[sdiy] Mixed-signal problems (dsPIC digital delay project)

Neil Johnson neil.johnson71 at gmail.com
Fri Jan 27 23:19:14 CET 2017

On 27 January 2017 at 21:35, Tom Wiltshire <tom at electricdruid.net> wrote:
> It's a stomp box circuit, so it's a standard audio job, and TL072 is a standard audio op-amp. It'll run at 9V, so no problem. If I was fussed about the current draw, I'd have used a TL062, but in this case, the op-amps are negligible compared to the uP, so there's really no point. In some ways "TL072" is used here as code for "whatever dual op-amp you have in your drawer". I don't care if people fit a LF353 instead. It shouldn't make any odds.
> Why do you ask? You think a different amp would solve my noise problems? Seems unlikely...

Yes, unlikely to be the cause of your noise problem, but I would have
used a rail-to-rail op-amp, at least one who's inputs can go down to
the ground rail.  The op-amp immediately after the DSP sees a
common-mode voltage of about 1.65V on the input, which after the
application of Vref pulls the op-amp inputs to a common of around
2.8V.  That's quite low for the TL072.  The rest of the circuit biases
the inputs to a common of 4.5V which is much better.


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