Electronic – Connecting ADC pin to op. amp. output distorts signal

adcdistortionmicrocontrollervoltage measurement

I am using a STM32 Cortex M3 microcontroller and have configured the ADC. I have verified that the ADC works by measuring different voltages on the micro controller eval. board.

When I connect my ADC to the output of the op amp (MCP6002) the signal gets distorted. A picture showing how the signal looks at the output before and after I connect the ADC is shown below.

The ADC pin is configured as analog in (as it should). When connecting pins with other configuration the signal is not distorted. The op. amp. is powered by the microcontroller eval. board.

Any help is much appreciated! =)

EDIT:
Added schematic.
Actual op. amp. used is MCP6002 instead of those shown in circuit. When I connect ADC1 to the ADC on my microcontroller the signal gets distorted.

EDIT2:
Additional observation: When scoping and placing the ADC to the 3.3V power rail provided by the microcontroller the scope reads 3.3V. When doing the same with GND i read 0V. But when connecting it to the (3.3V / 2) virtual GND made by one of the MCP6002 op. amps. the result is the same as the distorted picture below.

EDIT3:
I just set up a new ADC with identical settings (I will compare them agian) and it works. I assume I will find some error in code/configuration somewhere. I will let you know.

EDIT4/Solution:
I am again amazed by myself. The same pin has been configured as a digital push-pull somewhere else in code, but no longer used by this module. This code has been present for over a year, but since it probably always have been initialized before the ADC, the ADC have been overwriting the pin configuration and everything has been working fine. This was until the ADC initialization had been moved and done before the other configuration, making the ADC configuration overwritten as a digital input push-pull. I wish to thank all for the great help and input and their time.

Before on top and after on bottom

Schematic

Best Answer

A few things to make sure of:

Make sure you have decoupling capacitors (e.g. 100nF) on all opamp power pins.
As a unity gain configuration is sensitive to capacitive load (the ADC input pin), try isolating it using a series resistor (between opamp output and ADC input), value say between 220Ω and 820Ω.
Make completely sure there is nothing else affecting the ADC input, and it is set correctly as an analog input - check it with a known source such as a signal generator to confirm the same does not happen.
Make sure the analog section and the uC share a common ground.
Check none of the opamps are oscillating at any point.
Check supply rails are okay (check under load also)

Let us know how it goes, it should be a reasonably easy issue to track down, but no doubt Murphy will have something to say about that ;-)
A proper schematic with all parts present would be helpful (i.e. I'm assuming the opamps have decoupling caps but you did not include them on the LTSpice schematic)

Related Solutions

General Questions about PIC24 MCU Input and Output Pin Impedance, Voltage, and Current

So I'm not too sure I follow this first question here, but I'll take a stab at it: if Vin = 3.3V (i.e. your P-channel mosfet does not connect source and drain) then "TRIGGER" will essentially be disconnected, yes. This assumes that "TRIGGER" is connected to a high-impedance source, like an input pin on a standard microcontroller. As a quick note "essentially disconnected" means that there is a high impedance threshold to get across the source-drain junction, but it is not infinite. If Vin = 0V (i.e. source and drain are connected), then trigger will be close to (but slightly less than) 3.3V
So an input pin on a PIC24F is high impedance. This means that it will behave like a resistor with around 1MΩ of resistance to ground. If you check the "Electrical Characteristics" section in the "DC Characteristics: I/O Pin Input Specifications" subsection in the datasheet for the chip you're using, you should find a table which gives you the "Input Leakage Current." Using the dsPIC33FJ256GP710A as an example (it was a datasheet I happened to have on my machine), it lists 0 to 3.5 μA as the leakage current into or out of a pin within the limits of those pins.

So as a note, you'll also see that in the "Electrical Characteristics" section under the "Absolute Maximum Ratings" subsection, there will be a list which says something to the effect of "Voltage on any pin that is not 5V tolerant with respect to VSS(4) .... -0.3V to (VDD + 0.3V)". This is telling you that you can't put more than ~3.6V on any pin without damaging the pin. This doesn't mean that you should put a current-limiting resistor in front of the input pin, this means you should put a voltage divider in front of the input pin to limit the maximum voltage to less than VDD + 0.3V.

