stm32f103的DAC模块配置完毕后，用万用表测PA4电压输出始终为0,

这是DAC的配置函数
//  1.GPIO配置
  GPIO_InitStruct.GPIO_Pin = GPIO_Pin_4;
  GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AIN;//模拟输入
  GPIO_Init(GPIOA,&GPIO_InitStruct);



//2.DAC模块配置


      void DAC_Configuration(void)
     {
        DAC_InitTypeDef DAC_InitStructure;
       
        DAC_InitStructure.DAC_Trigger = DAC_Trigger_Software;//软件触发
        DAC_InitStructure.DAC_WaveGeneration = DAC_WaveGeneration_None;
        DAC_InitStructure.DAC_LFSRUnmask_TriangleAmplitude = DAC_LFSRUnmask_Bit0;
        DAC_InitStructure.DAC_OutputBuffer = DAC_OutputBuffer_Disable;
       
        DAC_Init(DAC_Channel_1,&DAC_InitStructure);//初始化


         DAC_Cmd(DAC_Channel_1,ENABLE);//开启DAC
       
    }

//3.输出特定的电压函数

void DAC_OutVoltage(float Voltage)
{
  u16 data = 0;
  data = (u16)(Voltage/3.3)*4095;//数值转换
  DAC_SetChannel1Data(DAC_Align_12b_R,data);//将数据送入DAC
  DAC_SoftwareTriggerCmd(DAC_Channel_1,ENABLE);//软件开启转换
}


希望大家帮忙看一下，实在找不出哪点有问题了