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stm32f4discovery, continues adc, noise

Question asked by belomutskiy.andrey on Aug 8, 2013
Latest reply on Aug 12, 2013 by de_haas.volker
I'm sorry, that's probably quite a beaten topic :(

I have continues mode ADC running on stm32f4discovery board.
static void ADC_Config(void) {
    ADC_InitTypeDef ADC_InitStructure;
    ADC_CommonInitTypeDef ADC_CommonInitStructure;
    DMA_InitTypeDef DMA_InitStructure;
    GPIO_InitTypeDef GPIO_InitStructure;
 
    /* Enable ADCx, DMA and GPIO clocks ****************************************/
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA2, ENABLE);
    RCC_AHB1PeriphClockCmd(ADCx_CHANNEL_GPIO_CLK, ENABLE);
    RCC_APB2PeriphClockCmd(ADCx_CLK, ENABLE);
 
    /* DMA2 Stream0 channel2 configuration **************************************/
    DMA_InitStructure.DMA_Channel = DMA_CHANNELx;
    DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t) ADCx_DR_ADDRESS;
    DMA_InitStructure.DMA_Memory0BaseAddr = (uint32_t) &uhADCxConvertedValue;
    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(DMA_STREAMx, &DMA_InitStructure);
    DMA_Cmd(DMA_STREAMx, ENABLE);
 
    /* Configure ADC3 Channel7 pin as analog input ******************************/
    GPIO_InitStructure.GPIO_Pin = GPIO_PIN;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
    GPIO_Init(GPIO_PORT, &GPIO_InitStructure);
 
    /* ADC Common Init **********************************************************/
    ADC_CommonInitStructure.ADC_Mode = ADC_Mode_Independent;
    ADC_CommonInitStructure.ADC_Prescaler = ADC_Prescaler_Div4;
    ADC_CommonInitStructure.ADC_DMAAccessMode = ADC_DMAAccessMode_Disabled;
    ADC_CommonInitStructure.ADC_TwoSamplingDelay = ADC_TwoSamplingDelay_20Cycles;
    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_ExternalTrigConv = ADC_ExternalTrigConv_T1_CC1;
    ADC_InitStructure.ADC_DataAlign = ADC_DataAlign_Right;
    ADC_InitStructure.ADC_NbrOfConversion = 1;
    ADC_Init(ADCx, &ADC_InitStructure);
 
    /* ADC3 regular channel7 configuration *************************************/
    ADC_RegularChannelConfig(ADCx, ADC_CHANNEL, 1, ADC_SampleTime_480Cycles);
 
    /* Enable DMA request after last transfer (Single-ADC mode) */
    ADC_DMARequestAfterLastTransferCmd(ADCx, ENABLE);
 
    /* Enable ADC3 DMA */
    ADC_DMACmd(ADCx, ENABLE);
 
    /* Enable ADC3 */
    ADC_Cmd(ADCx, ENABLE);
    print("ADC_Cmd ENABLE\r\n");
}

My board is powered via the mini-USB plus I have a UART-USB dongle connected to the same desktop. My ADC input is connected to a voltage divider made of two 150 ohm resistors between GND and 3V on the board.

My loop prints values to console:

while (1) {
   Delay(200); // 200 ms
   print("ADC: %d\r\n", uhADCxConvertedValue);
}

My question is about the noise. The values I see on the console are jumping in the 1957-1971 range, that's a 1% precision - that's a loss of 4 bits. I would like to reduce this noise.

Unfortunately I am a software developer, not an electrical engineer. I've tried a 0.1uf capacitor between the ADC input and ground and this does not reduce the noise. As far as I know I do not have an option to use "external reference voltage" with this board unless I modify it. Other than digital filtering, could anything be done here?

Would it help if I modify the board to provide external ADC reference voltage? Would you recommend switching to external ADC chip altogether? Is there another dev board I should look at?

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