Incorrect ADC values received

[Aamir]

Hello,

I am using NRF52840 Preview DK, I am running peripheral/SAADC example on the board, I shorted Channel 0 and GND on the board and I am receiving negative values such as -4 -5 -6 etc.

I used an external variable power supply, shorted boards ground to power supply's ground and gradually increased voltage over channel, in this case as well I got negative values for  0 volts and it was behaving predictibly uptil 3 volts but, as I increased the values further (3.1, 3.2, 3,3 volts) the ADC reading I got was around 500 which was half of expected value

what could be the reason for such odd behavior over GND and upper voltage levels ?

[awneil]

I wouldn't think that very small negative values are particularly "odd" - could be explained by ground offsets ?

What gain & reference settings are you using?

[Aamir]

it's is default from the example

Vref  = internal 0.6 V

Gain = 1/6

and what about, adc values over 3V ? why is it dropping to half