# Input Impedance of A0-A5

**URL:** <https://forum.arduino.cc/t/input-impedance-of-a0-a5/64820>\
**Category:** General Electronics\
**Created:** [June 27, 2011, 9:41pm UTC](https://forum.arduino.cc/t/input-impedance-of-a0-a5/64820 "2011-06-27T21:41:06Z")\
**Posts on this page:** 1\
**Showing post:** 7

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**Author:** ![Wawa](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/wawa/32/1211303_2.png) [@Wawa](https://forum.arduino.cc/u/Wawa)\
**Post date:** [March 25, 2016, 9:35pm UTC](https://forum.arduino.cc/t/input-impedance-of-a0-a5/64820/7 "2016-03-25T21:35:21Z")

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> gerdes1723:  
> it makes sense that the capacitor didn't have time to charge or drain after the multiplexer switched.

That's why you use a 100n capacitor from the tap of the voltage divider to ground.  
When the A/D muxer comes past to measure the divider, it sees a solid voltage (the 100ncap) to sample from.  
Now you can even use 10Megohm resistors.

[http://jeelabs.org/2013/05/16/measuring-the-battery-without-draining-it/](http://jeelabs.org/2013/05/16/measuring-the-battery-without-draining-it/)

Edit  
I seems the 100Megohm input impedance is just a number grabbed out of a hat.  
I have measured \>\>1000Megohm, at room temp, with nothing else connected to adjacent pins.  
Charge a low leakage large value film cap to e.g. 3.3volt, and measure it every second.  
With noting else connected than the cap, the A/D value will stay there for days.  
Leo..

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