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From: Wanderer on 14 Jul 2010 16:40
On Jul 14, 2:54 pm, "Jacques St-Pierre" <multe...(a)globetrotter.net>
wrote:
> "Wanderer" <wande...(a)dialup4less.com> wrote in message
>
> news:4ecb4dea-3df5-4050-996a-d3433713895a(a)d8g2000yqf.googlegroups.com...
> On Jul 14, 11:23 am, "Jacques St-Pierre" <multe...(a)globetrotter.net>
> wrote:
>
> > When you say you power the circuit from a separate supply are you
> > including the ADC and Vref? My guess is the noise would becoming
> > through the reference either through Vref or Ground. Try putting a cap
> > to ground at the non inverting input on the first opamp.
>
> Yes when we use a separate supply for our analog +5Vdc supply, it's include
> the analog section of the A/D too.
> We try to put a 10uF Tantalum capacitor on Vref, the positive input and
> output of the first Opamp with no impact on the problem.
> We try to raise the T filter capacitors from 47uF to 100uF on the analog
> +5Vdc with no result.
> We try to raise the 10uF capacitor to 100uF on the power pin of the Opamp
> with no result.
> We change the 100K to 1K with no result.
> The only thing that appear to stop the problem, is to put a large capacity
> on Vout, but it can not be use since it destroy the audio signal.
>
> We try to catch the glitch on the Vref and power pin of the Opamp, but we
> did not catch it yet with our digital scope.
>
> Bye
> Jacques

10uF and 100uF caps are kind of large for decoupling caps. They may
not have the frequency response you're looking for. Try using some
0.1uF or 0.01uF ceramic caps in parallel with the Tantalums. This link
has some information on caps.

http://dataweek.co.za/news.aspx?pklNewsId=27008
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