I made a mistake in my description above. From the datasheet:

"In either case, when the conversion is completed, the GO/DONE bit is cleared, the ADIF flag is set and the A/D begins sampling the currently selected channel again. If an acquisition time is programmed, there is nothing to indicate if the acquisition time has ended or if the conversion has begun."

It appearts that I should just stay in a loop with an acquisition delay of 2-3 us each time. The reason I ask is that everything appears to work either way, with or without delaying each sample.