Calculation Adc value based on CubeMx

jjj0214 · ‎2016-03-15

Posted on March 16, 2016 at 02:42

Hello guys.

I am using STM32F051K8 with CubeMX. While using ADC conversion, the adc value is working weird.

The value I measured is adc value from position sensor. the range is from 0 to 2.9V(measured with Oscilloscope).

When the value is reached in 2.9v, the measured ad value is 3600 which is correct(3600/4095 * 3.3v). However the ad value became 0 when it measured below than 0.7v.

Is there any calculation that I missed?

For example, if 1v is measured, I think the ad value should be 1240 (1v/3.3v*4095(12bits)) but the real ad value is 280.

Please help me with this. Thank you for reading guys.

#adc #stm32cube #bug

raptorhal2 · ‎2016-03-15

Posted on March 16, 2016 at 03:07

Try setting the sampling time to max cycles. If that substantially improves the result, your position sensor probably has a high output impedance.

If increasing the sampling time is not the answer, provide more details on the sensor, adc channel and your code.

Cheers, Hal

jjj0214 · ‎2016-03-15

Posted on March 16, 2016 at 03:38

I tried to increase the sampling but it wont help neither :(

I am using the 357 potentionmeter from Vishay.

I attached a datasheet in pdf.

Here is thecircuit diagram

And, for the source code, I didnt do anything.

What I put is only

__IO uint16_t ADC_val[2]=0;

HAL_ADC_Start_DMA(&hadc,(uint32_t*)&ADC_val, 2);

and for Adc Init()

ADC_ChannelConfTypeDef sConfig;

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

*/

hadc.Instance = ADC1;

hadc.Init.ClockPrescaler = ADC_CLOCK_SYNC_PCLK_DIV2;

hadc.Init.Resolution = ADC_RESOLUTION12b;

hadc.Init.DataAlign = ADC_DATAALIGN_RIGHT;

hadc.Init.ScanConvMode = ADC_SCAN_DIRECTION_FORWARD;

hadc.Init.EOCSelection = EOC_SINGLE_CONV;

hadc.Init.LowPowerAutoWait = DISABLE;

hadc.Init.LowPowerAutoPowerOff = DISABLE;

hadc.Init.ContinuousConvMode = ENABLE;

hadc.Init.DiscontinuousConvMode = DISABLE;

hadc.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;

hadc.Init.DMAContinuousRequests = ENABLE;

hadc.Init.Overrun = OVR_DATA_PRESERVED;

HAL_ADC_Init(&hadc);

/**Configure for the selected ADC regular channel to be converted.

*/

sConfig.Channel = ADC_CHANNEL_7;

sConfig.Rank = ADC_RANK_CHANNEL_NUMBER;

sConfig.SamplingTime = ADC_SAMPLETIME_239CYCLES_5;

HAL_ADC_ConfigChannel(&hadc, &sConfig);

/**Configure for the selected ADC regular channel to be converted.

*/

sConfig.Channel = ADC_CHANNEL_VREFINT;

HAL_ADC_ConfigChannel(&hadc, &sConfig);

I hope it can be fixable..

________________

Attachments :

potentionmeter-357.pdf : https://st--c.eu10.content.force.com/sfc/dist/version/download/?oid=00Db0000000YtG6&ids=0680X000006HthB&d=%2Fa%2F0X0000000aVr%2F0IlA_P6gKwMQoP54t7SoTQsgk9304.OEx78P_DKmUVM&asPdf=false

jjj0214 · ‎2016-03-16

Posted on March 16, 2016 at 08:45

Self Answer:

I figured that it occurs because of the potentiometer.

in 360 degrees, this potentiometer can use only 250~260 degrees.

because of the high resistance, around 100~110 degrees are not usable.

and calculation of ad value is different with normal ad calculation.

In this situation, I am just using ad value to get degree not voltage value.

Thanks for Hal to answer my question, anyway it was helpful for me too.

raptorhal2 · ‎2016-03-16

Posted on March 16, 2016 at 18:27

The ADC sampling circuit has a 1K input resistance in series with the 470 Ohm and pot resistance. This causes the mysterious 0.7V drop.

When using a resistive signal source, a buffer amplifier to the ADC channel is recommended.

Cheers, Hal