LM4040 AREF Pin Voltage Drop

[DVDdoug]

I can think of couple of possibilities - There is an internal resistor -

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.

Or, you might be "too close" to Vcc. I'm pretty sure the external reference has to be less than Vcc.