On page 77 of the MSP430F5659 data sheet, below the table "REF, External Reference", are several footnotes. These seem to suggest that when one uses any external reference below aVcc, accuracy is reduced. "Lower reference voltage levels may be applied with reduced accuracy requirements."
This statement is puzzling.
First, I might assume that a 2,5V reference on VeREF+ (with VeREF- at AGND) will yield 12 bits across 0 to 2.5V (610 microvolts/bit). Whereas, a 3.3V VeREF+ (or using AVCC set at 3.3V) will yield 850 microvolts/bit.
So, is TI telling us that even if we use a VeREF+ that's lower than aVCC, and can therefore resolve the input to greater resolution, our "accuracy" will be only as good as the full span of aVcc?
Also, puzzlement as to the use of VeREF-. Is this to allow inputs slightly below aGND? I thought these were illegal and would violate pin voltage limits. What other purpose might VeREF- serve?
