STM32F4 Discovery ADC schwankt stark

void ADC_init(void)

{

  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);

  RCC_APB2PeriphClockCmd(RCC_APB2Periph_ADC3, ENABLE);

  //GPIO initialisieren

  GPIO_InitTypeDef  GPIO_InitStructure;

  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_1 | GPIO_Pin_2;

  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;

  GPIO_InitStructure.GPIO_OType = GPIO_OType_OD;

  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;

  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_2MHz;

  GPIO_Init(GPIOA, &GPIO_InitStructure);

  //erweiterte ADC Funktionen

  ADC_CommonInitTypeDef ADC_CommonInitStruct;

  ADC_CommonInitStruct.ADC_Mode = ADC_Mode_Independent;

  ADC_CommonInitStruct.ADC_Prescaler = ADC_Prescaler_Div4;

  ADC_CommonInitStruct.ADC_DMAAccessMode = ADC_DMAAccessMode_Disabled;

  ADC_CommonInitStruct.ADC_TwoSamplingDelay = ADC_TwoSamplingDelay_5Cycles;

  ADC_CommonInit(&ADC_CommonInitStruct);

  //als ADC initialisieren

  ADC_InitTypeDef ADC_InitStruct;

  ADC_InitStruct.ADC_Resolution = ADC_Resolution_12b;

  ADC_InitStruct.ADC_ScanConvMode = DISABLE;

  ADC_InitStruct.ADC_ContinuousConvMode = ENABLE;

  ADC_InitStruct.ADC_ExternalTrigConvEdge = ADC_ExternalTrigConvEdge_None;

  ADC_InitStruct.ADC_DataAlign = ADC_DataAlign_Right;

  ADC_InitStruct.ADC_NbrOfConversion = 1;

  ADC_Init(ADC1, &ADC_InitStruct);

  ADC_Cmd(ADC3, ENABLE);

}

uint16_t ADC_Read(uint8_t kanal)

{

  uint16_t messwert = 0;

  ADC_RegularChannelConfig(ADC3, kanal, 1, ADC_SampleTime_480Cycles);

  ADC_SoftwareStartConv(ADC3);

    while(ADC_GetFlagStatus(ADC3, ADC_FLAG_EOC)==RESET);

    messwert = ADC_GetConversionValue(ADC3);

    return(messwert);

}