#define Fosc     16000000
#define Fdescr  4000

#define ADC_CONTROL (1 << ADEN)|(1 << ADIE)|(1 << ADPS2)|(1 << ADPS1)
#define MUX_CONTROL (1 << REFS1)|(1 << REFS0)

void adc_Init(void)
{
// init sampling timer
    TIMSK |= (1 << OCIE1A);
    OCR1A = Fosc / Fdescr; // for 4Khz sampling
    TCCR1B = (1 << WGM12) | 1; // запуск таймера
    
// turn off analog comparator
    ACSR = 0x80;
    
// init Adc in single conv mode
    ADCSRA = ADC_CONTROL;
    ADMUX = MUX_CONTROL;
}


/********************************************************
* adcBeginConversion()                                  *
* ---> channel - particular MUX channel                 *
* <--- returns nothing                                  *
********************************************************/
void adcBeginConversion(U8 channel)
{
    ADMUX = (MUX_CONTROL | channel);
    ADCSRA = ADC_CONTROL | (1 << ADSC);
}


/********************************************************
*       Sampling Timer Interrupt service routine        *
********************************************************/
#pragma vector=TIMER1_COMPA_vect
__interrupt void T1OCAISRHandler(void)
{
    adcBeginConversion( 0 );
}

/********************************************************
*            ADC Interrupt service routine              *
********************************************************/
#pragma vector=ADC_vect
__interrupt void AdcISRHandler(void)
{
    U16 Val = ADC;
    ....
}