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

[Dave Manley]

Also if the pic has digital outputs see if there is an option for slew rate control.  Fast edge rates can cause current spikes on the supply.

-Dave

On January 26, 2017 8:04:05 AM PST, yo at vacoloco.net wrote:
>Tom,
>
>  I have to admit I've never been a fan of star grounds, principally 
>because you end up making the return paths long and overly complex so 
>you get an increase in ground resistance and hence a larger
>differential 
>between the two, and often noise too.
> I assume this is a two layer board? If you can, I'd suggest rolling a 
>four layer board, not much more cost but *SO* much better for noise.
>Top 
>layer is signal, 2nd layer is a solid common ground, 3rd layer is
>power, 
>again using big tracks/planes and the lower layer is signal again. I'd 
>also ground fill top and bottom layer.
>
>You might also look at adding some inductors before the grounds hit the
>
>common 'star' earth to try and stop any noise coming along the ground 
>that way. Similarly look at inductors on the input to the regulators 
>(REG1 and REG2), and even go as far as adding one for each of the rails
>
>that supplies the opamp (in this case the ground here is effectively 
>your -ve supply rail.
>
>  Not sure that helps.
>P
>
>
>On 2017-01-26 15:34, Tom Wiltshire 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
>> 
>> 
>> 
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org
>> http://synth-diy.org/mailman/listinfo/synth-diy
>_______________________________________________
>Synth-diy mailing list
>Synth-diy at synth-diy.org
>http://synth-diy.org/mailman/listinfo/synth-diy