RPi Pico ADC Characterization and Issues

[ferret_guy]

I was disappointed in the details provided for the RP2040 ADC. So I decided to test it myself! I did some testing of a few ADC specs and posted them here: http://pico-adc.markomo.me/

The biggest thing is that the ADC has some significant DNL issues. It looks like those were caused by a systematic error in the capacitors for the DAC portion of the SAR ADC.



I also discovered the "Low-Noise" mode of the DC-DC converter on the Pico actually has more noise (and the same or worse performance) than the high-efficiency mode.

To head off some complaints, I wanted to test the Pico; I was not attempting to measure the RP2040 chip itself, so all the noise, etc... from the Pico is included.

Below is a listing of the specs I measured and their results (along with links to the relevant posts)

Specification	Test Method	Result PSM	Result PWM
Power Supply Noise and Spectrum	Oscilloscope	1.0mV RMS	1.3mV RMS
Input-Referred Noise	Histogram	0.97 LSB RMS	1.36 LSB RMS
Noise Free Resolution	Histogram	9.3 Bits	8.8 Bits
Effective Resolution (Not ENOB)	Histogram	12 Bits	11.6 Bits
Effective Number of Bits (ENOB)	Sine Curve Fit	8.64 Bits	8.48 Bits
Effective Number of Bits (ENOB)	Coherent Sampling/FFT	7.89 Bits	7.84 Bits
Signal to Noise and Distortion (SINAD)	Sine Curve Fit	53.77 dB	52.82 dB
Signal to Noise and Distortion (SINAD)	Coherent Sampling/FFT	49.27 dB	48.96 dB
Differential Nonlinearity	Sine Histogram	8.9 LSBs	Unchanged
Integral Nonlinearity	Sine Histogram	7.85 LSBs	Unchanged