STM32F446 ADCs Fighting Inputs

[joesnyder]

Posted on March 09, 2017 at 16:59

I am using a single channel on ADC1, and a single channel on ADC2, both using DMA.  Everything code-wise is working, but the ADCs appear to be fighting/resisting the inputs.  For instance, with nothing connected, each ADC pin floats at ~1V.  But then when you try to ground either input through a 10K resistor, I get a ~250mV triangle wave at 1MHz.  Image is attached below, along with CUBE generated ADC initialization code.  Any ideas on what could be causing that?  I would think that grounding an ADC input through a 10K would cause the input to go to zero. 

[code]

static 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_CLOCK_SYNC_PCLK_DIV6;  hadc1.Init.Resolution = ADC_RESOLUTION_12B;  hadc1.Init.ScanConvMode = ENABLE;  hadc1.Init.ContinuousConvMode = ENABLE;  hadc1.Init.DiscontinuousConvMode = DISABLE;  hadc1.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;  hadc1.Init.ExternalTrigConv = ADC_SOFTWARE_START;  hadc1.Init.DataAlign = ADC_DATAALIGN_RIGHT;  hadc1.Init.NbrOfConversion = 1;  hadc1.Init.DMAContinuousRequests = ENABLE;  hadc1.Init.EOCSelection = ADC_EOC_SEQ_CONV;  if (HAL_ADC_Init(&hadc1) != HAL_OK)  {    Error_Handler();  }

    /**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;  //sConfig.SamplingTime = ADC_SAMPLETIME_480CYCLES;  if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK)  {    Error_Handler();  }

}

/* ADC2 init function */

static void MX_ADC2_Init(void){

  ADC_ChannelConfTypeDef sConfig;

    /**Configure the global features of the ADC (Clock, Resolution, Data Alignment and number of conversion)

    */  hadc2.Instance = ADC2;  hadc2.Init.ClockPrescaler = ADC_CLOCK_SYNC_PCLK_DIV6;  hadc2.Init.Resolution = ADC_RESOLUTION_12B;  hadc2.Init.ScanConvMode = ENABLE;  hadc2.Init.ContinuousConvMode = ENABLE;  hadc2.Init.DiscontinuousConvMode = DISABLE;  hadc2.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;  hadc2.Init.ExternalTrigConv = ADC_SOFTWARE_START;  hadc2.Init.DataAlign = ADC_DATAALIGN_RIGHT;  hadc2.Init.NbrOfConversion = 1;  hadc2.Init.DMAContinuousRequests = ENABLE;  hadc2.Init.EOCSelection = ADC_EOC_SEQ_CONV;  if (HAL_ADC_Init(&hadc2) != HAL_OK)  {    Error_Handler();  }

    /**Configure for the selected ADC regular channel its corresponding rank in the sequencer and its sample time.

    */  sConfig.Channel = ADC_CHANNEL_1;  sConfig.Rank = 1;  //sConfig.SamplingTime = ADC_SAMPLETIME_480CYCLES;  sConfig.SamplingTime = ADC_SAMPLETIME_3CYCLES;  if (HAL_ADC_ConfigChannel(&hadc2, &sConfig) != HAL_OK)  {    Error_Handler();  }

}

[/code]

[S.Ma · Accepted solution]

Posted on March 09, 2017 at 17:59

If looking at a single channel per ADC, why use scan mode?

hadc1.Init.ScanConvMode = ENABLE;

What happens if the sample and hold time is increased? (because the input impedence is not low)

Otherwise, for testing pull-up/down, the internal GPIO ones can be used instead of an external physical one?

Are VDDA/VSSA well connected to board common ground/supply?

[joesnyder]

Posted on March 09, 2017 at 20:20

YOU...are awesome.  I had been using scan mode initially, and that was left enabled.  I switched it to DISABLE, and all is well.  Thanks for the help!