# How many ohm is the ADC port internal resistance of the atmega8? [duplicate]

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.

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

• It should be in it's datasheet. Have you looked there? – Eugene Sh. Aug 5 '16 at 2:05

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