I am using the DAC on the NUCLEO-L433RC-P board for development and have also designed a custom PCB using the STM32L433.
The DAC is used for audio voice prompts (12 bit, 44.1kHz stored in external flash) and fed into an audio amplifier (TI LM48100Q).
However, I am getting hiss and interference from a flashing LED on my custom PCB (but not so much on the NUCLEO-L433RC-P board).
I decided to try using a bench PSU to power the VDDA and VSSA pins on the NUCLEO-L433RC-P board, in case the SMPS on my custom PCB was causing the hiss.
Bbut I was suprised that the DAC still worked even though the VDDA and VSSA pins were floating after I modified the board.
Why does the DAC work when the VDDA and VSSA pins are floating ?
I measured the voltage on the VDDA pin (with reference to the VSSA pin) and it was around 1.6V.
When I did connect the STM32 VDDA and VSSA pins to the bench PSU and supplied it with 3.3V, it made no difference to the behavior.
Below are the modifications I made to the NUCLEO-L433RC-P board...
A concise explanation of the problem can be found in this post...
