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This question already has an answer here:

I am trying to link a digital pin to a pad that's already wired to an ADC. I was wondering how many ohm the ADC port internal resistance is.

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marked as duplicate by Tom Carpenter, Community Aug 5 '16 at 3:28

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    \$\begingroup\$ It should be in it's datasheet. Have you looked there? \$\endgroup\$ – Eugene Sh. Aug 5 '16 at 2:05
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Page 310 of the datasheet gives you your answer:

Analog Input Specification

The analogue ports have an input resistance of \$100\space\mathrm{M}\Omega\$.


However, that is not the whole story. As with all ADCs, a sample and hold capacitor is used as part of the ADC in order to hold the sample value during conversion. This circuitry has its own characteristics. From Page 244 the following diagram is taken:

ADC Input Diagram

Here we see a 14pF sample/hold capacitor with series resistor to half the Vcc rail. As a result of this circuitry, you must use a low impedance source to feed the ADC.

The ADC is optimized for analog signals with an output impedance of approximately 10k or less.

A \$10\space\mathrm{k}\Omega\$ or less source resistance is recommended, otherwise the low pass filter effect of the capacitor with the source resistance becomes a major issue, requiring a longer sampling time for conversion and as a result limiting the maximum frequency. A unity gain buffer can be used if the source is high impedance in order to negate the effect of the sample capacitor.

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  • \$\begingroup\$ Never mind, just read the link you posted, seems they posted a similar answer as this. \$\endgroup\$ – Tom Carpenter Aug 5 '16 at 3:27

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