Is AREF a stable output Ref voltage with analogReference(INTERNAL)
No, it is not acceptable.

The datasheet states:


  AVCC is connected to the ADC through a passive switch. The internal 1.1V reference is generated from the internal bandgap reference (VBG) through an internal amplifier. In either case, the external AREF pin is directly connected to the ADC, and the reference voltage can be made more immune to noise by connecting a capacitor between the AREF pin and ground. VREF can also be measured at the AREF pin with a high impedance voltmeter. Note that VREF is a high impedance source, and only a capacitive load should be connected in a system.


The 1.1V reference is only presented on the AREF pin for the purpose of adding an external bypass capacitor, or for measuring it using a very high impedance meter. Any load applied to the pin will cause the reference voltage to droop.

While it may work in certain circumstances it's certainly not recommended. If you want to use it externally you should buffer it through a high-impedance unity-gain voltage follower:







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