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

[Mike Beauchamp]

> Also, you use Vref for the wrong purposes in many places all over your
> schematics.  While that may not be the main contributor of noise, it
> doesn't help doing daft things.

Vref on this schematic is just his Virtual Ground, so I don't think it's 
being misused - just mislabelled. Maybe I'm missing it but I don't see 
it ever being used as a proper reference voltage for the DAC/ADC.

 From what I see, there's a 47uF cap on the fake -4.5V rail, but no 
filtering on the +4.5V rail or the 9V input. Also, no decoupling caps as 
mentioned previously.

Some simple things:

Are you testing this circuit with batteries, or a power supply? What 
does the noise on the power supply measure? What's the lighting in your 
shop and does your circuit noise measure the same with those turned off? 
Is this noise you actually hear once the pedal is in real-world use (in 
an enclosure,etc ) or just noise you see on a scope? What does the chip 
datasheet say the noise floor should be?

After that, my only suggestion would be to add your noise measurements 
directly onto your schematic at a bunch of different places and then 
post that to a few different electronics forums.

Mike