Schau mal in die Bascom-Hilfe unter CONFIG ADC!
Code:
'For some chips you can use the additional param : 
'Config Adc = Single , Prescaler = Auto, Reference = Internal 
'The reference param may be:
'OFF      : AREF, internal reference turned off
'AVCC     : AVCC, with external capacitor at AREF pin
'INTERNAL : Internal 2.56 voltage reference with external capacitor at AREF pin 
'Using the additional param on chip that do not have the internal reference will have no effect.