Amplifier for voltage measurement
The answers are all in the datasheet. Mostly in Table 1.

http://www.analog.com/media/en/technical-documentation/data-sheets/AD8206.pdf

1)  The maximum power supply voltage is 5.5V.
  2)  The amplifier output voltage can go to VCC - 0.3V (which is 5.2V).
  3)  The gain of the amplifier is fixed at 20V/V.
  4)  Therefore the maximum difference you can measure is 5.2V / 20 = 0.26V.

But analog devices has plenty of other difference amplifiers that would work.

The LT6375 is pretty good.