Вынимательное чтение доки дало еще вот это: Internal voltage
reference (Vrefint) The internal voltage reference (Vrefint)
provides a stable (bandgap) voltage output for the ADC and
Comparators. Vrefint is internally connected to the ADCJN17 input
channel. The precise voltage of Vrefint is individually measured
for each part by ST during production test and stored in the system
memory area. It is accessible in read-only mode.