Issue in getting persistent ADC Reading NCS 2.4.1 (nRF52832)

[X4ce]

Hi,

I am using a custom nRF52832 board with multiple ADC channel readings sequentially. I am trying to read thermistor (NTC) value on channel 7 and not getting accurate reading. As soon as I plug the thermistor, the value goes on decreasing/ stabilizing which goes below the actual/ anticipated reading that could be due to hardware/ software issue in my setup.

The schematic of that specific channel is:

The DTS for that specific channel is:

	// Thermistor
	channel@7 {
		reg = <7>;
		zephyr,gain = "ADC_GAIN_1_5";
		zephyr,reference = "ADC_REF_INTERNAL";
		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
		zephyr,input-positive = <NRF_SAADC_AIN7>;
		zephyr,resolution = <10>;
		zephyr,oversampling = <8>;
	};

This is the output of channel 7 readouts after 1 sec interval:

ADC reading of device adc@40007000, channel 7: 555 = 1625 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 545 = 1596 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 538 = 1576 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 533 = 1561 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 527 = 1543 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 523 = 1532 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 513 = 1502 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 510 = 1494 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 509 = 1491 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 511 = 1497 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 504 = 1476 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 504 = 1476 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 501 = 1467 mV
ADC Thread fired!

ADC reading of device adc@40007000, channel 7: 505 = 1479 mV

The ADC reading falls/ stabilizes from 555 to 505 and I expect a value around 540 (measuring thermistor from meter then converting to raw adc value).

I have tried setting VDD and different ACQ times but didn't help.

Please suggest a solution.

Regards

[hmolesworth]

The starting point is to know that using the internal reference provides an absolute measurement of voltage on AIN7; with a perfect internal reference and perfect ADC with no errors you get a perfect voltage measurement but unfortunately that is not what is required to measure this NTC. Should the 3.3V change by (say) 1% or have noise then that "perfect" voltage measurement will vary accordingly.

Instead try a ratiometric measurement such that any variation in the 3.3V supply causes an equal variation in the SAADC reference voltage which largely eliminates the effect on the measured voltage. Ironically to get the best results use minimal or no capacitance on the NTC input as the AIN7 input should exactly track the noise on 3.3V rail and not be filtered; ie set the reference to other than ADC_REF_INTERNAL and test both with and without the 10nF capacitor.

Once you have those results measure using differential mode using AIN7 and (say) AIN6 where AIN6 is connected to the lower end of the 10k. Measure twice with P AIN7, N AIN6 and P AIN6 and N AIN7. The average will give a more accurate reading with residual SAADC offset removed.

Edit: I wrote some code here which might be informational, though it is bare-metal code:

nrf52833-adc-reference-and-buffers

Related ticket:

rtd-measurement-with-wheatstone-bridge-and-adc