Hi ilan,
The Max. ADC VREFH is VDDA, which could up to 3.6V.
The Min. ADC VREFL is VSSA, which in general same with digital ground (VSSA power for MCU and analog circuits. Filtered from GND), the delta number is the VSS to VSSA differential voltage.
The typical ADC refer circuit design is below:

Wish it helps.
best regards
Ma Hui
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