Adding an ADC, what happens to input impedance
The ADC is affected by the impedance, not usually the other way around.

Most cheap ADCs will have a sampling capacitor on their input. This connects to the outside world for a short period of time, charges up, then disconnects and the conversion begins inside this device.

Every time the ADC takes a sample, it has to charge up this capacitor for enough time constants that the difference in voltage with what you're measuring is ideally less than 1 step of the output. As such when there is more and more "impedance" between the ADC and the "to be measured" voltage, you start losing meaningful resolution in your output.

Most ADCs in their datasheet will quote what the maximum input impedance can be for a given sampling speed, with the general rule being lowering the ADC clock speed by half will allow for more impedance because the sampling switch is open for longer. But if you get too slow, the sampling caps leakage may start affecting the measurements.

Below is a schematic of what is usually inside an actual microcontroller ADC:



Image source: Open Music Labs - ATmega ADC tutorial