/group/beam>" on the web.
      >
      > * To unsubscribe from this group, send an email to:
      > beam-unsubscribe@yahoogroups.com
      > <mailto:beam-unsubscribe@yahoogroups.com?subject=Unsubscribe>
      >
      > * Your use of Yahoo! Groups is subject to the Yahoo! Terms of
      > Service <http://docs.yahoo.com/info/terms/>.
      >
      >
      > ------------------------------------------------------------------------
      >
    • Joseph Charles
      Thanks Wilf, Dave and Leo. I think I m getting the idea. Cheers Jo
      Message 2 of 5 , Apr 1, 2006
        Thanks Wilf, Dave and Leo. I think I'm getting the idea.

        Cheers

        Jo
      Your message has been successfully submitted and would be delivered to recipients shortly.