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Yes—the RP2040’s 12-bit ADC has a documented silicon erratum. It can show unusually large differential-nonlinearity (DNL) errors around four code regions and may become non-monotonic, meaning the reported value can occasionally fail to rise as the input voltage rises. The problem was investigated and publicly discussed in 2021, then documented as erratum RP2040-E11.
That does not make every Pico ADC reading unusable. The internal ADC remains suitable for many low-cost, slow-changing measurements, but it should not be treated as an ideal 12-bit precision instrument. Applications requiring guaranteed monotonicity or tightly specified linearity should use calibration, signal conditioning, or an external ADC.
What Raspberry Pi investigated
In January and February 2021, developers reported unexpectedly large DNL excursions in the RP2040 ADC, including possible non-monotonic behavior. A technical report filed in Raspberry Pi’s pico-feedback repository on February 13, 2021 described measurements that were inconsistent with ordinary small ADC noise.
Raspberry Pi representatives said the behavior was understood and that additional characterization would be added to the datasheet. Raspberry Pi subsequently attributed the apparent root cause to a mismatch between the capacitors used in the ADC’s capacitive digital-to-analog converter (DAC) in production hardware and the values used during simulation. The issue was later formalized as RP2040-E11 in the RP2040 datasheet.
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The important current distinction is that this is not an ongoing 2021-style investigation. It is a documented characteristic of RP2040 silicon.
The short technical explanation
An ADC divides an input-voltage range into digital code bins. For an ideal 12-bit converter there are 4,096 possible codes, from 0 through 4,095. At a 3.3-volt reference, one ideal code represents approximately:
3.3 V / 4096 = 0.8057 mVThat is nominal resolution, not guaranteed accuracy. Real ADC performance also depends on effective number of bits (ENOB), integral and differential non-linearity, reference accuracy, noise, temperature, source impedance, grounding, and the quality of the signal being measured.
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What DNL means
Differential non-linearity describes how much the width of an individual code bin differs from the ideal one-LSB width. A large positive DNL error means a code represents an unusually wide input interval. A sufficiently negative DNL error can produce a missing code. In severe cases, the transfer curve can become non-monotonic: increasing the analog input can produce the same code, an unexpectedly large step, or occasionally a lower code.
This is different from random noise. Noise makes repeated readings vary around a value. DNL is a defect in the converter’s transfer function. Averaging can reduce random noise, but it cannot reliably reconstruct a code transition that is intrinsically irregular.
Which RP2040 ADC codes are affected?
The published characterization identifies prominent DNL regions around:
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| Approximate code | Hexadecimal | Ideal voltage with a 3.3 V reference |
|---|---|---|
| 512 | 0x200 | 0.4125 V |
| 1,536 | 0x600 | 1.2375 V |
| 2,560 | 0xA00 | 2.0625 V |
| 3,584 | 0xE00 | 2.8875 V |
These voltage figures are only ideal conversions. The RP2040 ADC uses its analog supply as its reference, so the corresponding voltages move when that supply changes. The Pico datasheet provides the relevant hardware details.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsDo not interpret the list as four isolated “bad output values” that can simply be discarded. The irregularity concerns the neighborhoods around the transitions and the widths of the surrounding code bins. A reading of exactly 512, for example, is not automatically invalid; the input may legitimately be near that transition, and the error may affect adjacent behavior as well.
Does the flaw affect every Pico?
The issue is in the RP2040 ADC design, not in one particular Pico PCB revision. It can therefore affect Raspberry Pi Pico, Pico W, and other boards built around RP2040 silicon.
That does not mean every board will produce equally visible errors. Analog-supply filtering, grounding, layout, input-source impedance, temperature, and measurement equipment can make the observed result better or worse. Those factors can reduce additional noise, but they do not remove the underlying RP2040-E11 erratum.
The Raspberry Pi Pico 2 uses the RP2350 rather than the RP2040. Its silicon and errata are different, so reports about one device should not automatically be applied to the other.
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No. Three error sources should be separated:
- RP2040-E11: a transfer-function problem associated with the capacitive DAC and its DNL behavior.
- Reference variation: the ADC reference is tied to the analog supply, so supply tolerance and ripple directly affect voltage conversion.
- Board-level noise and settling: switching regulators, digital activity, grounding, wiring, GPIO configuration, and high source impedance can add further errors.
A 2025 Raspberry Pi forum discussion shows why diagnosis matters: unexpected readings can also result from GPIO pulls, supply behavior, or source impedance. Those are separate failure modes from the silicon erratum. Before blaming RP2040-E11, configure the pin correctly and verify the signal source.
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What “12-bit ADC” means in practice
The RP2040 still returns a 12-bit raw code. It has not literally been reduced to an 8-bit converter. However, Raspberry Pi’s current Pico SDK documentation describes approximately 8.7 effective bits.
ENOB is a summary of practical noise-and-linearity performance; it is not the same measurement as DNL and does not mean the lowest four bits are always worthless. The safer interpretation is that the converter’s useful performance is closer to an ideal 8.7-bit converter than to an ideal 12-bit instrument.
The simple conversion below is therefore only an estimate:
voltage = raw * 3.3 / 4096It assumes the ADC reference is exactly 3.3 V. In a real design, the analog supply may differ from 3.3 V or contain ripple. Divider tolerance, ground offsets, sensor error, temperature drift, and ADC gain and offset errors may exceed one ideal LSB by a wide margin.
