Microcontroller › AVR › Problem with moisture sensor interfacing with ADC › It is given in the datasheet
April 12, 2012 at 12:15 pm #7426
It is given in the datasheet like “The user is recommended to only use low impedance
sources with slowly varying signals, since this minimizes the required charge transfer to the S/H
So try changing the value of resistors to small value, still keeping the ratio same.
If its not working, may be your sensor is getting loaded while connecting to the ADC. Try connecting a high valued resistor like 100k in series.