I'm a bit lost inbetween the arduino standard calls and what the DUE is or does,
but I learned that that MCK/2 and prescaler determine the speed of the ADC processing.
given that (clock) speed, taking a sample takes a few clock cycles.
but what determines the sampling timing is what the datasheet calls a trigger.
the trigger can be an external signal or e.g. a timer.
I have not yet gotten that to work yet, however.
If / when I do, I'll report. but I'm following the datasheet, not the arduino calls, so the code is extensive.