void initUltimateTimer()
{
  noInterrupts();
  TCCR2A = (1 << WGM21);  //CTC-Mode Activation
  TCCR2B = (1 << CS21);    //Prescaler 8
  OCR2A = 250;            //125 µs periodicity using 2 MHz counter clock
  TIMSK2 = (1 << OCIE2A); //Compare Match A interrupt is enabled
  interrupts();
}

ISR(TIMER2_COMPA_vect)
{
  ADCSRA |= (1 << ADSC);  //An AD-Conversion is started
}

void initADC()
{
  noInterrupts();
  ADCSRA = 0; // Setting the ADC  Set ADC prescaler to 128 - 125KHz sample rate @ 16MHz
  ADCSRA |= (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);
  ADMUX = 0;
  // Set voltage reference
  // REFS1 REFS0 Voltage Reference Selection
  //   0     0   AREF, Internal Vref turned off
  //   0     1   AVCC with external capacitor at AREF pin
  //   1     0   Reserved
  //   1     1   Internal 1.1V Voltage Reference with external capacitor at AREF pin
  ADMUX |= (1 << REFS0);  // Enable ADC and ADC interrupt
  ADCSRA |= (1 << ADEN) | (1 << ADIE);  // Start first conversion which takes 25 cycles
  ADCSRA |= (1 << ADSC); // Start conversion  
  while(!(ADCSRA & ADIF));    // Wait for the first conversion to complete
  interrupts();
}

//Low Pass Filter ****************************
double lp_filter_value = 0.0;
double dT = 125e-6;
double Tau = 2;                    //Sekunden, Zeitkonstante Tau
double alpha = dT / Tau;
//*********************************************

double adc_input = 0;
double filtered_adc_input = 0;

ISR(ADC_vect)
{
  adc_input = ADC;
  filtered_adc_input = alpha * adc_input  + (1 - alpha) * filtered_adc_input;
}

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  initADC();
  initUltimateTimer();
}

void loop()
{
  delay(100);
  //Serial.print(alpha);
  Serial.print(adc_input); Serial.print(",");
  Serial.println(filtered_adc_input);
}