Yep, the datasheet is indeed your best resource here. Looking at the 12LF1824 datasheet, page 373 you'll find that the minimum VRef voltage (where VRef is specified as VRef+ minus VRef-) is 1.8V so you can't use the fixed voltage reference when its set to 1.024V, you need to switch it to atleast 2.048V (see note 5), which with your 0.5V signal only gives you ~25% of the available range - bummer.

So the gotcha in this case is the fact the VRef can't be lower than 1.8V.

/Henrik.