Lowest voltage applicable on AREF pin on Uno and Mega I need to measure an Analog signal in the range of 5-30 mV.

Naturally, using the internal default 5V reference leads to very poor measurement resolution. I would like to use the capability to use an external reference voltage of 50mV on the AREF pin (using an Agilent precision power supply)

However, when I tried this in the lab, I noticed something strange. The ADC readout on the channel remained stuck at 1023. If I used anything above 0.5V on the AREF pin, the readout on the ADC is correct, and the counts reliably go up and down when I ramp up and down the Analog input accordingly.

Any idea how to resolve my issue?
According to the datasheet (page 375) the minimum voltage you are allowed to have as a reference voltage for the ADC is 1V.