Read the reference: analogReference() - Arduino Reference
There is a "Notes and Warnings" paragraph, be sure to follow that when applying a voltage to the AREF pin.
Some protect the AREF pin with a resistor of 1k. You can do that during testing, but I don't like to make a good reference voltage weaker.
You can use the internal reference of 2.56V, I think that will be almost just a good as a external reference voltage. The signal has often noise, so the full 10-bits accuracy is hard to reach. Therefor the reference of 2.56V will be almost as accurate as 2.00V.
If you happen to have a better reference of 2.0V, then you can connect the reference voltage to the AREF pin.
Is your signal with certain defined voltages or is it ratiometric to a power supply ?