The voltage from DAC lower than usual

#include "stm32f10x.h"

volatile uint32_t Timer=0;
int16_t vout;

/* Init Structure definition */
DAC_InitTypeDef   DAC_InitStructure;
GPIO_InitTypeDef  GPIO_InitStructure;

void Delay(volatile uint32_t ms) {
	volatile uint16_t i,j;
	for(i=0;i<ms;i++)
	for(j=0; j<3442; j++) ;
}

void GPIO_setup()
{
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA |	
                         RCC_APB2Periph_GPIOC, ENABLE);  
  // Configure the GPIO Switch B1 pin (PA0)
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
  GPIO_Init(GPIOA, &GPIO_InitStructure);

  // Configure the GPIO Switch SW2(PC13)
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_13;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
  GPIO_Init(GPIOC, &GPIO_InitStructure);
}

void GPIO_DAC1_setup()
{
  // Configure PA4 (DAC_OUT1) to Analog in
  GPIO_InitStructure.GPIO_Pin =  GPIO_Pin_4;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AIN;
  GPIO_Init(GPIOA, &GPIO_InitStructure);
}

void DAC1_setup()
{
  // DAC Periph clock enable 
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_DAC, ENABLE);

  // DAC channel1 Configuration 
  DAC_InitStructure.DAC_Trigger = DAC_Trigger_Software;
  DAC_InitStructure.DAC_WaveGeneration = DAC_WaveGeneration_None;
  DAC_InitStructure.DAC_OutputBuffer = DAC_OutputBuffer_Disable;
  DAC_Init(DAC_Channel_1, &DAC_InitStructure);

  /* Enable DAC Channel1: Once the DAC channel1 is enabled, PA.04 is 
     automatically connected to the DAC converter. */
  DAC_Cmd(DAC_Channel_1, ENABLE);
}

/*--------------------------------------------------------------
  MAIN function
 *------------------------------------------------------------*/
int main (void) 
{
	uint8_t  reading1, previous1=0;
  vout=1000;    
	GPIO_setup();
  GPIO_DAC1_setup();
  DAC1_setup();

   while(1) 
  {
		reading1 = GPIO_ReadInputDataBit(GPIOA,GPIO_Pin_0);
    if((reading1 ==1) && (previous1 !=1)  )
    {
      Delay (10);            // delay 10 msec for debounce
			if(GPIO_ReadInputDataBit(GPIOA,GPIO_Pin_0)==1)
			{
				vout=vout+1000;
				if(vout>=4095) 
					vout=4095;
				DAC_SetChannel1Data(DAC_Align_12b_R, vout);
				DAC_SoftwareTriggerCmd(DAC_Channel_1, ENABLE);
			}
		}
		Delay(100);
    previous1=reading1;
	}
}