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

Jason Tribbeck jason at tribbeck.com
Thu Jan 26 17:11:46 CET 2017

You don't seem to have any decoupling capacitors for the op-amps.

And I would also suggest small inductors (or even ferrite beads) going from
the 9v line to the input of each of the regulators.

On Thu, 26 Jan 2017 at 15:41 Tom Wiltshire <tom at electricdruid.net> wrote:

> Hi All,
> I'd like some advice. I recently did a PCB for my DigiDelay project:
> http://electricdruid.net/diy-digital-delay/
> This is a commercial project in as much as I'm selling chips and PCBs. If
> my asking for help for that bothers you, please stop reading here and
> accept my apologies.
> The pedal is good, but it has more hiss than I'd like. A certain amount is
> going to be inevitable because the circuit uses the dsPIC's on-chip 12-bit
> ADC for audio, and the delay processing is only 16-bit. But that's not the
> problem I'm seeing.
> The problem I've got is high frequency noise on the 9V power supply rails.
> This looks like spikes at the 64KHz sample rate (the audio is sampled every
> other sample, the interleaved samples are used for the control pots). The
> spikes are causing an oscillation at around 2.7MHz. This comes out as basic
> white noise on the audio.
> There's an image of the top of the PCB here:
> http://electricdruid.net/wp-content/uploads/2016/12/DigiDelayComponentValues.jpg
> I thought I'd done as much as I could to reduce interference between the
> digital and the analog sides of the circuit: The PCB divides the two areas
> completely (analog on the left, digital on the right), both have separate
> ground planes. The grounds only meet at a single point back at the power
> supply input. The dsPIC's analog and digital 3.3V supplies are generated by
> separate regulators. This is all supposed to help, right?!
> It seems to me there must be some simple thing I can do to remove such a
> high frequency. The audio from the dsPIC is already heavily filtered once
> it's back in the analog world. This helps (there's less noise after the
> filters than before them), but it's not eliminating it, I guess since the
> noise is getting in through the op-amps' power pins.
> http://electricdruid.net/wp-content/uploads/2016/10/DigiDelay-Schematic-Pg1.jpg
> http://electricdruid.net/wp-content/uploads/2016/10/DigiDelay-Schematic-Pg2.jpg
> http://electricdruid.net/wp-content/uploads/2016/10/DigiDelay-Schematic-Pg3.jpg
> Any pointers or things I could try would be appreciated.
> I'd like to get this properly sorted since I'd like to do a rack version
> with delay time modulation like the old Ibanex DM2000.
> Thanks,
> Tom
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