I have a Mega32C1 board where I read from the ADC. Having the internal Vref on without connecting the external Vref (AREFEN = 0, REFS1 = 1, REFS0 = 1, datasheet page 210) gives me a correct reading.
If I instead want to use the external Vref I have select it as refenrence source instead (AREFEN = 1, REFS1 = 0, REFS0 = 0), I always read maximum ADC value. The external Vref is 1.25V and the internal is 2.56V, but my measurement signal is roughly 0.9V +- 0.3V.
Is there something I am missing? Is there a misstake often done that gives this symptoms?
Greatful for any help!