LPC1768 ADC not functioning

I am using LPC1768 for my project. I used the following code to test the micro-controller's ADC on a training board. The code reads the channel 5 of the ADC and puts its value on DAC(polling strategy). However using a new designed board for the project the code does not run correctly on the new board. I'm sure that the program enters the ADC's interrupt service routine since un-comenting the commented lines of the code leads to generating a ramp signal in the DAC output (which is done in the ADC_IRQHandler). On the other hand the signal is valid on the related ADC pin (checked on oscilloscope). It seems the problem is related to an internal setting of LPC1768. Any clues?
Thanks!

#define CLKDIV 38

uint32_t counter;

void ADC_Init (void) {

  LPC_SC->PCONP       |=  (1<<12);         /* Enable power to ADC block */

  LPC_PINCON->PINSEL1 &= ~(3UL<<14);       /* P1.23 is GPIO */
  LPC_PINCON->PINSEL1 |=  (1UL<<14);       /* P1.23 is AD0.0 */

  LPC_ADC->ADCR        =  (1UL << 0) |    /* select AD0.0 */
                          (CLKDIV << 8) |  /* ADC clock */
                          (1UL << 16) |      /* ADC is in the burst mode */
                          (1UL << 21);       /* enable ADC */

   LPC_ADC -> ADINTEN = 0x00000001;         /* enable interrupt on channel 0*/
}

void ADC_IRQHandler (void)
{
        uint32_t addr0;
        addr0 = LPC_ADC->ADDR0;
        addr0 = addr0 >> 6;
//      counter += 1;

        LPC_DAC->DACR = (addr0 & 0x3ff) << 6;
//      LPC_DAC->DACR = ((counter) & 0x3ff) << 6;
}

void DACInit(void)
{
  /* setup the related pin to DAC output */
  LPC_PINCON->PINSEL1 = 0x00200000;            /* set p0.26 to DAC output */
  LPC_DAC->DACCTRL = (1UL << 1);                  /* double-buffering is enabled */
  return;
}

int main(void)
{
    SystemInit();
    ADC_Init();
    DACInit();
    NVIC_EnableIRQ(ADC_IRQn);
    while (1)
    {

    }
}