[sdiy] Differential 12dB/oct filter

Richie Burnett rburnett at richieburnett.co.uk
Wed Apr 27 18:11:41 CEST 2016


> The F373 is pretty cool in that its analog section...

It's a shame they didn't match the 16-bit Sigma-Delta ADC performance with 
the DACs.  Were you able to over-sample the output and dither the DACs to 
mitigate some of the quantisation noise?

I've also found the dsPIC audio DAC outputs to be quite noisy, and it to be 
necessary to follow it with a proper differential amplifier to at least 
cancel out some of the common-mode noise that they push out.

Directed towards any others reading this thread, I would say that good 
layout with separate analogue and digital ground and power planes is a must 
if you want to get close to 16-bit accuracy.

Whilst I agree that the dsPIC DAC isn't fantastic for audio use, I've had 
excellent results using the on-board 12-bit successive-approximation ADC in 
a demanding instrumentation application just by paying close attention to 
design and layout of the PCB.

-Richie,


-----Original Message----- 
From: Eric Brombaugh
Sent: Wednesday, April 27, 2016 4:55 PM
To: Synth-diy at synth-diy.org
Subject: Re: [sdiy] Differential 12dB/oct filter

On 04/27/2016 08:24 AM, Tom Wiltshire wrote:
> Well, I don't know about "excessively", but I'd certainly like less noise. 
> I'm playing with using the 33FJ64GP802 for short delays. I've got audio 
> input into the 12-bit ADC (at 32KHz sampling rate), and audio output from 
> the 16-bit DAC at 64KHz, so it's currently a one-chip-wonder!
> 12-bit audio isn't too bad (after all, 12-bit/32KHz was an expensive 
> studio rack mount delay processor in the middle 1980s!!) and the 16-bit 
> output gives me headroom for processing.

I've done something that's similar, but with STM32F373:

http://ebrombaugh.studionebula.com/synth/f373_mod/index.html

That uses two of the 16-bit SDADCs for input and two of the 12-bit DACs
for output. It sounds pretty good, but suffers from some quantization at
low output levels of course. The F373 is pretty cool in that its analog
section is one of the best I've come across in embedded MCUs - very low
noise. Downside is that there's only 32kB of on-chip SRAM, so you can't
do huge delay effects with it. It does run most kinds of DSP pretty well
though - I've got a few different effects running on it, selectable
through the OLED / button interface.

> It's funny, I haven't had such noise problems with it when doing 
> oscillator designs, so I think perhaps some of my noise is coming from the 
> ADC or the signal going into it. I should probably do everything I can to 
> ensure that the input is smooth and clean before worrying about the 
> output.

The 12-bit ADCs on the dsPIC are pretty noisy. They quote 12-bit in the
spec sheet but I typically see about 2lsb of noise on them. I'm not
surprised that you can hear it.

The 16-bit DACs are no great shakes either. There's a considerable
amount of dithering and limit cycling on the outputs - I can recall
driving a steady DC output signal and seeing noticeable dithering on the
scope trace at the oversample rate. Good reconstruction filtering is a must.

Eric

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