[sdiy] PCB layout tips

[Seb Francis]

I thought the reason why the bigger caps (i.e. electrolytics) are needed is to prevent resonance due to the LC network formed by all the bypass caps and power bus inductance.  The ESR of electrolytics helps damp the resonance.

The ESR of electrolytics also means that they are not so effective at decoupling very fast current transients.

Personally I use ferrite beads in series where the power comes onto each board, then 20uF+ electrolytics after the ferrite beads, and 100nF multi-layer ceramics (low ESR) as close as possible to each chip.

Seb


Jay Schwichtenberg wrote:

> While we're on the subject of power bus/distribution I want to bring up a
> pet peeve of mine. I look at so many designs and see little or no bypass
> caps in a circuit. Anything that needs speed (which includes our VCOs and
> VCFs) or is switching really should have at least small bypass caps. And if
> they suck power some bigger caps should be put in with little ones. IMHO the
> rules are: digital every chip or two, comparators always, analog depends on
> the layout but every 2 or 3 chips, analog VLZ circuits on all the chips and
> drivers put some bigger caps in there. Also the proper way of doing this is
> power -> cap -> pin/pins not power -> pin/pins -> cap. The latter can in
> some cases turn the lead from the pin to cap into an antenna or possibly
> cause oscillations. It doesn't hurt to get the cap as close to the power pin
> as possible. For grounding I usually don't do an overall star ground system
> (if I did tubes I would). I take each sub-circuit (for example
> inverter/summer or filter) and build a star ground for it and bring that out
> to the main ground. Purists (audiophiles) will argue on doing star grounding
> that way though.
>
> Thanks for the soap box everyone.
> Goooood holidays to one and all.
> Jay
>
> > -----Original Message-----
> > From: owner-synth-diy at dropmix.xs4all.nl
> > [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Scott Juskiw
> > Sent: Sunday, December 15, 2002 6:16 PM
> > To: synth-diy at dropmix.xs4all.nl
> > Subject: [sdiy] PCB layout tips
> >
> >
> > In AN-15 (Electronotes), BH suggests putting +ve and -ve supply lines
> > in two parallel rows underneath the chips. This sounds like a good
> > idea to me. Yet when I look at PCBs of various modules (from people
> > on this list no less), I never see this in practice. Is there some
> > reason why this is in fact a really bad thing to do?
> >
> > Is there a PCB layout "do's and don'ts" list available? Would anyone
> > care to share any layout tips for a newbie?
> >
> >