Thought I'd share a few spectra I've been getting since connecting a Prop2 smartpin in random dithered DAC mode to
an ADS8885 18 bit SAR ADC.
The main conclusions are that the switchmode 3.3V supply isn't suitable for audio (no surprise there), and that its pretty soft
at audio frequencies (not regulating well), due to the low power operation with no other loads. You get modulation of audio
bandwidth signals from the DAC pin showing up as sidebands a few kHz either side that vary a lot with temperature.
The intermodulation distortion from the DAC pin at standard drive levels is about -70 dB.
Loading the DAC reduces IM distortion somewhat (presumably due to lower voltage excursion) - I have tried both 500 ohm
AC coupled load and 5k AC coupled load, the loading with 500 ohms notably reduces amplitude but slightly improves IM
Disclaimer: The ADC circuit is pretty primitive with rudimentary reference voltage (blue LED+capacitor), and no buffering
on the inputs. The ADC circuit is powered directly from one of the VIO pins, either connected to linear or switch mode.
5k load, linear regulator for DAC pin and for ADC circuit (separate pin group), two tones about 11.2 and 11.8kHz, 0.5V rms
or so (the top of screen is nominally about 3.5Vrms)
With heavier load the amplitude goes down (scale is 10dB per division, 1.25kHz/division):
With both DAC and ADC on switch mode supply we see modulation effects around the tones
After the board warms up the modulation effects reduce in intensity:
With one powered by linear regular, other by switch mode, the spurious signals are larger as the rails
are different, which ever way round we configure the linear/switch mode: