[sdiy] dare I say - decoupling?

[James Patchell]

And just to reinforce what Paul says...

If a rail to rail bypass seems to work better than rail to ground...you 
better look at your grounds!

I did a linear power amplifier a few years ago that had a persistent 
oscillation.  Replacing the bypass cap from rail to ground with a rail to 
rail got rid of the problem...sort of.  Despite the ground traces being 0.2 
inches wide in 2 oz copper...they were inadequate for the job...the rail to 
ground just made matters actually worse...but what this was telling me was 
"Hey, you dummy...you need to add another layer that is a ground 
plane!"...which I did, finally (I didn't want to...this increased the 
cost)...but the ground plane solved the oscillation problem and the rail to 
ground bypasses worked like they were supposed to.

Harry likes to use the analogy of "Ground == Ocean"....very important 
here...In general, you do not need a ground plane.  But when things don't 
seem to be working they way they should...well...

By the way, that power amplifier was for a servo system and would put out 
about +/- 45 volts at 27 amps with a bandwidth of about 20KHz...I might 
have been expecting just a bit too much from .2 inch wide ground traces 
(they were not expected to carry 27 amps, but they were the return for the 
bypass caps on the rails which did carry that current....).

-Jim

At 10:46 AM 9/3/2005 -0500, Paul Schreiber wrote:
>Rail-to-ground.
>Rail-to-rail doesn't do *anything* worthwhile. In fact, it can actually add
>noise :)
>
>Paul S.
>
>
>----- Original Message ----- From: "Michael Ruberto" <frankentron at hotmail.com>
>To: <synth-diy at dropmix.xs4all.nl>
>Sent: Saturday, September 03, 2005 10:28 AM
>Subject: [sdiy] dare I say - decoupling?
>
>
>>Well we know who the decoupling experts are here, and they do shine so 
>>brightly. At the risk of sounding like the moron I am I must pose 2 
>>questions to the decoupling kings.
>>which is better, decoupling caps from each rail to ground, from rail to 
>>rail, or both?
>>what's the difference?
>>what is the air-speed velocity of an unladen swallow?
>>thanks in advance oh wise ones ;-)
>>M. A. Ruberto

         -Jim
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