Problem with ADC precision

[Louis Hatte]

Hello everyone,

We are using the nrf52840 Dev Kit (PCA10056 1.1.0 2019.19 683465548)

SDK Version: 15.3

OS: Fedora

When we are generating a 1V voltage our adc returns values between [260, 280]. We use this function to convert it into µV:

#define ADC_REF_VOLTAGE_IN_MILLIVOLTS     600
#define ADC_RES_10BIT                     1024
#define ADC_PRE_SCALING_COMPENSATION      6

int32_t adc_result_to_uv(int16_t adc_value) {
    int32_t   adc_uv = ((((adc_value) * 600000) / 1024) * 6);
    return adc_uv;
}

We get values around 940 000 µV instead of 1 000 000µV.

We tried to change the resolution into 14 BIT like above:

#define ADC_REF_VOLTAGE_IN_MILLIVOLTS     600
#define ADC_RES_14BIT                     16384
#define ADC_PRE_SCALING_COMPENSATION      6

int32_t adc_result_to_uv(int16_t adc_value) {
    int32_t   adc_uv = ((((adc_value) * 600000) / 16384) * 6);
    return (int32_t) adc_uv;
}

But we get strange results:

- adc_value between [4200, 4400]

- adc_uv around -631554 µV

We don't know what we are doing wrong and how to get a better precision...

Any ideas?

[awneil]

Louis Hatte said:

When we are generating a 1V voltage

How are you generating it?

How precise is it?

Louis Hatte said:

We get values around 940 000 µV instead of 1 000 000µV.

Assuming your "1V" really is 1 000 000 uV, that's an error of 60 000 uV - which is 6%

In the nrf52840  SAADC electrical specs, the gain error is 3-4%

So it doesn't seem that you're wildly off ?

https://infocenter.nordicsemi.com/topic/ps_nrf52840/saadc.html?cp=3_0_0_5_22_9#unique_1950216936

See also: https://devzone.nordicsemi.com/f/nordic-q-a/48492/saadc-error

[haakonsh]

My guess is that you've probably not calibrated the offset, or your 1V source is not really 1.00V.