Datasheet

Nominally an ADC moves from one digital value to the next digital value, colloquially expressed as “no missing codes”.
However, if the ADC skips a value bin this would cause a spike in the Differential Non-Linearity (DNL) error. These types
of error often only occur at specific codes due to the design of the ADC.
The RP2040 ADC has a DNL which is mostly flat, and below 1 LSB. However at four values — 512, 1,536, 2,560, and
3,584 — the ADC’s DNL error peaks, see Figure 117
Figure 117. ATE
machine results for
DNL (RP2040).
The INL and DNL errors come from an error in the scaling of some internal capacitors of the ADC. These capacitors are
small in value (only tens of femto Farads) and at these very small values, chip simulation of these capacitors can
deviate slightly from reality. If these capacitors had matched correctly, the ADCs performance could have been better.
These INL and DNL errors will somewhat limit the performance of the ADC dependent on use case (See Errata RP2040-
E11).
4.9.5. Temperature Sensor
The temperature sensor measures the Vbe voltage of a biased bipolar diode, connected to the fifth ADC channel
(AINSEL=4). Typically, Vbe = 0.706V at 27 degrees C, with a slope of -1.721mV per degree. Therefore the temperature
can be approximated as follows:
T = 27 - (ADC_voltage - 0.706)/0.001721
As the Vbe and the Vbe slope can vary over the temperature range, and from device to device, some user calibration
may be required if accurate measurements are required.
The temperature sensor’s bias source must be enabled before use, via CS.TS_EN. This increases current consumption
on ADC_AVDD by approximately 40 μA.

NOTE
The on board temperature sensor is very sensitive to errors in the reference voltage. If the ADC returns a value of
891 this would correspond to a temperature of 20.1°C. However if the reference voltage is 1% lower than 3.3 V then
the same reading of 891 would correspond to 24.3°C. You would see a change in temperature of over 4°C for a small
1% change in reference voltage. Therefore if you want to improve the accuracy of the internal temperature sensor it
is worth considering adding an external reference voltage.
RP2040 Datasheet
4.9. ADC and Temperature Sensor 584