PCB worries/ decoupling

Roel Das Roel.Das at student.groept.be
Mon Mar 13 18:35:24 CET 2000


I'm currently working on the pcb of my midiCV project. I need ALL the tips
you can give me. It's a mixed digital/analog design, and I haven't got any
experience on pcb design. First time I ever do this, actually.
I use the eagle lite software for it. It's got lotsa libraries, and is quite
easy to work with. The size is somewhat small (160x100mm) but I can split up
the schemo easily.

And a question on decoupling capacitors. Which values should I use? Will 100
nF do fine? And which kind of capacitors? The DAC716 datasheets suggests 1uF
Tantalum capacitors at the power pins. Does the tantalum part matter? Why?

Thanks
Roel


----- Oorspronkelijk bericht -----
Van: "Harry Bissell" <harrybissell at prodigy.net>
Aan: "Per Mattsson" <XinclXsynth-diy at mkv.mh.se>;
<synth-diy at node12b53.a2000.nl>
Verzonden: zaterdag 11 maart 2000 18:08
Onderwerp: Re:unwanted sync


> Mosk "unwanted sync" events involve iceberg, submarine, etc
>
> see "titanic", "lusitania", etc...
>
> But seriously folks....
> The things to watch for are mostly power supply routes. Use the routes to
force
> current
> to flow where you want it. The best layout would have the oscillators
> physically separated
> at three corners of the board, with common circuits in the fourth...
(makes
> sense yes??)
> The power should start from a single point (the main filter caps right
where
> power enters the board) and run separately to each oscillator. Use
decoupling
> caps for each active (IC) device very near the chip (keeps the capacitance
near
> the current drain, with very little inductance). Inductance will stop the
caps
> from delivering current in time to meet demand, causing voltage drop...
> unstable operation....
>
> IMHO the lack of attention to power supply concerns (esp. placement of
> decoupling) is the greatest cause of a good (schematic) design to perform.
>
> The NEXT BIGGEST ERROR is to share a single IC (maybe a quad op-amp)
between
> separate VCO's. This guarantees that the power supply distribution is WRON
G!
> and also
> puts sensitive circuits within microns of each other, instead of mils
(.001").
> A sure prescription for interaction !!!
>
> Good circuit boards are art. A proper layout looks good, feels good.
Keeping
> symmetry
> between all three VCO's will assure similar performance. Think about
thermal
> issues too, the VCO nearest the top of the enclosure is probably the
warmest
> one...
>
> OTOH... some circuits work because of (intended or unintended)
interaction. The
> MXR
> Bluebox is a prime example.... the 'sneak' path through the power supplies
> actually makes the unit perform 10X better. The designer was either very
lucky
> or very skilled.
>
> IMHO I perfer to keep circuit functions separate... so that changing a
gain
> resistor doesn't
> screw up three other parameters.
>
> Are you going to build and sell these... or are they a kit ??? SMT can be
hard
> with a
> "100 watt American Beauty" (soldering iron).
>
> H^)  Harry
>
> Per Mattsson wrote:
>
> > Hi guys!
> >
> > I'm drawing loose sketches on a three ASM VCO board. To reduce size I
hope
> > to make use of as many SMDs as possible.
> >
> > Are there any specific "demands" on that (or any) VCO board design? A
part
> > from the lin-log components beeing close/glued together. Will I get any
> > unwanted soft sync effects from traces going parallell to long? Or are
the
> > traces on a PCB the size of a ASM VCO simply to short to have any
effects
> > of that kind at all? Or does unwanted sync things has more to do with
> > porely regultated/filtered power than "badly" placed components .
> >
> > This will be a real compact design since I aim to build a four voice,
three
> > oscillator module.  I hope to make room for "global" CV ins as well
voice
> > specific CVs . I have yet no idea on how to patch this unit together
with
> > other polyphonic modules. Maybe matrix switch chips or five pole DIN
> > suggested on the list before.
> >
> > If I were to give board a name it samply has to be  'a ASM VCO SMD PCB',
> > which I find rather fun...
> >
> > ---Per Mattsso
>
>




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