STM32F4Discovery + ADC/DMA, instabile Werte

Hallo zusammen, ich bekomme sehr komische und instabile ADC Werte. Am 
ADC Eingang liegt eine Spannung vom Labornetzteil im Bereich 0-3V. Die 
Funktion ADC_Config() habe ich aus den FW Beispielen entnommen. Ich habe 
viel damit experementiert, Kanaele gewechselt, DMA ein und ausgemacht, 
doch leider ohne Erfolg, die ADC Werte wackeln viel zu sehr. IN 
stm32f4xx.h ist der HSE Wert auf 8MHz. Hier ist der Code:

void ADC_Config(void)
{
  ADC_InitTypeDef       ADC_InitStructure;
  ADC_CommonInitTypeDef ADC_CommonInitStructure;
  DMA_InitTypeDef       DMA_InitStructure;
  GPIO_InitTypeDef      GPIO_InitStructure;

  /* Enable ADC3, DMA2 and GPIO clocks 
****************************************/
  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA2 | RCC_AHB1Periph_GPIOC, 
ENABLE);
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_ADC3, ENABLE);

  /* DMA2 Stream0 channel2 configuration 
**************************************/
  DMA_InitStructure.DMA_Channel = DMA_Channel_2;
  DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t)&ADC3->DR;
  DMA_InitStructure.DMA_Memory0BaseAddr = (uint32_t)&ADC3ConvertedValue;
  DMA_InitStructure.DMA_DIR = DMA_DIR_PeripheralToMemory;
  DMA_InitStructure.DMA_BufferSize = 1;
  DMA_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
  DMA_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Disable;
  DMA_InitStructure.DMA_PeripheralDataSize = 
DMA_PeripheralDataSize_HalfWord;
  DMA_InitStructure.DMA_MemoryDataSize = DMA_MemoryDataSize_HalfWord;
  DMA_InitStructure.DMA_Mode = DMA_Mode_Circular;
  DMA_InitStructure.DMA_Priority = DMA_Priority_High;
  DMA_InitStructure.DMA_FIFOMode = DMA_FIFOMode_Disable;
  DMA_InitStructure.DMA_FIFOThreshold = DMA_FIFOThreshold_HalfFull;
  DMA_InitStructure.DMA_MemoryBurst = DMA_MemoryBurst_Single;
  DMA_InitStructure.DMA_PeripheralBurst = DMA_PeripheralBurst_Single;
  DMA_Init(DMA2_Stream0, &DMA_InitStructure);
  DMA_Cmd(DMA2_Stream0, ENABLE);

  /* Configure ADC3 Channel12 pin as analog input 
******************************/
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_2;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL ;
  GPIO_Init(GPIOC, &GPIO_InitStructure);

  /* ADC Common Init 
**********************************************************/
  ADC_CommonInitStructure.ADC_Mode = ADC_Mode_Independent;
  ADC_CommonInitStructure.ADC_Prescaler = ADC_Prescaler_Div2;
  ADC_CommonInitStructure.ADC_DMAAccessMode = 
ADC_DMAAccessMode_Disabled;
  ADC_CommonInitStructure.ADC_TwoSamplingDelay = 
ADC_TwoSamplingDelay_5Cycles;
  ADC_CommonInit(&ADC_CommonInitStructure);

  /* ADC3 Init 
****************************************************************/
  ADC_InitStructure.ADC_Resolution = ADC_Resolution_12b;
  ADC_InitStructure.ADC_ScanConvMode = DISABLE;
  ADC_InitStructure.ADC_ContinuousConvMode = ENABLE;
  ADC_InitStructure.ADC_ExternalTrigConvEdge = 
ADC_ExternalTrigConvEdge_None;
  ADC_InitStructure.ADC_DataAlign = ADC_DataAlign_Right;
  ADC_InitStructure.ADC_NbrOfConversion = 1;
  ADC_Init(ADC3, &ADC_InitStructure);

  /* ADC3 regular channel12 configuration 
*************************************/
  ADC_RegularChannelConfig(ADC3, ADC_Channel_12, 1,
    ADC_SampleTime_3Cycles);

 /* Enable DMA request after last transfer (Single-ADC mode) */
  ADC_DMARequestAfterLastTransferCmd(ADC3, ENABLE);

  /* Enable ADC3 DMA */
  ADC_DMACmd(ADC3, ENABLE);

  /* Enable ADC3 */
  ADC_Cmd(ADC3, ENABLE);

  ADC_SoftwareStartConv(ADC3);

}

int main(void)
{
  ADC_Config();

  while(1){
  }
}


Hat Jemand irgendeine Idee warum die ADC Werte sehr instabil sind? 
Vielen Dank!

Ilya