byte backup = ADMUX; // store the arduino settings
ADMUX = _BV(REFS1) | _BV(MUX3) | _BV(MUX0); // internal 1.1V reference, differential: ADC1+ ADC0-, gain: 10x
ADCSRA |= _BV(ADSC);  // start conversion

while ((ADCSRA & _BV(ADSC)) != 0);  // wait for conversion end (ADSC is cleared after the end by hardware)

int val = ADC; // read the result
ADMUX = backup; // restore original settings (from arduino)