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

yo at vacoloco.net yo at vacoloco.net
Thu Jan 26 17:13:59 CET 2017


I forgot to add a couple of useful links;

http://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.html

http://www.cn-william.com/uploadfile/Documents/e2v/Application%20Note/Design%20Considerations%20for%20Mixed-Signal.pdf

Hope that helps too

P

On 2017-01-26 16:04, 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
>> 
>> 
>> 
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