measuring dc voltage giving output through dac

hai everybody,
i am using c8051350 controller in a board to measure dc voltage upto 600v and give DAC output proportionaly upto 9.22v maximum , while measuring voltages upto 60v the dac is giving output then it is getting reduced and after 300v the DAC is giving 0v output.the controller is having current output DAC we are converting it to voltage using resistor,what can be the reason can anyone help me?

post your circuit and code please

rohit here is my code

void main (void)
{

PCA0MD &= ~0x40;//disable watchdog timer
initialise();
//present_voltage = 0x22;
while(1)
{
read_adc();
delay();
IDA1 = ((present_voltage)*(0x02));
delay();
//IDA1 = 0x90;
}
}

void read_adc()
{
//while(AD0BUSY);

while(!AD0INT); // Wait till conversion complete
AD0INT = 0; // Clear ADC0 conversion complete flag
present_voltage = ADC0L;
present_voltage = ADC0M;
present_voltage = ADC0H;

}
void adc_init(void)
{
REF0CN |= 0x01;
ADC0CN = 0x00;
ADC0CF = 0x00;
ADC0CLK = (SYSCLK/MDCLK)-1;
ADC0DEC = ((unsigned long) MDCLK / (unsigned long) OWR /
(unsigned long) 128) - 1;
ADC0MUX = 0x08;

ADC0MD = 0x83;
}

now i suspect the ADC is getting hang, did anyone experienced this problem before?