ESP32-C3 ADC issue - reading 4095 at 2.8V

[Mattgo]

Hello, I would like to ask you for help with ESP32-C3 built in ADC.
I tested multiple channels on ADC1 and still reading 4095 at 2.8V

Regarding datasheet effective range for 11db attenuation (ATTEN3, which should be default) is 0-2500mv and reference voltage is somewhere between 1000-1200mV. I have no idea where the 2.8V is coming from. I tried 2 different C3 dev boards and both perform about same except ESP32-S3 which read 4095 at 3.3V as expected.

I need to figure out how to calculate / set ADC values properly because 2.8V is just rough estimation which fit best but overall ADC output seems to have quite big non-linear error and it looks like something is wrong there. I am not able to find any additional information in documentation.
Why is it reading 4095 at 2.8V and not at 2500mV or 3.3V? Will it be same for all C3 MCUs or do I have to "calibrate" each unit individually?

Parameter Description Min Max Unit Total error
ATTEN0, effective measurement range of 0 ~ 750 –10 10 mV
ATTEN1, effective measurement range of 0 ~ 1050 –10 10 mV
ATTEN2, effective measurement range of 0 ~ 1300 –10 10 mV
ATTEN3, effective measurement range of 0 ~ 2500 –35 35 mV

I am using Arduino 1.8.16 with latest ESP32 v2.0.9.

[lbernstone]

https://docs.espressif.com/projects/esp ... alibration