How to set ADC clock in low power mode?
Because in low power mode only the LSI clock is running and the ADC is working with the MSI clock.


Actually, in low power mode the system runs on MSI, and the ADC runs by default on HSI16. You can however change the latter with the CKMODE[1:0] bits of the ADC_CFGR2 register. See 13.4.4 ADC clock (CKMODE, PRESC[3:0], LFMEN) in the Reference Manual.