Posted on June 19, 2018 at 14:13Hello everyone,
I'm currently trying to get as accurate ADC values as possible from a STM32F072. I'm using the NUCLEO-F072RB and wanted to use the internal stored reference voltage value for calibration. But I'm already off when I use this to calculate my actual VDDA for further ADC measurement. I double checked all my voltages with a calibrated multimeter and oscilloscope directly on the pins and with smalles possible ground loops. There are no spikes/drops in the voltage due to the ADC measurement and the ripple and noise is at around 5 mVpp. I also added 2x 10 nF and 1x 10 uF to C30 (100 nF) on AVDD on the nucleo. I tried all sampling times for the ADC without any change in the results and of course use 12-Bit. I do not want to read the value of the internal Vref-Voltage as it is not accurate (min 1.2V, typ 1.23V, max 1.25V).
So even after taking care of any possible error sources I'm getting a wrong value for my calculated AVDD voltage. I measured this with two multimeters to be 3.310V at 25�C. The calculated value from the STM32 is ~3.296 V, which is 14 mV off. This might be too much for measurements of voltages after a voltage divider, even when considering the 1% tolerance of the resistors. I read about a accuracy of ~1 mV (without voltage divider), so I'm curious how to achieve this when the reference value is not accurate.
This are parts of the code from the used test program:
// ADC self calibration, has to be done before any ADC Start/Enable
while(HAL_ADCEx_Calibration_Start(&hadc) != HAL_OK);
// values for calculation of actually VDDA-supply and reference voltage
volatile uint32_t VREFINT_CAL, VREF_3V3 = 330000;
const uint16_t *p = (uint16_t *)0x1FFFF7BA; // 2 Byte at this address is VRefInt @3.3V/30�C
VREFINT_CAL = *p; // read the value at pointer address
// Start ADC with DMA support
volatile uint16_t adc_value[4] = {0, 0, 0, 0};
HAL_ADC_Start_DMA(&hadc, (uint32_t*)adc_value, 4);
while (1)
{
//toggle LED
HAL_GPIO_TogglePin(LD2_GPIO_Port, LD2_Pin);
HAL_Delay(50);
// calculate 3.3 V supply voltage with the factory stored VREFINT_CAL value
// adc_value[2] contains the raw ADC_IN17 value of the internal voltage reference (~1530)
VREF_3V3 = (330000 * VREFINT_CAL) / adc_value[2];
// VREFINT_CAL = 1529
// adc_value[2] = 1530
// VREF_3V3 = ~329568 (~3.29568 V calculated)
}
#accuracy #nucleo-f072rb #adc #calibration #stm32f0 #internal-voltage-reference
