ADC conversion question

bee2 · ‎2013-04-13

Posted on April 13, 2013 at 17:39

I have a potential divider circuit set up using the 3V and GND pins on the discovery board and two 100ohm pots, I measure a constant divided voltage of 1.25 (from ground to the ADC input) but when I debug in Uvision my converted voltage drifts between 1.08-1.13 V.

Below is the ADC/DMA setup I'm using plus my voltage calculation.

I'm wondering if input impedance could cause the difference in my measured value and the ADC converted value?

Also any ideas why the drifting occurs, is it down to junction temperatures?

Calculation used in main:

//convert the ADC value (from 0 to 0xFFF) to a voltage value (from 0V to 2.5V)

ADC3ConvertedVoltage = ADC3ConvertedValue *(float)2.5/4096;

ADC/DMA setup

/* Includes ------------------------------------------------------------------*/

#include <stdio.h>

#include ''stm32f4xx.h''

#include ''stm32f4_discovery.h''

/* Private define ------------------------------------------------------------*/

#define ADC3_DR_ADDRESS ((uint32_t)0x4001224C)

/* Private variables ---------------------------------------------------------*/

volatile uint16_t ADC3ConvertedValue=0;

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

* @brief ADC3 channel12 with DMA configuration

* @param None

* @retval None

*/

void ADC3_CH12_DMA_Config(void)

{

ADC_InitTypeDef ADC_InitStructure;

ADC_CommonInitTypeDef ADC_CommonInitStructure;

DMA_InitTypeDef DMA_InitStructure;

GPIO_InitTypeDef GPIO_InitStructure;

/* 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 = 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(DMA2_Stream0, &DMA_InitStructure);

DMA_Cmd(DMA2_Stream0, ENABLE);

/* Configure ADC3 Channel12 pin as analog input ******************************/

GPIO_InitStructure.GPIO_Pin = GPIO_Pin_2;

GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;

GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL ;

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 = DISABLE;

ADC_InitStructure.ADC_ContinuousConvMode = ENABLE;

ADC_InitStructure.ADC_ExternalTrigConvEdge = ADC_ExternalTrigConvEdge_None;

ADC_InitStructure.ADC_DataAlign = ADC_DataAlign_Right;

ADC_InitStructure.ADC_NbrOfConversion = 1;

ADC_Init(ADC3, &ADC_InitStructure);

/* ADC3 regular channel12 configuration *************************************/

ADC_RegularChannelConfig(ADC3, ADC_Channel_12, 1, 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);

}

bee2 · ‎2013-04-18

Posted on April 18, 2013 at 20:21

Hi Knik

Thanks for reply again, Yea I had tried the pull up/down options. I've just found that the problem is to do with the Embest expansion board I'm using, as soon as I split the discovery board from it the values stay steady, bit of a learning curve for me. I'm just going to spend some time trying to find out why it causes the problem.

Thanks everyone for ideas etc.

Cheers

Expansion board I've been using

http://uk.farnell.com/stmicroelectronics/stm32f4dis-bb/expansion-stm32f4-cortex-m4-discovery/dp/2250205