Basic Pico ADC setup
The official SDK sequence initializes the ADC, configures the GPIO as an ADC input, selects a channel, and reads the result:
#include <stdio.h>
#include "pico/stdlib.h"
#include "hardware/adc.h"
int main() {
stdio_init_all();
adc_init();
// GPIO26 is ADC input 0.
adc_gpio_init(26);
adc_select_input(0);
while (true) {
uint16_t raw = adc_read();
float voltage = raw * 3.3f / 4096.0f;
printf("raw=%u voltage=%.4f Vn", raw, voltage);
sleep_ms(100);
}
}RP2040 user ADC inputs are GPIO26 through GPIO29, corresponding to ADC inputs 0 through 3. ADC input 4 is connected to the internal temperature sensor. The SDK documentation specifies a maximum sample rate of up to 500 kS/s, but maximum speed does not imply precision or monotonicity.
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Practical ways to reduce problems
Configure the ADC pin correctly
Use adc_gpio_init() for the selected input. Check that digital pulls are not left enabled and that the pin is not being configured elsewhere in the program. A GPIO pull or an incorrect alternate function can make a correct ADC appear defective.
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Follow Raspberry Pi’s recommended analog-ground and power arrangement. Keep noisy digital currents away from sensitive analog paths and provide appropriate local supply decoupling. A cleaner analog supply improves reference stability and repeatability, but it does not cure RP2040-E11.
Keep source impedance low enough
The ADC samples through an internal switching network. A high-value resistor divider or high-impedance sensor may not charge the sampling capacitance quickly enough. Use a lower-impedance divider, an appropriately selected capacitor, or a buffer amplifier when the application requires it.
For a battery divider, resistor tolerance can also dominate the result. A 1% divider may introduce far more error than one ideal ADC count. Use precision resistors and calibrate the assembled product if absolute voltage matters.
Average slow signals carefully
For battery voltage, temperature, light level, and similar slow signals, multiple samples followed by averaging or low-pass filtering can improve repeatability:
uint32_t sum = 0;
for (int i = 0; i < 32; ++i) {
sum += adc_read();
sleep_us(100);
}
uint16_t average = sum / 32;A trimmed mean can reject an occasional outlier:
uint16_t samples[16];
uint32_t sum = 0;
uint16_t minimum = 4095;
uint16_t maximum = 0;
for (int i = 0; i < 16; ++i) {
samples[i] = adc_read();
sum += samples[i];
if (samples[i] < minimum) minimum = samples[i];
if (samples[i] > maximum) maximum = samples[i];
sleep_us(100);
}
uint16_t trimmed_average = (sum - minimum - maximum) / 14;Filtering reduces uncorrelated noise and improves repeatability. It is not a complete mathematical repair for deterministic DNL or non-monotonicity.
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Calibrate the complete signal chain
Calibration can compensate for gain and offset errors, divider ratios, and some repeatable system behavior. It cannot guarantee recovery of information lost at a non-monotonic transition. Calibrate at the temperatures, supply conditions, and operating range that matter to the product.
Reduce effective resolution when appropriate
If the application values monotonic behavior more than fine resolution, discarding the lowest four bits can be a practical workaround:
uint16_t raw = adc_read();
uint8_t result8 = raw >> 4;This deliberately reduces the result to roughly 8-bit granularity and may avoid visible small-scale discontinuities. It should be validated on the actual device and application; it is not a guarantee that every reduced result is perfectly accurate.
Keep critical operating ranges away from transitions
If a design measures only a narrow voltage range, calculate where that range maps in ADC codes. Changing the divider ratio or signal scaling may keep a critical threshold away from one of the four prominent discontinuity regions. This is not a solution for applications that must accurately cover the full ADC range.
When is the internal ADC good enough?
| Application | Likely decision | Reason |
|---|---|---|
| Rough threshold detection | Internal ADC may be adequate | Exact linearity may not matter. |
| Light, temperature, or battery indication | Use with filtering and calibration | Signals are often slow and modest accuracy is acceptable. |
| Closed-loop motor, heater, or power control | Evaluate carefully; external ADC may be preferable | Non-monotonic readings can destabilize a control decision. |
| Precision instrumentation | Use an external ADC | Documented INL/DNL, reference, and monotonicity may be required. |
| Safety-critical threshold | Do not rely on uncharacterized internal ADC behavior | The consequence of an irregular code transition may be unacceptable. |
When an external ADC is the better answer
Choose an external converter when monotonicity is mandatory, absolute accuracy is tighter than the Pico signal chain can support, or the sensor needs a precision reference, differential inputs, programmable gain, or documented INL/DNL performance.
Typical options include:
- TI ADS1115 for slow, higher-resolution I²C measurements, differential inputs, and programmable gain. Its relatively low sample rate makes it unsuitable for high-speed sampling.
- Microchip MCP3008 for multiple single-ended SPI channels. It is not automatically more accurate in every system; reference quality and layout still matter.
- Adafruit’s ADS1115 breakout or SparkFun ADS1015/ADS1115 boards for prototype-friendly wiring.
An external reference, buffer amplifier, or better resistor network can improve the surrounding signal chain. None of those components removes the RP2040 ADC’s silicon DNL defect by itself.
Bottom line
The Raspberry Pi Pico remains a capable, inexpensive microcontroller board, but its RP2040 ADC should be treated as an approximately 8–9-effective-bit peripheral with a known nonlinearity erratum—not as a precision 12-bit instrument. For slow sensors and threshold decisions, correct setup, a clean analog supply, suitable source impedance, filtering, and calibration may be enough. For precision measurement or control that requires guaranteed monotonicity, use an external ADC designed and specified for that job.
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