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 ?

[Jørgen Holmefjord]

Hi,

The SAADC peripheral has a Full scale input range (0 to VDD). If you measure VDD on the DK, this should be 3.0 V. Trying to sample voltages above VDD will give undefined results.

Negative values at GND are caused by noise and offset error.

Best regards,
Jørgen

[Jørgen Holmefjord]

The SAADC were designed for nRF52832, which has a supply voltage range from 1.7 - 3.6 V, if you increase the supply voltage to the chip, you will also be able to sample higher voltages.

[Aamir]

Thanks !