SDK 15.2: Reading Analog Sensor Data using ADC gives value out of range

[tBStar]

Hi,

I need some help calibrating my moisture sensor analog readings using 3.3V regulated power source (VDD = 3.3). Here is my set up, borrowed from the proximity example.

#define ADC_REF_VOLTAGE_IN_MILLIVOLTS 600

#define ADC_PRE_SCALING_COMPENSATION 6 

#define DIODE_FWD_VOLT_DROP_MILLIVOLTS 270

#define ADC_RES_10BIT 1024 

#define ADC_RESULT_IN_MILLI_VOLTS(ADC_VALUE)  ((((ADC_VALUE) * ADC_REF_VOLTAGE_IN_MILLIVOLTS) / ADC_RES_10BIT) * ADC_PRE_SCALING_COMPENSATION)

the ADC_RESULT_IN_MILLI_VOLTS is in the 3500 mV range which is not accurate. While 3.3 / 1024 * adc_result (900) = 2.9V which is right on the money.

How can I calibrate my SAADC using the proximity example?

Thanks

[Edvin]

Hello,

I don't understand exactly what you mean:

 

the ADC_RESULT_IN_MILLI_VOLTS is in the 3500 mV range which is not accurate

 So what should it be?

 

While 3.3 / 1024 * adc_result (900) = 2.9V which is right on the money.

 

So are what you are trying to say that you don't get 900 as adc result?

BR,

Edvin

[tBStar]

When I measure the voltage, I get 2.9V, your formula gives me 3500mV, how do I go from 3500mV or 3.5V to 2.9V?

Thanks

[Edvin]

I don't think I understand your question. 

The value that you are measuring, is it really 2.9V or 3.5V? And are you asking how to scale from 3.5V to 2.9V?

What is your nRF Supply voltage?

BR,

Edvin

[tBStar]

Hi,

The value that you are measuring, is it really 2.9V or 3.5V? 2.9V

And are you asking how to scale from 3.5V to 2.9V? YES

What is your nRF Supply voltage? As mentioned before, VDD is 3.3V

[Edvin]

Ok, so you have nrf_saadc_channel_config_t config.gain = NRF_SAADC_GAIN1_6?

If so, you should keep ADV_PRE_SCALING_COMPENSATION 6, but you should change ADC_REF_VOLTAGE_IN_MILLIVOLTS from 600, as this suggests that you have a higher VDD voltage. 

If you need adc_result * 3.3 / 1024, and ADC_PRE_SCALING_COMPENSATION = 6, then ADC_REF_VOLTAGE_IN_MILLIVOLTS = 3300/6 = 550.