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STM32F4 ADC Impedance Calculation

Question asked by bryan costanich on Nov 5, 2017
Latest reply on Nov 15, 2017 by bryan costanich

Hey folks!


I’m trying to calculate the impedance and current draw of the ADC in an STM32F4 that’s on the Netduino. The data sheet says that the max External Input Impedance is 50KΩ, but gives the following equation for calculating the actual impedance:


ADC input impedance equation


I’ve having a hard time tracking down some of these variables, though. For instance, k is supposed to be the number of sampling periods, but I don’t know where that register is defined. I can’t find it anywhere in the firmware source. However, I have found some of the other things:


  • MCU Frequency: 168MHz
  • ADC Clock: set to default, which is peripheral clock / 2 (according to the note next to this in source). I’m pretty sure from my reading that the peripheral clock is the same as the MCU clock. So 168MHz / 2 = 84MHz. However, the data sheet says that the ADC Clock Frequency (fADC) is between 30 and 36MHz for VDDA of 2.5V to 3.6V, and the VDDA on the Netduino should be around 3.3V. So what is the actual value here?
  • ADC Sampling Time: 28 cycles (I found this as a note in the source somewhere)
  • N: 12 (because it’s 12 bit resolution)
  • RADC: This is the sampling switch resistance, and is stated to be 6kΩ in the data sheet.
  • CADC: this is the internal sample and hold capacitor, which has a stated value of between 4 and 7 pF.I assume I can just use 5.5pF for calculations.


Hoping that you folks can help clear up the ADC Clock value based on the other items so I can do the calculation. Or, if there's some other value I need to know, please let me know so I can track it down.