nRF9160 ADC Reference Values

[Gurkan]

Hello,

I'm trying to work with ADC on nRF9160DK and I'm using 3V for VDD IO;

With below configuration 

static const struct adc_channel_cfg m_1st_channel_cfg = {
	.gain = ADC_GAIN_1,
	.reference = ADC_REF_INTERNAL,
	.acquisition_time = ADC_ACQUISITION_TIME,
	.channel_id = 0,
#if defined(CONFIG_ADC_CONFIGURABLE_INPUTS)
	.input_positive = NRF_SAADC_INPUT_AIN0,
#endif
};

I'm getting full scale reading at 640mV

When I've switch to

static const struct adc_channel_cfg m_1st_channel_cfg = {
	.gain = ADC_GAIN_1,
	.reference = ADC_REF_VDD_1_4,
	.acquisition_time = ADC_ACQUISITION_TIME,
	.channel_id = 0,
#if defined(CONFIG_ADC_CONFIGURABLE_INPUTS)
	.input_positive = NRF_SAADC_INPUT_AIN0,
#endif
};

I'm getting full scale reading at 600mV, which I think it should 750mV due to 1/4th of VDD.

And another problem that I'm having is when I set gain to "ADC_GAIN_1_5" after 2.5V adc reading value start going down instead of going up. Like at 2.5V i'm reading 14600 and at 2.7V it is 9500.

Is there anything wrong with my configuration?

Regards,

Gurkan

[Gurkan]

Apperantly, the fault at output value of ADC is known issue, It has been pointed out at Errata.
Errata link

[Jared]

Could you try implementing the workaround and see if it works? 

[Gurkan]

Hey Jared,

I've switched to 1.8V and my observations;

ADC Settings;

Resolution: 14 bit

Gain: ADC_GAIN_1

Reference: ADC_REF_INTERNAL

Oversampling: NRF_SAADC_OVERSAMPLE_256X

at 640mV I can get 16380 which is maximum, it does not go over 16380

##############################################################

Resolution: 14 bit

Gain: ADC_GAIN_1

Reference: ADC_REF_VDD_1_4

Oversampling: NRF_SAADC_OVERSAMPLE_256X

at 580mV(I think this supposed to be 450mV since our VDD 1.8V) can get 16380 which is maximum, it does not go over 16380

################################################################

Gain: ADC_GAIN_1_3

Reference: ADC_REF_INTERNAL

Oversampling: NRF_SAADC_OVERSAMPLE_256X

AN0 pin connected to VDD (1.8V)

Resolution 14 bit - > ADC value 16130

Resolution 12 bit - > ADC value 4030

Resolution 10 bit - > ADC value 1008

[Jared]

Hi,

Could you try this example and see if you get the expected output?

Jared

[Gurkan]

Hi Jared,

That sample was the first thing I have tried actually.

Btw, I'm getting better readings if comment out "NRF_SAADC_NS->TASKS_CALIBRATEOFFSET = 1;"

And when I don't comment out that line I'm trying to check if the calibration is finished with "NRF_SAADC_NS->EVENTS_CALIBRATEDONE" but this register returns always 0.

[Jared]

Hi,

Are you experiencing wrong results even after the first result? Could you try clearing the the NRF_SAADC_NS->EVENTS_CALIBRATEDON field by setting it to 0 before writing NRF_SAADC_NS->TASKS_CALIBRATEOFFSET = 1 and see if the flag is set?

[Gurkan]

Hi.

As I have mentioned earlier I always read the EVENTS_CALIBRATEDONE as 0, before  setting TASK_CALIBRATEOFFSET or after.

    printk("Calibration has started\n");
	NRF_SAADC_NS->EVENTS_CALIBRATEDONE = 0;
	NRF_SAADC_NS->TASKS_CALIBRATEOFFSET = 1;
	while (!NRF_SAADC_NS->EVENTS_CALIBRATEDONE)
		;

For example it never passes the above check.

[Gurkan]

Another inconsistency that I have seen is about gain:

I'm measuring 577mV signal;

If I setup the gain as;

"ADC_GAIN_1" -> ADC Value 984, calculated voltage 577mV

"NRF_SAADC_GAIN1" - > Configuration error - Selected ADC gain is not valid

"ADC_GAIN_1_2" -> ADC Value 497. calculated voltage 582 mV

"NRF_SAADC_GAIN1_2" -> same as "ADC_GAIN_1_2"

"ADC_GAIN_1_3" -> ADC Value 333, calculated voltage 585 mV

"NRF_SAADC_GAIN1_3" -> same as "ADC_GAIN_1_3"

"ADC_GAIN_1_5" -> ADC Value 204, calculated voltage 595 mV

"NRF_SAADC_GAIN1_5" -> same as "ADC_GAIN_1_5"

"ADC_GAIN_1_6" -> ADC Value 171, calculated voltage 601 mV

"NRF_SAADC_GAIN1_6" -> same as "ADC_GAIN_1_6"