Posted on November 18, 2016 at 12:31If 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.
