Linearity of the ESP32 ADC

The ESP32 Analog to Digital Converter (ADC) is not linear. We measured the value returned by the ADC and the real voltage on the pin. We compared these signal to better understand the properties of the ADC.

The following has been performed with an ESP32 FireBettle DFR0654.

Measures

We measured twice the real voltage on the ADC pin with an oscilloscope and with a multimeter.

The oscilloscope voltage is considered as the more accurate measurement since the device average a large number of measurements. In the following, the oscilloscope voltage is considered as the real voltage.

The following data can also be found in this Google Sheet.

Oscilloscope Voltage [V]	Multimeter Voltage [V]	ADC Raw	ADC Voltage [V]	Error [V]
0.000 V	0.000 V	0	0.000 V	0.000 V
0.098 V	0.100 V	0	0.000 V	0.098 V
0.193 V	0.200 V	115	0.093 V	0.100 V
0.296 V	0.300 V	240	0.193 V	0.103 V
0.388 V	0.400 V	364	0.293 V	0.095 V
0.486 V	0.500 V	479	0.386 V	0.100 V
0.581 V	0.600 V	605	0.488 V	0.093 V
0.677 V	0.700 V	725	0.584 V	0.093 V
0.784 V	0.800 V	849	0.684 V	0.100 V
0.875 V	0.900 V	973	0.784 V	0.091 V
0.980 V	1.000 V	1103	0.889 V	0.091 V
1.070 V	1.100 V	1227	0.989 V	0.081 V
1.170 V	1.200 V	1351	1.089 V	0.081 V
1.270 V	1.300 V	1473	1.187 V	0.083 V
1.370 V	1.400 V	1602	1.291 V	0.079 V
1.470 V	1.500 V	1726	1.391 V	0.079 V
1.570 V	1.600 V	1844	1.486 V	0.084 V
1.670 V	1.700 V	1967	1.585 V	0.085 V
1.770 V	1.800 V	2093	1.687 V	0.083 V
1.880 V	1.900 V	2214	1.784 V	0.096 V
1.980 V	2.000 V	2343	1.888 V	0.092 V
2.080 V	2.100 V	2459	1.982 V	0.098 V
2.180 V	2.200 V	2586	2.084 V	0.096 V
2.270 V	2.300 V	2711	2.185 V	0.085 V
2.380 V	2.400 V	2835	2.285 V	0.095 V
2.470 V	2.500 V	2961	2.386 V	0.084 V
2.570 V	2.600 V	3115	2.510 V	0.060 V
2.680 V	2.700 V	3262	2.629 V	0.051 V
2.770 V	2.800 V	3433	2.767 V	0.003 V
2.870 V	2.900 V	3642	2.935 V	-0.065 V
2.970 V	3.000 V	3872	3.120 V	-0.150 V
3.070 V	3.100 V	4094	3.299 V	-0.229 V
3.180 V	3.200 V	4094	3.299 V	-0.119 V
3.270 V	3.300 V	4094	3.299 V	-0.029 V

Charts

The following chart represents the real voltage (oscilloscope voltage) vs the ADC voltage:

The next chart is the error vs pin voltage (oscilloscope voltage).

Conclusion

If you need an accurate measurement, you have several options:

measure only voltage between 0.2 and 2.5V and apply a linear correction ;
measure only voltage between 0.2 and 3V and apply a non-linear correction ;
use an external ADC (I²C or SPI external ADC).

Linearity of the ESP32 ADC

Measures

Charts

Conclusion

See also