Power Supply Noise and Spectrum Testing
Overview
The performance of an ADC starts with its power supplies. The RP2040 chipset has a separate power supply input for the ADCs, which should be as low noise as possible for best performance. The Pico can change its regulator’s mode to reduce noise. As we will see below, it turns out that the analog supply has more noise in the low-noise setting!
Contents
Schematic
AVDD
The Raspberry Pi Pico has an RC filter on the ADC Power supply to filter out noise from the 3V3 converter. It has a cutoff frequency of 360kHz to try and filter out the switching regulator noise. The ADC_VREF pin can also be connected directly to a shunt regulator, like the suggested LM4040 (it even comes in an easy to breadboard PTH package).
Switching Regulator
The Raspberry Pi Pico has a RT6150B-33GQW DC-DC converter to generate the 3.3V rail. It can operate in one of two modes, Power Save Mode (PSM) and Pulse Width Modulation (PWM) mode. PSM mode has more noise (see below) than the PWM mode; however, it is more efficient at light load.
Test Methodology
To measure power supply noise, you need to keep the loop area as small as possible. I measured all noise with a short spring clip. Using an MSO5000 with a 20MHz BW Limit (20MHz is a standerd bandwidth for power supply noise measurements). The output spectrum was collected with a PXIe-5164, which has a much lower noise floor and is better suited for spectral analysis.
Surprising Results
RMS and Pk-Pk values
Click on the links to see the oscilloscope screenshots.
| Power Rail | RMS Noise | Pk-Pk Noise |
|---|---|---|
| VBUS (USB In) (Power Save Mode) | 5.1mV | 44.0mV |
| VBUS (USB In) (PWM Mode) | 3.9mV | 30.7mV |
| 3V3 (Power Save Mode) | 11.1mV | 114.8mV |
| 3V3 (PWM Mode) | 3.9mV | 30.7mV |
| ADC_VREF (Power Save Mode) | 1.0mV | 10.0mV |
| ADC_VREF (PWM Mode) | 1.3mV | 7.1mV |
You can see something very interesting in the above data, the RMS and Pk-Pk noise is lower in PWM mode (“low noise mode”) than PSM mode except for the noise on the ADC_VREF line, which is the point of controlling the mode of the converter.
The RMS noise is the noise figure that will drive degradation in the output quality.
AVDD Output Spectrum
Looking at the spectrum, you can see that most of the Power Save mode’s energy is focused at lower frequencies, where the PWM mode has several very high-frequency peaks. The higher frequency of the noise, the harder it is for the ADC to reject. In addition, sharp frequency spikes may show up on spectral outputs as noise or tones.
Conclusion
It looks like the system may have higher performance when it is NOT in the “Low Noise” PWM mode rather than the Power Save mode. Further testing will be needed.