DAC无电压输出，求解！

DAC配置如下：
void Dac1_Init(void)
{
        DAC_InitTypeDef DAC_InitStruct;
        GPIO_InitTypeDef GPIO_InitStructure;
        RCC_APB1PeriphClockCmd(RCC_APB1Periph_DAC, ENABLE);
        RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);

        /* Configure DAC channe1 outputs pins */
        GPIO_InitStructure.GPIO_Pin = GPIO_Pin_4;   
        GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
        GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AIN;
        GPIO_Init(GPIOA, &GPIO_InitStructure);
        DAC_DeInit();
        //DAC_Cmd(DAC_Channel_1,ENABLE); //
    //DAC_Cmd(DAC_Channel_2, ENABLE);
        DAC_StructInit(&DAC_InitStruct);//将DAC_Struct的值设为默认
        DAC_InitStruct.DAC_Trigger=DAC_Trigger_Software;//由软件触发
        DAC_InitStruct.DAC_WaveGeneration=DAC_WaveGeneration_None;//关闭波形生成
        DAC_InitStruct.DAC_LFSRUnmask_TriangleAmplitude=DAC_TriangleAmplitude_4095;
        DAC_InitStruct.DAC_OutputBuffer=DAC_OutputBuffer_Disable;//使能DAC通道缓存   
        DAC_Init(DAC_Channel_1,&DAC_InitStruct);
    /* Enable DAC Channel1 */
        DAC_DualSoftwareTriggerCmd(ENABLE);//使能软件触发,更新DAC的值
        DAC_SetChannel1Data(DAC_Align_12b_R,4000);//12位右对齐,
        DAC_SoftwareTriggerCmd(DAC_Channel_1, ENABLE);
        DAC_Cmd(DAC_Channel_1, ENABLE);
}
主函数中：
    Dac1_Init();            DAC_SetChannel1Data(DAC_Align_12b_R, 4000);    DAC_SoftwareTriggerCmd(DAC_Channel_1, ENABLE); 测PA4无电压输出啊！求解！

定一下，单付款啦放假啊