RE: Read a values of anolg input ( nRF52840, S140 )

AmbystomaLabs — Mon, 03 Sep 2018 18:31:33 GMT

I should add that the reason you don't see a fullscale reading when connected to vdd is due to both the prescaler and the saadc reference. Normal the saadc uses an internal reference so the ref voltage doesn't vary with vdd. This is an issue in battery operated designs. However, in the config register you can choose vdd as the reference if you choose.

RE: Read a values of anolg input ( nRF52840, S140 )

AmbystomaLabs — Mon, 03 Sep 2018 18:24:32 GMT

When you short to ground you will see the dc offset of the saadc. All adc's have some offset. There is a cal process in the documentation that will reduce the offset. The reason it fluctuates a little is because the quant noise floor is random. This is why most adc designs throw away the bottom 1 or 2 bits.

The max number when connected to nrf_vdd is determined by the prescaler value and the bit width of the sample.

For your analog generator, you need to have the max frequency of the generator inside the nyquist window of the saadc. This is basic sampling theory and would be useful to familiarize yourself with. When this is not true, since the input of an saadc is switched capacitive you will only see the mean value which for an ac signal is zero. Or in this case a couple of quanta.