The default for the Arduino A/D capability is a 5v reference. Given the 10-bit A/D converter, this is 4.88mV per number from 0 to 1023. The Arduino also supports setting the analog reference voltage to 1.1V, giving an accuracy approximately 5 times higher.
I wanted to try increasing that further. The Arduino has an internal 32k ohm resistor on AREF, so I connected a 1M ohm resistor to Vcc. The voltage reads 0.16V, but then A/D converter doesn't work. Presumably there is a lower limit. With 100k, it works, but the reference voltage is more than 1.1V, so there isn't much point.
Is it possible to set the reference voltage to less?