Electronic – ADC works with pots but not with piezos

I have ADC on stm32f429 with 3 channels turned on(writing data with DMA). Though I suspect this question may be not STM32-specific, probably a general ADC question.

THE "ANSWER"
I found "solution". I tried to use channel 6 and 7 instead of 1 and 2 – problem is gone. Apparently channel 1 and 2 are
"special", though I couldn't find why in datasheet(or I missed it).
Maybe they are more picky on the input power, idk. If there is someone
who can give some information – feel free, I will accept it as an
answer(if it is good enough). Maybe it IS stm32 specific, who knows…

• All 3 channels get correct ADC conversion value if I connect them to potentiometer(or directly to Vcc(4095 value) or GND(0 value))
• If I leave them floating then things start to get weird – channel 0 has value ~400 on it and channel 1 and 2 have value 0. What is going on?
• And the real problem is – I have 3 sensors that I want to connect to ADC, but for some reason only ADC channel 0 works and channel 1 and 2 do not work with sensor(even though they were working with pots!)

What I have done/noticed:

1. This seems has nothing to do with the fact that I use three sensors – I tried to test one at a time – the conclusion is that sensors work great with channel 0 and don't work at all at channel 1 and 2, even when tested one by one without making problems to each other.
2. This is really weird – when I probe with the oscilloscope the output of my sensor it works OK without ADC connected at all or with Channel 0 connected to it. Now as soon as I connect channel 1 or 2 to it – oscilloscope shows zero regardless of what I do with the piezo.

My sensor is setup is basically a piezo sensor with some diodes to clip it to 3.3 V and omit negative part(I also tried to put 1M resistor in parallel to cap, but this solved nothing).

^{simulate this circuit – Schematic created using CircuitLab}

On this screenshot you can see the debug window in Keil uVision, it shows value 403 on the ADC ch 0 even if nothing is connected to it. I measured it is ~200-250 mV. Because Channel 0 is the only one working with sensor I assume other channels should have the same ~250mV on them, but they are not.

Here are my configurations for stm32f429 using HAL library. Every channel should be connected the same, but I wonder if something is wrong in configs…

void MX_ADC1_Init(void)
{

  ADC_ChannelConfTypeDef sConfig;

    /**Configure the global features of the ADC (Clock, Resolution, Data Alignment and number of conversion) 
    */
  hadc1.Instance = ADC1;
  hadc1.Init.ClockPrescaler = ADC_CLOCKPRESCALER_PCLK_DIV4;
  hadc1.Init.Resolution = ADC_RESOLUTION12b;
  hadc1.Init.ScanConvMode = ENABLE;
  hadc1.Init.ContinuousConvMode = ENABLE;
  hadc1.Init.DiscontinuousConvMode = DISABLE;
  hadc1.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;
  hadc1.Init.DataAlign = ADC_DATAALIGN_RIGHT;
  hadc1.Init.NbrOfConversion = 3;
  hadc1.Init.DMAContinuousRequests = ENABLE;
  hadc1.Init.EOCSelection = EOC_SEQ_CONV;
  HAL_ADC_Init(&hadc1);

    /**Configure for the selected ADC regular channel its corresponding rank in the sequencer and its sample time. 
    */
  sConfig.Channel = ADC_CHANNEL_0;
  sConfig.Rank = 1;
  sConfig.SamplingTime = ADC_SAMPLETIME_3CYCLES;
  HAL_ADC_ConfigChannel(&hadc1, &sConfig);

    /**Configure for the selected ADC regular channel its corresponding rank in the sequencer and its sample time. 
    */
  sConfig.Channel = ADC_CHANNEL_1;
  sConfig.Rank = 2;
  HAL_ADC_ConfigChannel(&hadc1, &sConfig);

    /**Configure for the selected ADC regular channel its corresponding rank in the sequencer and its sample time. 
    */
  sConfig.Channel = ADC_CHANNEL_2;
  sConfig.Rank = 3;
  HAL_ADC_ConfigChannel(&hadc1, &sConfig);

}

And stm32f4xx_hal_msp.c file

void HAL_ADC_MspInit(ADC_HandleTypeDef* hadc)
{

  GPIO_InitTypeDef GPIO_InitStruct;
  if(hadc->Instance==ADC1)
  {
  /* USER CODE BEGIN ADC1_MspInit 0 */

  /* USER CODE END ADC1_MspInit 0 */
    /* Peripheral clock enable */
    __ADC1_CLK_ENABLE();

    /**ADC1 GPIO Configuration    
    PA0/WKUP     ------> ADC1_IN0
    PA1     ------> ADC1_IN1
    PA2     ------> ADC1_IN2 
    */
    GPIO_InitStruct.Pin = GPIO_PIN_0|GPIO_PIN_1|GPIO_PIN_2;
    GPIO_InitStruct.Mode = GPIO_MODE_ANALOG;
    GPIO_InitStruct.Pull = GPIO_NOPULL;
    HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

    /* Peripheral DMA init*/

    hdma_adc1.Instance = DMA2_Stream0;
    hdma_adc1.Init.Channel = DMA_CHANNEL_0;
    hdma_adc1.Init.Direction = DMA_PERIPH_TO_MEMORY;
    hdma_adc1.Init.PeriphInc = DMA_PINC_DISABLE;
    hdma_adc1.Init.MemInc = DMA_MINC_ENABLE;
    hdma_adc1.Init.PeriphDataAlignment = DMA_PDATAALIGN_WORD;
    hdma_adc1.Init.MemDataAlignment = DMA_MDATAALIGN_WORD;
    hdma_adc1.Init.Mode = DMA_CIRCULAR;
    hdma_adc1.Init.Priority = DMA_PRIORITY_HIGH;
    hdma_adc1.Init.FIFOMode = DMA_FIFOMODE_DISABLE;
    HAL_DMA_Init(&hdma_adc1);

    __HAL_LINKDMA(hadc,DMA_Handle,hdma_adc1);

  /* Peripheral interrupt init*/
    HAL_NVIC_SetPriority(ADC_IRQn, 0, 0);
    HAL_NVIC_EnableIRQ(ADC_IRQn);
  /* USER CODE BEGIN ADC1_MspInit 1 */

  /* USER CODE END ADC1_MspInit 1 */
  }

}

EDIT 1
I also tried just to use piezo sensor with 1M resistor – same thing, as soon as I connect ch 1 or 2 – oscilloscope shows zero.

EDIT 2
One more strange thing – I beeped ch1 and ch2 pins and they seem to be grounded. O_o