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sbertuletti
sbertuletti
Associate II
Nov 17, 2016 5:26 PM UTC
Question

ADC reading value

Posted on November 17, 2016 at 18:26

Why if i put a voltage equal to 1V to the ADC input of the microcontroller, i read a value equal to 1.4 V??

It seems that the Voltage_Actual_Value=0.7*ADC_Value

Thanks

#more-facts #!stm32-!adc #please.
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    6 replies

    AvaTar
    AvaTar
    Senior III
    Nov 18, 2016 8:26 AM UTC
    Posted on November 18, 2016 at 09:26

    >Why if i put a voltage equal to 1V to the ADC input of the microcontroller, i read a value equal to 1.4 V??

     

    What makes you think so ?

    slimen
    slimen
    Visitor II
    Nov 18, 2016 8:30 AM UTC
    Posted on November 18, 2016 at 09:30

    Hello,

    Which board are you using?

    You may refer to your product datasheet or reference manual for more clarification.

    Regards

    sbertuletti
    sbertulettiAuthor
    Associate II
    Nov 18, 2016 8:36 AM UTC
    Posted on November 18, 2016 at 09:36

    I used different boards: STM32F411RE_NUCLEO, STM32F401RE_NUCLEO and STM32L071 microcontroller. 

    AvaTar
    AvaTar
    Senior III
    Nov 18, 2016 9:48 AM UTC
    Posted on November 18, 2016 at 10:48

    > Which board are you using?

    This is less a question of board(s) involved, but rather of missing knowledge at the O.P.'s side about the inner workings of an ADC.

    ColdWeather
    ColdWeather
    Senior
    Nov 18, 2016 11:31 AM UTC
    Posted on November 18, 2016 at 12:31

    If you read an ADC value out, you read no voltage but some number between 0 and 4095 (for 12-bit ADC). The weight of the lowest bit depends on the reference voltage Vref used by the ADC. In the most cases that is the VCC - the supply voltage of the CPU. Mostly 3.3V, some Discovery boards have 3.0V. Thus, the actual voltage measured by the ADC is

    V = Vref * ADC_Value / 4095

    for the Vref = VCC = 3.3V:

    V = 3.3 * ADC_Value / 4095

    The VCC can vary. To solve this, you can use alternatively the internal voltage reference Vint = 1.2V to find the weight of the lowest ADC bit out. Measure the Vint by the ADC, you'll get some number ADC_Int:

    Weight = Vint / ADC_Int = 1.2 / ADC_Int (volts per bit)

    then the measured external voltage would be

    V = Weight * ADC_Value = Vint * ADC_Value / ADC_Int

    and the VCC used as the Vref:

    VCC = 4095 * Weight = 4095 * Vint / ADC_Int = 4914 / ADC_Int .

    Check this on your boards.

    sbertuletti
    sbertulettiAuthor
    Associate II
    Nov 18, 2016 11:57 AM UTC
    Posted on November 18, 2016 at 12:57

    Thanks for the explanation. I'll check it on my board

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