Hi,
I am using the SAADC (AIN2) to get temperature values from a Pt1000, and I am experiencing sometimes strange behaviour after the offset calibration is done.I have quite simple requirements (only one analog value, repeat sampling every 2 ms until I have 50 samples, than stop). Wait for some time, than repeat. This is my initialization:
nrfx_saadc_config_t nrfConfig;
nrf_saadc_channel_config_t nrfChannelConfig;
nrfConfig.resolution = NRF_SAADC_RESOLUTION_12BIT;
nrfConfig.oversample = NRF_SAADC_OVERSAMPLE_DISABLED;
nrfConfig.interrupt_priority = APP_IRQ_PRIORITY_LOW;
nrfConfig.low_power_mode = false;
err = nrfx_saadc_init(&nrfConfig, &pt1000EventHandler);
if (err != NRFX_SUCCESS) { ... }
nrfChannelConfig.resistor_p = NRF_SAADC_RESISTOR_DISABLED;
nrfChannelConfig.resistor_n = NRF_SAADC_RESISTOR_DISABLED;
nrfChannelConfig.gain = NRF_SAADC_GAIN1_5;
nrfChannelConfig.reference = NRF_SAADC_REFERENCE_INTERNAL;
nrfChannelConfig.acq_time = NRF_SAADC_ACQTIME_40US;
nrfChannelConfig.mode = NRF_SAADC_MODE_SINGLE_ENDED;
nrfChannelConfig.pin_p = NRF_SAADC_INPUT_AIN3;
nrfChannelConfig.pin_n = NRF_SAADC_INPUT_DISABLED;
nrfChannelConfig.burst = NRF_SAADC_BURST_DISABLED;
err = nrfx_saadc_channel_init(0, &nrfChannelConfig);
if (err != NRFX_SUCCESS) { ... }
I start by doing one calibration and getting one measurement if the internal temperature sensor. While normal operation is going on, I repeat the temperature measurement every minute, and if the temperature has changed more then 10°C, I repeat the calibration and store the temperature value for later comparison.
The 10°C threshold is taken from the recommendation in the PS 1.1, 6.23.8.
To test temperature stability, I connect a 1kOhm resistor to my circuit and start heating up my device. I expected to see some drift, countered by the calibrations occurring when temperature has been changed by more then 10K. What I found was little drift, but changes of up to11 digits after calibration (see excel sheet) .The values from column C are calculated by aggregating 50 values as mentioned above and than converted to °C according to the circuity. Than they are sampled to my PC every 30 seconds. The first calibration is not visible in the graph, because it is done directly after startup, the second calibration pushes the measured values up by 0.6K (about 11 ADC digits), the third calibration pulls the measured values down by 0.4K (about 7 ADC digits) and the third calibration has no visible effect. I should add, that this effect does not occur on every test run. I do not get any error code on calling nrfx_saadc_calibrate_offset(), and I always get the NRFX_SAADC_EVT_CALIBRATEDONE event delivered to the event handler.
To me it looks a bit like the second calibration miscalculated a too large offset, and the third calibration then corrected the error.
Is there any explanation of this behaviour? Do I overlook something simple? I am also missing some more detailed explanation, what this calibration really does. What reference is used internally to determine an offset? I could not find a register where I can observe the calculated offset. Would it help to do the calibration more often (either by time interval or by reducing the temperature threshold from 10 K to maybe 1-2 K)?
Regards
Dirk
