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

[Dave Manley]

I took a quick look at the schematics and don't see a way to put a series filter component on either supply.  You might want to build one with the power leads for the pic bent up and add a ferrite bead in series on either or both of the analog and digital supplies.

Add cap(s) to ground after the ferrite.

It's always good practice to allow for this in the PCB.  You can always use a 0 ohm resistor if you don't need the FB.

-Dabe

On January 26, 2017 7:34:22 AM PST, 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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