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

[yo at vacoloco.net]

I totally missed that, too busy looking at PSU problems and grounds.
yes, decoupling caps (but they'll need to be to your virtual ground).

On 2017-01-26 16:11, Jason Tribbeck wrote:
> 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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