[nRF52] Best practise for ultra low power SAADC 4 channel conversion

[Mateusz Gancarczyk]

What is the best practise for multichannel SAADC conversion in ultra low power devices? For example I have to measured 4 channel in interval 250ms. Below what I try:

1. I'm set RTC for interupt 250ms. In RTC handler I initialized SAADC channel 0 and start conversion. In SAADC end of conversion handler i uintialized SAADC, get result from channel 0 and initialized for channel 1 and start conversion. Same for channels 3 and 4 but in channel 4 I'm not start new conversion but Im set RTC for 333ms Handler. I try it and I get strange result.

If I have only one channel I've get current consuption:

avarage current is around 10uA ( it's ok for my aplication ). but if I enabled conversion for 4 channel, one by one ( as I descripted above ) I've got something like that:

Current is 2 times higher! I don't know why density of the current spikes is in that level.

Mayby I should use PPI? i read this topic: devzone.nordicsemi.com/.../

or maybe different method?

[Mateusz Gancarczyk]

The reason why I'm not using scan mode is tha I can't do this in low power. For example if I try this example: github.com/.../nRF52-ADC-examples for one channel I get 4uA witch it's ok but if I add another channel:

 channel_config2.reference = NRF_SAADC_REFERENCE_INTERNAL;                              //Set internal reference of fixed 0.6 volts
    channel_config2.gain = NRF_SAADC_GAIN1_6;                                              //Set input gain to 1/6. The maximum SAADC input voltage is then 0.6V/(1/6)=3.6V. The single ended input range is then 0V-3.6V
    channel_config2.acq_time = NRF_SAADC_ACQTIME_10US;                                     //Set acquisition time. Set low acquisition time to enable maximum sampling frequency of 200kHz. Set high acquisition time to allow maximum source resistance up to 800 kohm, see the SAADC electrical specification in the PS. 
    channel_config2.mode = NRF_SAADC_MODE_SINGLE_ENDED;                                    //Set SAADC as single ended. This means it will only have the positive pin as input, and the negative pin is shorted to ground (0V) internally.
    channel_config2.pin_p = NRF_SAADC_INPUT_AIN1;                                          //Select the input pin for the channel. AIN0 pin maps to physical pin P0.02.
    channel_config2.pin_n = NRF_SAADC_INPUT_DISABLED;                                      //Since the SAADC is single ended, the negative pin is disabled. The negative pin is shorted to ground internally.
    channel_config2.resistor_p = NRF_SAADC_RESISTOR_DISABLED;                              //Disable pullup resistor on the input pin
    channel_config2.resistor_n = NRF_SAADC_RESISTOR_DISABLED;                              //Disable pulldown resistor on the input pin    
	
    //Initialize SAADC channel
    err_code = nrf_drv_saadc_channel_init(0, &channel_config);                         0 with the channel configuration
    APP_ERROR_CHECK(err_code);
		   err_code = nrf_drv_saadc_channel_init(1, &channel_config2);                            
    APP_ERROR_CHECK(err_code);
    if(SAADC_BURST_MODE)
    {
        NRF_SAADC->CH[0].CONFIG |= 0x01000000;    
NRF_SAADC->CH[1].CONFIG |= 0x01000000;   				
    }

current goes to 900uA. So I decide to try convert channel on by one.

What I'm doing worong in this example?