The ADC will finish a conversion before going to Idle/Standby sleep mode. The ADC can start conversions in Idle sleep mode if the START bit field in the Command (ADCn.COMMAND) register is configured to start a conversion on an event trigger. This is also possible in Standby sleep mode if the RUNSTDBY bit is set in the Control A (ADCn.CTRLA) register.
If both the LOWLAT and RUNSTDBY bits in the Control A register are set, the ADC will keep all required modules ON during Standby sleep mode to start a conversion faster, at the expense of increased power consumption during sleep.
When the system enters POWERDOWN, the ADC will abort an ongoing conversion and enter sleep mode immediately. It is recommended to make sure conversions have completed before entering Power-Down mode.