# Introduction

The RP2040 datasheet has a woefully underspecified Analog to Digital Converter, only 3 lines:

Compare this to the Digital IO Characteristics (Table 631); it has 18 lines! And ADCs have much more complexity and nuance than a Digital IO (however, Digital IO blocks have surprising complexity).

In this project, I will explain different ADC specifications, how they can be characterized, and the results of those tests on the PR2040.

## Issues Found

During this project, I found some issues and inconsistencies with the ADC.

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.

The other thing I discovered is the “Low-Noise” mode of the DC-DC converter on the Pico has more noise (see below for the performance impacts) than the high-efficiency mode.

## Specifications:

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 | 8.92 Bits | 8.84 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 |

Input Bandwidth | Sine Sweep | TBD | |

Aperture Jitter? | |||

Power Supply Rejection Ratio? |

## Posts

### Integral and Differential Nonlinearity (INL/DNL)

### Signal to Noise and Distortion (SINAD)

### Effective Number of Bits (ENOB)

### Input-Referred Noise and Noise-Free/Effective Resolution

### Power Supply Noise and Spectrum Testing

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