need help understanding Vref / Vdda

[deep_rune]

I'm trying to find what voltage reference my ADC is using, as it is not going up to 0xFFF. Along the line, I have become quite confused about the Vref - I used to use other chips where the Vref is just a reference voltage for the ADC, but with STM32 it seems it's more complicated.

I'm confused by the function of the Vref / Vdda pin on the STM32L412 - is this a power pin, or a voltage reference pin? I looked through the reference manual but the information was ambiguous. If it's a power pin, how much current does it need? And how does the Vref work?

[TDK]

VDDA is a power pin. It must be the same as VDD for most (all?) STM32 chips.

VREF+ is a voltage reference pin.

On smaller packages, these two pins are bonded together, which means VREF+ = VDDA. This means it is both a power pin and a voltage reference pin.

[deep_rune]

ok.. so if my chip runs off 3.3V but I want to run the ADC of a 3v reference, I can only do this on a chip with separate VDDA and Vref+ pins?

[TDK]

That is correct.

[Tesla DeLorean]

There is an internal intVRef (1.25V as I recall) that can attach to the ADC and can be used as a calibration point

[waclawek.jan]

My take on the topic.

JW

[S.Ma]

It should be the bandgap voltage, works like an internal regulator. By measuring it, vdda can be deducted.

[waclawek.jan]

> in practice I'm getting an error of around 30mV when reading a 1393mV source.

Did you perform ADC calibration before measurement, as outlined in the ADC chapter in RM?

Also, next time, don't hijack an old thread, start your own.

JW

[DCarr.1]

I believe so, although I could be doing it wrong:

main.c

int main(void) {
 HAL_Init();
 SystemClock_Config();
 MX_GPIO_Init();
 MX_ADC1_Init();
 
 if (HAL_ADCEx_Calibration_Start(&hadc1, ADC_SINGLE_ENDED) != HAL_OK) {
	 Error_Handler();
 }
 
 while (1) {
		if (HAL_ADC_Start(&hadc1) != HAL_OK) {
		 Error_Handler();
		}
 
		if (HAL_ADC_PollForConversion(&hadc1, 100) != HAL_OK) {
			Error_Handler();
		}
 
	 float adc = (float)HAL_ADC_GetValue(&hadc1);
 
	 float vref = 2500.0f;
	 float vadc = (adc * vref) / 4096.0f;
 
	 float R14 = 47000;
	 float R15 = 100000;
	 float vbat = vadc * (R14 + R15) / R15;
 
	 __NOP();
 }