user40642: 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?

user4143: According to the datasheet (page 375) the minimum voltage you are allowed to have as a reference voltage for the ADC is 1V.

40642: <p>I need to measure an Analog signal in the range of 5-30 mV.</p>

<p>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)</p>

<p>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.</p>

<p>Any idea how to resolve my issue?</p>
 4143: <p>According to <a href="http://www.atmel.com/Images/Atmel-42735-8-bit-AVR-Microcontroller-ATmega328-328P_Datasheet.pdf" rel="nofollow noreferrer">the datasheet</a> (page 375) the minimum voltage you are allowed to have as a reference voltage for the ADC is 1V.</p>
