What is the ADC Input impedance for connecting sensors?

#1

Hi, I'm trying to optimally filter a sensor while I read it from my Axoloti. I was wondering what the effective impedance is on the ADCs so I can design the last stage of my output filter. Thank you :grinning:

#2

The short answer is 50kOhm, the long answer is found in the microcontroller datasheet, in section 6.3.21: 12-bit ADC characteristics.

#3

For the long answer, there's the equation which involves F_ADC and k, the number of sampling periods in ADC_SMPR1. I was wondering if you could tell me those values? I appreciate the effort and I know players will be grateful for the extra sensitivity I can give them through knowing this.

#4

ADC_SMPR1 is configured for 84 samples (in axoloti_board.c)
The ADC clock is the APB2 clock (84MHz) divided by 4 (ADC_CCR_ADCPRE_DIV4 in mcuconfig.h), so I believe F_ADC is 21MHz. So the datasheet equation tells, using quoted typical Cadc and Radc values:

Rain = (84-0.5)/(21M*4p*ln(2^(12+2)))-6k
... = 83.5/(84E-6 *ln(16384))-6k
... = 96436 ohm

Input resistance/impedance of GPIOs in AnalogIn Mode?