mauried:
Its important to read the spec sheet for the Atmega328 when looking at the internal voltage referance specs.
The 1.1 volt referance can vary anywhere between 1.0 to 1.2 volts, which is 20% variation.
Its not a stabilised voltage referance.
If you want a stabilised referance voltage for things like calibrating voltmeters, then you need something like this.
ADR4525 Datasheet and Product Info | Analog Devices
Voltage guaranteed to be stable within 0.02%.
They aint cheap though.
I'm not sure I can agree with you that the internal voltage reference is not a stabilized voltage reference. There can be device to device variation for the nominal 1.1vdc band-gap reference, but for any specific chip it will be a stable voltage reference that will be somewhere within that stated range. And once one does actually measure the specific bandgap voltage of their specific chip then steps can be developed a calibration factor one might apply to ADC readings.
Lefty