ok, also

Code:
// Externe Referenzspannung verwenden (also 5 V)
ADMUX &= ~(1<<REFS1);
ADMUX |= (1<<REFS0);

Verdrahtung
GND --> GND

AVCC
|--> 10µH --> VCC
|--> 100nF --> GND

AREF --> 100nF --> GND