RTD measurement with Wheatstone Bridge and ADC

Question

Hello,

I've developed a PCB that connects an RTD (PT100) to a Wheatstone bridge, set to 100 ohms at 0°C. The bridge terminals (as seen in the diagram) connect to AIN4 and AIN6 of the nRF52840. However, no matter what devicetree configuration I try (differential mode, single mode, gain, vref, etc), the measurements are very unstable, and I can't calculate a correct temperature value. What could be the cause of this problem?

Schematic (RTD_V1 to AIN6 and RTD_V2 to AIN4):

Device tree snippets:

	channel@4 {
		reg = <4>;
		zephyr,gain = "ADC_GAIN_1_6";
		zephyr,reference = "ADC_REF_INTERNAL";
		zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
		zephyr,input-positive = <NRF_SAADC_AIN4>; /* P0.03 */
		zephyr,input-negative = <NRF_SAADC_AIN6>; /* P0.03 */
		zephyr,resolution = <12>;
	};

Application snippets:

#if !DT_NODE_EXISTS(DT_PATH(zephyr_user)) || \
	!DT_NODE_HAS_PROP(DT_PATH(zephyr_user), io_channels)
#error "No suitable devicetree overlay specified"
#endif

#define DT_SPEC_AND_COMMA(node_id, prop, idx) \
	ADC_DT_SPEC_GET_BY_IDX(node_id, idx),


static const struct adc_dt_spec adc_chan4 = ADC_DT_SPEC_GET_BY_IDX(DT_PATH(zephyr_user), 1);

static const struct gpio_dt_spec rtd = GPIO_DT_SPEC_GET(DT_PATH(zephyr_user), rtd_gpios);

int main(void)
{
	int err;
	uint32_t count = 0;
	uint16_t buf_1, buf_2;
	struct adc_sequence sequence_1 =
		{
			.buffer = &buf_1,
			/* buffer size in bytes, not number of samples */
			.buffer_size = sizeof(buf_1),};

	int rc;


	rc = gpio_pin_configure_dt(&rtd, GPIO_OUTPUT);
	if (rc < 0)
	{
		printf("Could not configure sw0 GPIO (%d)\n", rc);
		return 0;
	}
	gpio_pin_set_dt(&rtd, 0);


	if (!adc_is_ready_dt(&adc_chan4))
	{
		printk("ADC controller device %s not ready\n", adc_chan4.dev->name);
		return 0;
	}

	err = adc_channel_setup_dt(&adc_chan4);
	if (err < 0)
	{
		printk("Could not setup channel (%d)\n", err);
		return 0;
	}
;
	while (1)
	{
		int32_t val_mv_1 = 0;


		(void)adc_sequence_init_dt(&adc_chan4, &sequence_1);

		err = adc_read_dt(&adc_chan4, &sequence_1);
		if (err < 0)
		{
			printk("Could not read (%d)\n", err);
			continue;
		}

		val_mv_1 = (int32_t)((int16_t)buf_1);
		
		err = adc_raw_to_millivolts_dt(&adc_chan4,
									   &val_mv_1);


			if (err < 0)
		{
			printk(" (value in mV not available)\n");
		}
		else
		{

			printk(" = %d mV\n", val_mv_1);
		}

		k_msleep(1000);
	}
	return 0;
}

RTT:

00> = -4 mV
00> = -9 mV
00> = 10 mV
00> = -13 mV
00> = -9 mV
00> = -4 mV
00> = -8 mV
00> = 0 mV
00> = -6 mV
00> = -4 mV
00> = -2 mV
00> = 1 mV

and go on.

Thanks!!

hmolesworth · Accepted Answer

I wrote some extensive code to get very accurate samples from an RTD with SAADC 12-bits effective ENOB using alternate differential measurements; the code is low-level bare-metal, but you can follow the guidance I provide and use either nrfx or zephyr (if you must). The code is valid for nRF52832, nRF52840 and nRF52833: 
 seeking-more-information-on-nrf52833-adc-reference-and-buffers

RTD measurement with Wheatstone Bridge and ADC

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