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strange behavior of the adc

Question asked by marino.fernando on Mar 12, 2013
Latest reply on Mar 12, 2013 by baird.hal.001
hi!
I've built my simple application reading two analog input using a single adc and transferring all into memory with dma. Anyway the code not always works. Sometimes I get consistent readings, some others I get values that exceed a lot the theoretical upper bound of 4095, getting completely wrong readings. I'm not familiar with electronics concept so I can't decide well on the configuration settings of the ports.
Here is my hw init code:

void peripheralInit(void)
{
    GPIO_InitTypeDef GPIO_InitStructure;
    DAC_InitTypeDef  DAC_InitStructure;
    ADC_InitTypeDef       ADC_InitStructure;
    ADC_CommonInitTypeDef ADC_CommonInitStructure;
    DMA_InitTypeDef       DMA_InitStructure;
 
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_DAC, ENABLE);
 
    /* DAC channel 1 (DAC_OUT1 = PA.4)(DAC_OUT2 = PA.5) configuration */
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_4;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;
    GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
    GPIO_Init(GPIOA, &GPIO_InitStructure);
 
    DAC_InitStructure.DAC_Trigger = DAC_Trigger_None;
    DAC_InitStructure.DAC_WaveGeneration = DAC_WaveGeneration_None;
    DAC_InitStructure.DAC_OutputBuffer = DAC_OutputBuffer_Disable;
    DAC_Init(DAC_Channel_1, &DAC_InitStructure); // PA4
 
    DAC_Cmd(DAC_Channel_1, ENABLE);
 
  /* Enable ADC3, DMA2 and GPIO clocks ****************************************/
  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA2 | RCC_AHB1Periph_GPIOC, ENABLE);
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_ADC3, ENABLE);
 
  /* DMA2 Stream0 channel0 configuration **************************************/
  DMA_InitStructure.DMA_Channel = DMA_Channel_2; 
  DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t)ADC3_DR_ADDRESS;
  DMA_InitStructure.DMA_Memory0BaseAddr = (uint32_t)&ADC3ConvertedValue;
  DMA_InitStructure.DMA_DIR = DMA_DIR_PeripheralToMemory;
  DMA_InitStructure.DMA_BufferSize = 2;
  DMA_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
  DMA_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Enable;
  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_Enable;
  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_Pin_1;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_DOWN ;
  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 = ENABLE;
  ADC_InitStructure.ADC_ContinuousConvMode = ENABLE;
  ADC_InitStructure.ADC_ExternalTrigConvEdge = ADC_ExternalTrigConvEdge_None;
  ADC_InitStructure.ADC_DataAlign = ADC_DataAlign_Right;
  ADC_InitStructure.ADC_NbrOfConversion = 2;
  ADC_Init(ADC3, &ADC_InitStructure);
 
  /* ADC3 regular channel12 configuration *************************************/
  ADC_RegularChannelConfig(ADC3, ADC_Channel_12, 1, ADC_SampleTime_3Cycles);
  ADC_RegularChannelConfig(ADC3, ADC_Channel_11, 2, 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);
}

to pc2 I've connected a pressure sensor that has an output impedance of 1kohm.

Additionally, can you please tell me if the dac settings is correct? to pa4 port I've connected an opamp circuit (not designed by me! )

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