To tie this into question #1: this means that when a digital input pin is connected to a high-impedance output source like Fig. 1, you will see oscillating input values (1 and 0 randomly) based on when that very small drain removes or adds enough electrons to jump from one digital state to another. This is called a floating input
So I just answered this question in the previous one, but I should tell you: the "Maximum output current" is not internally limited. So although the PIC24F can "only output 18mA" this means that if you connect an output pin which is high directly to ground, it will output lots more than 18mA before it burns out! The 18mA is the maximum it can safely output without overheating/damaging the chip. This is true for both sourcing and sinking, there is no fuse which will stop you at 18mA, only the magic blue smoke.

Just to reiterate: you can safely connect any voltage to a digital input pin within its range (-0.3V to VDD+0.3V for the dsPIC33F). It would not blow the pin to connect it directly to +3.3V, nor would it blow the pin to connect directly to ground.
Just answered this in #3, but again, it's not the output impedance of the pin which limits this... Or, more accurately, it is the output impedance, but it will limit the currently by melting.
And yes, you can safely remove the 1kΩ resistor from Fig. 2 without damaging your PIC.

What does ADCXX_INYY represent? Configuring ADC on certain pins from STM32F207ZG Board

So it turns out that ADCXX_INYY, means that XX - the type of ADC's that you can configure to (i.e. 12 = can configure to ADC1 or ADC2). YY represents that GPIO channel where you can configure ADC.

Also, you need to do ADC_CommonInitStructure only once.

Here is the new and improved version of the code for further convenience.

static volatile Int32U TimingDelay;


int i;


__IO Int16U Adcdata[3];


__IO float Adc1=0;
float Adc1_avg=0;


__IO float Adc2=0;
float Adc2_avg=0;


__IO float Adc3=0;
float Adc3_avg=0;

/**************************************************************************************/

void RCC_Configuration(void)
{
  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA2 | RCC_AHB1Periph_GPIOC | RCC_AHB1Periph_GPIOB | RCC_AHB1Periph_GPIOA | RCC_AHB1Periph_GPIOE, ENABLE);
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_ADC1 | RCC_APB2Periph_ADC2 | RCC_APB2Periph_ADC3, ENABLE);
}

/**************************************************************************************/


void DelayResolution100us(Int32U Dly)
{
  TimingDelay = Dly;


  while(TimingDelay != 0);
}


/**
  * @brief  Decrements the TimingDelay variable.
  * @param  None
  * @retval None
  */
void TimingDelay_Decrement(void)
{
  if (TimingDelay != 0x00)
  {
    TimingDelay--;
  }
}


void GPIO_Configuration(void)
{
  GPIO_InitTypeDef GPIO_InitStructure;

  /* ADC Channel  8 -> PB0  ADC12_IN8
     ADC Channel  15 -> PC5 ADC12_IN15
     ADC Channel  0 -> PA0 ADC123_IN0
  */

  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;

  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0;
  GPIO_Init(GPIOB, &GPIO_InitStructure);
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_5;
  GPIO_Init(GPIOC, &GPIO_InitStructure);
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0;
  GPIO_Init(GPIOA, &GPIO_InitStructure);
}

/**************************************************************************************/

void ADC_Configuration(void)
{

  ADC_DeInit();

  ADC_CommonInitTypeDef ADC_CommonInitStructure;
  ADC_InitTypeDef ADC_InitStructure;

  /* ADC Common Init - COMMON - Do it once, do it first */
  ADC_CommonInitStructure.ADC_Mode = ADC_TripleMode_RegSimult;
  ADC_CommonInitStructure.ADC_Prescaler = ADC_Prescaler_Div2;
  ADC_CommonInitStructure.ADC_DMAAccessMode = ADC_DMAAccessMode_1; // 2 half-words one by one, 1 then 2 then 3
  ADC_CommonInitStructure.ADC_TwoSamplingDelay = ADC_TwoSamplingDelay_6Cycles; //6 or 5?
  ADC_CommonInit(&ADC_CommonInitStructure);

  ADC_InitStructure.ADC_Resolution = ADC_Resolution_12b;
  ADC_InitStructure.ADC_ScanConvMode = DISABLE; // 1 Channel - why is it DISABLE?
  ADC_InitStructure.ADC_ContinuousConvMode = ENABLE; // Continuous Conversions
  ADC_InitStructure.ADC_ExternalTrigConvEdge = ADC_ExternalTrigConvEdge_None;
  //ADC_InitStructure.ADC_ExternalTrigConv = ADC_ExternalTrigConv_T2_TRGO; // Ignored - why include it if ignored?
  ADC_InitStructure.ADC_DataAlign = ADC_DataAlign_Right;
  ADC_InitStructure.ADC_NbrOfConversion = 1;
  ADC_Init(ADC1, &ADC_InitStructure);
  ADC_Init(ADC2, &ADC_InitStructure); // Mirror on ADC2
  ADC_Init(ADC3, &ADC_InitStructure); // Mirror on ADC3

