STM32L452 ADC Vref Voltage Difference Between Old and New Boards

[MidhunRaj]

We are working with a board featuring the STM32L452 microcontroller, configured for ADC readings. On my old board with the same MCU, the Vref (reference voltage for ADC) measures 3.3 V even without flashing any code, which is as expected. However, in the new hardware, Vref is only 0.3 V. The design is identical between both boards.

Could anyone please help me understand why this voltage difference is occurring? What steps should I take to diagnose and resolve this issue?

[waclawek.jan]

You can't leave the VDDA/VREF+ pin floating, i.e. without having it connected to external power source.

In 'L4, you can connect it to 0V, if you don't use any analog functionality.

I guess the consequence of leaving it floating may result in any voltage on it, due to various possible parasitic paths.

JW

[MidhunRaj]

No, In the old board also it is left like this without connecting to VDD.

[waclawek.jan]

I've edited my reply, please re-read it. Sorry for the confusion.

Try to load the VDDA pin on the old board (with mcu erased), using e.g. a 1kOhm resistor to ground (in parallel to the capacitor). IMO its voltage will drop to or close to zero.

JW

[TDK]

You can use VREFBUF to generate a VREF+ voltage, but max will be 2.5 V. Otherwise, you need to supply an external voltage reference as @waclawek.jan says.