Arduino (weird) ADC problem
However, does not work for:
 ADMUX = 0xC0;


See the datasheet:

An ADMUX of 0xC0 will be using a voltage reference of &quot;Internal 1.1V Voltage Reference with external capacitor at AREF pin&quot;. Since you are supplying 2.5V that exceeds 1.1V and thus you get a reading of 1023.


  Serial.println((int)sensorValue);


Also, don't do serial prints inside an ISR.