How to read small DC signal reliably using Arduino Uno

Hello,

I am trying to read a small DC signal ranging from 11mv to 480mv using Arduino Uno, but failed to read it reliably especially at the low end (from 11mv to 50mv). Could someone provide a suggestion how to accomplish this.
Thanks,

Kiet

Try using the INTERNAL 1.1V reference, vs the default EXTERNAL 5V reference.

https://www.arduino.cc/reference/en/language/functions/analog-io/analogreference/

Then a full scale reading of 1023 will represent 1.1V, and a low end reading of 1 or 2 will represent 1 or 2mV, vs 4.88 to 9.7mV.