@mrks-tsk The capacitor has to be placed between ADC input and GND. In your diagram at the VBATT side.

@cocoa Thank you for your answer

@kjm So you are averaging over 1000 values. You can also use a low pass filter, which is a little bit simpler See also the lessons and analysis by Bernd Boser: https://github.com/bboser/IoT49 and especially https://github.com/bboser/IoT49/blob/master/doc/analog_io.md which shows a noise of about 25 LSB. So your averaging of 1024 values (25*25=625) matches that. B.t.w: good ADCs have a noise <= 1 LSB.