Shannon,
The spec table guarantees that in unbuffered mode, the input current is higher, but you can get within
50 mV of the rail. In buffered mode, the input current is lower, but you can only get within 250 mV
of the rail. This is guaranteed over temp, so if you design to these numbers, and build 10,000 systems a week
for ten years with parts coming from different fab runs, you will not have a problem. Obviously, there
are variations from part to part, over temperature, and some measurement error when we characterize
and test the parts. So you may have one part at 25 C, that goes down to 170 mV. You could
put another part on your board, and it might go down to 130 mV. Or a lot of parts go down
to 50 mV, but the DNL and INL degrade to 14 bits.
Harry