Changing from external to internal Aref

I want to use an external (3.072V derived from the 3.3V regulator output) for the Aref input on my UNO.
Do I have to disconnect that input when I change to the internal reference?


If you’re using an external reference on the AREF pin, you must set the analog reference to EXTERNAL before calling analogRead(). Otherwise, you will short together the active reference voltage (internally generated) and the AREF pin, possibly damaging the microcontroller on your Arduino board.

Alternatively, you can connect the external reference voltage to the AREF pin through a 5K resistor, allowing you to switch between external and internal reference voltages. Note that the resistor will alter the voltage that gets used as the reference because there is an internal 32K resistor on the AREF pin. The two act as a voltage divider, so, for example, 2.5V applied through the resistor will yield 2.5 * 32 / (32 + 5) = ~2.2V at the AREF pin.

Thank you, uxomm, for the much needed information! Now I see that my idea
will not work! The Aref input is not a standard CMOS input but, has a 38K
resistance to ground. Since I sometimes make a mistake, I will want to use
the 5K protection resistor, which will drop too much voltage from the 3.3
volt supply for the Aref voltage I want! Back to the drawing board!