Input Impedance or Resistance

[NoriLapp]

I am using pico-Scoppy oscilloscpe for ECG and heart pulse wave(fig.1 & 2). I tried to measure the input impedance(or resistance:Rin) of ADC0,ADC1(fig.3). I mesured the Rin of ADC as almost 10Mohm(fig.4). Is this correct?

[NoriLapp]

Oh No! This is the input resistance of the multimeter as about 11.1Mohm. Real input impedance of ADC-input of pico is over 10Mohm. May be it is almost infinite. Sorry, I made mistake!

[NoriLapp]

RinP : Input Resistance of ADC-input of Pico
RinM: Input Resistance of the multimeter
As fig.5 Rmeasured which I measured was the parallel resistance of RinP and RinM.
Wrong measured value of Rmeasured=10.88Mohm
RinM=11.10Mohm
I can calculate the real Rinp as following.
RinP=(Rmeasured*RinM)/(Rmeasured+RinM)=549Mohm
This calculation contains a very large error. Maybe RinP is infinete as Arduino ADC-input.

[NoriLapp]

I measured the ADC input resistance (Rin) of the Raspberry Pi Pico.
I connected a 500 MΩ resistor (Rs) between the 3.3V output and the ADC0 input (Fig1).
Under this condition, the input voltage (Vin) was 1.53V (average). I then calculated Rin as follows(Fig2):
Rin = Rs * Vin / (3.3 - Vin).
The calculated Rin was approximately 430 MΩ (Fig3).
Therefore, we can plan the front-end circuit without considering the ADC input resistance.