How to stop ADC before sys_poweroff ?

Hello~

I am developing nrf52840 with zephyr ncs tool chain v2.7.0 and SDK v2.6.1.

We had made our own pre-production PCB, and trying to lower down power consumption.

We happen to find that, in our code, ADC isn't turned off before calling sys_poweroff().

I mean, we just call sys_poweroff() directly, and leave ADC along. 

Should we do something on ADC to turn it off before sys_poweroff() ?

But we can't find any documents about turn off ADC.

Would you please tell us "How to turn off ADC?"