Oh, now I see.
Then what Henrik said....set a comparator to 'trip' (toggle) at anything higher than 0.4V... use that 'toggling' level to generate a comparator interrupt......do your ADC inside the comparator interrupt routine.
Which PIC are you using? You can likely use the PIC's own internal comparator & DT's CMP_int interrupt....
(http://darreltaylor.com/DT_INTS-14/intro.html )





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