ADC reading below 0°C

[ohnowhathaveidone]

Hi,

I have an application where we gather data from temperature and moisture sensors outdoors. Readings are taken every 15 minutes and between the readings the board goes into deep sleep. I use a SparkFun Thing Plus - ESP32-S2 WROOM board.

I am attaching some images of recent data. The sensors are buried in the ground with S0 being at a depth of around 1 m and S4 being barely covered with soil. S1 to S3 are on depths in between.

In order to take a measurement, the sensors are supplied with power through a transistor that is powered off afterwards.

I find it strange that i consistently get an offset of my moisture readings when the temperature crosses 0° C. Also, the temperature readings seem very noisy around 0°.

Is there some temperature dependency of the ADC? Am I missing something else?

Cheers!

[ESP_Sprite]

Well, the fact that your voltage graphs look sane but the temperature one doesn't, assuming both come from the same ESP, implies that something is going wrong in your voltage->temperature conversion.