nrf SAADC measure mv accuracy Voltage

[Muhammad]

Hi, 

I'm using saadc library to measure the voltage on nrf52832 DK. also I'm using soft device s132 on the project. I have several questions and will list below,

 I want to measure the voltage on pins AIN0 and AIN1 between 0 - 3.0. I'm currently using VDD4 reference and 12-bit ADC with 1/6 gain.

I have tried the scan mode to measure the voltages simultaneously, but I find that both pins voltage shows inaccurate readings like If I apply 3 volts on pin AIN0  and no voltage on the AIN1 then the digital readings on both pins are like

AIN0 = 2483,  AIN1 = 1135

As I did not apply any voltage on AIN1 but it still shows the value. I have tried using a single sample reading for both and multiple sample readings then took the average for the samples of corresponding changes but in both cases, I get the same pattern of results.

1)  I tried to measure only pin AIN0 reading and it's giving me 2500 on 3V but when I remove 3V it still gives me 150 - 200 values which turn out to be some 0.4 0.5V but I need the mv accuracy in analog reading and that I think I have not achieved. Can you guide me on how I can achieve the millivolts sensitivity in measurement? 

2) I want to measure the voltage using the 10useconds interval for each sample for AIN0 pin. How can I do that?

3) I want to measure the voltage on AIN1 after every 5 min and on AIN0 continuously, what you suggest would be the best practice to measure these. like measure using scan mode which is not giving me good results or should I measure 1 continuously and when I want to measure others after every 5 minutes disable the AIN0 enable AIN1 and measure voltage then again disable AIN1, enable AIN0. So please suggest how to do the second method if adopt it?

I have tried all the available data and examples on this forum but didn't succeed in measuring both voltages and measuring accurately. 

Thanks

[haakonsh]

1. Are you using the DK or a custom board? 

What is your source impedance? 

There should be no issues in using scan mode, what you are experiencing is not what we'd expect, something is terribly wrong with your set-up. 

2. In nrfx_saadc_channel_init you pass a nrf_saadc_channel_config_t. Set acq_time to NRF_SAADC_ACQTIME_10US.

3. I suggest you use one channel and re-configure it to take the one sample every 5 min, then re-configure it back to the continuous sampling. Ie change the input pin and buffer. 
Use nrfx_saadc_channel_uninit, nrfx_saadc_channel_init, and nrfx_saadc_buffer_convert.