Internal Reference voltage VREF of STM32L4R9

[MoniR]

Hi,

I am using STM32L4R9ZIJ6 microcontroller.

I have connected an external reference voltage of 3.3V to VREF+ pin and wanted to use internal reference voltage over this one for internal ADC read operation. Is that possible?

I tried configuring the MCU to enable VREFINT for internal ADC1 read but I can still see that the external 3.3V is being taken by the ADC for conversions. 

Can someone please answer this?

Thanks.

[TDK]

> I have connected an external reference voltage of 3.3V to VREF+ pin and wanted to use internal reference voltage over this one for internal ADC read operation. Is that possible?

No, this is not possible. The internal VREFBUF still uses the VREF+ pin and you must provide capacitors on this pin to stabilize the voltage. If it's connected to 3.3 V, you cannot use it.

(VREFINT is something different and not relevant here.)

[Aime]

Hi @MoniR

Regarding your question about the STM32L4R9ZIJ6 and the use of the internal reference voltage (VREFINT) versus an external reference voltage connected to the VREF+ pin for ADC conversions:

VREF+ Pin Functionality:

• The VREF+ pin is the analog reference input for the ADC.
• When you connect an external voltage (e.g., 3.3 V) to VREF+, the ADC uses this external voltage as its reference.
• This means the ADC conversion results are scaled relative to this external voltage.

Internal Reference Voltage (VREFINT):

• The internal reference voltage (VREFINT) is a fixed, stable voltage source inside the MCU (typically around 1.2 V).
• VREFINT is not used as the ADC reference voltage; instead, it is an input channel that the ADC can measure.
• You can enable and measure VREFINT as an ADC channel to monitor the internal reference voltage for calibration or monitoring purposes.

 

The ADC reference voltage is always the voltage applied to VREF+ pin (or internally connected to VDD if VREF+ is tied to VDD).

The internal VREFINT is only available as a measurement input channel to the ADC, not as a reference voltage source for conversions.

Since you connected 3.3 V to the VREF+ pin, the ADC uses this as the reference voltage.

When you enable the VREFINT channel for ADC, you are simply telling the ADC to measure the internal reference voltage as a signal input, not to use it as the ADC reference. Therefore, ADC conversions of other channels will still use the external 3.3 V reference.

 

If you want to use the internal reference voltage as the ADC reference, this is not supported on STM32L4 series.

 

Best regards,
Aime