  /* ADC3 regular channel 8 configuration */
  ADC_RegularChannelConfig(ADC1, ADC_Channel_8, 1, ADC_SampleTime_56Cycles); // PB0

  /* ADC2 regular channel 5 configuration */
  ADC_RegularChannelConfig(ADC2, ADC_Channel_15, 1, ADC_SampleTime_56Cycles); // PC5

  /* ADC3 regular channel 0 configuration */
  ADC_RegularChannelConfig(ADC3, ADC_Channel_0, 1, ADC_SampleTime_56Cycles); // PA0

  /* Enable DMA request after last transfer (Multi-ADC mode)  */
  ADC_MultiModeDMARequestAfterLastTransferCmd(ENABLE);

  /* Enable ADC1 */
  ADC_Cmd(ADC1, ENABLE);

  /* Enable ADC2 */
  ADC_Cmd(ADC2, ENABLE);

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

  ADC_DMACmd(ADC1, ENABLE); //enable ADC1 DMA since ADC1 is the master
}

/**************************************************************************************/

static void DMA_Configuration(void)
{
  DMA_InitTypeDef DMA_InitStructure;

  DMA_DeInit(DMA2_Stream0);  //Set DMA registers to default values

  DMA_InitStructure.DMA_Channel = DMA_Channel_0;
  DMA_InitStructure.DMA_Memory0BaseAddr = (uint32_t)&Adcdata;
  DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t)0x40012308; // CDR_ADDRESS; Packed ADC1, ADC2
  DMA_InitStructure.DMA_DIR = DMA_DIR_PeripheralToMemory;
  DMA_InitStructure.DMA_BufferSize = 3; // Count of 16-bit words
  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);

  /* DMA2_Stream0 enable */
  DMA_Cmd(DMA2_Stream0, ENABLE);
}

/**************************************************************************************/

int main(void)
{

  ENTR_CRT_SECTION();

  if(SysTick_Config(SystemCoreClock/10000))
 { 
    while (1);
  }

  EXT_CRT_SECTION();

  RCC_Configuration();

  GPIO_Configuration();

  ADC_Configuration();

  DMA_Configuration();

  /* Start ADC1 Software Conversion */
  ADC_SoftwareStartConv(ADC1);

  /*Turn on Backlight*/
  GLCD_Backlight(BACKLIGHT_ON);
  GLCD_SetFont(&Terminal_9_12_6,0x000F00,0x00FF0);
  GLCD_SetWindow(10,116,131,131);

  /*-------------------Display Readings-----------------------------------------*/


  GLCD_PowerUpInit((pInt8U)IAR_Logo.pPicStream);


  GLCD_Backlight(BACKLIGHT_ON);

  while(1){
    /***** Compute Gain and Phase ****/
        Adc3_avg = 0;
        Adc2_avg = 0;
        Adc1_avg = 0;

        DelayResolution100us(10000);

        for(i=1;i<=20;i++)
        {
          DelayResolution100us(10000);

          Adc1=(((float)Adcdata[0]) * 3.3)/4096;  //voltage value
          Adc1_avg = Adc1_avg + Adc1; //average gain

            //   Adc2con = (Adcdata[1] & 0x0FFF);
          Adc2=(((float)Adcdata[1]) * 3.3)/4096;  //voltage
          Adc2_avg = Adc2_avg + Adc2;//average phase

          //   Adc3con = (Adcdata[1] & 0x0FFF);
          Adc3=(((float)Adcdata[2]) * 3.3)/4096;  //voltage
          Adc3_avg = Adc3_avg + Adc3;//average phase
        }

        Adc1_avg = Adc1_avg/20;
        Adc2_avg = Adc2_avg/20;
        Adc3_avg = Adc3_avg/20;

        //Write to LCD
        GLCD_TextSetPos(0,0);
        GLCD_print("G:%.02f", Adc3_avg);
        GLCD_TextSetPos(7,0);
        GLCD_print("P1:%.02f", Adc2_avg);
        GLCD_TextSetPos(17,0);
        GLCD_print("P2:%.02f", Adc1_avg);
  }; // Don't want to exit
}

Best Answer

Related Solutions

General Questions about PIC24 MCU Input and Output Pin Impedance, Voltage, and Current

What does ADCXX_INYY represent? Configuring ADC on certain pins from STM32F207ZG Board

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