The microcontroller in the Arduino has a single sampling capacitor that can be connected to any one of the analog pins.
It will charge up to the voltage on the pin if enough charged particles are available on the pin (or a path less than about 10kohm exists to a source of current).
The cap is not discharged by the analog-to-digital conversion process or between samples except through leakage.
If no charge is available on the pin the capacitor will continue to slowly drain its charge from the previous sample.
It's normal and harmless unless you are in a high-voltage environment like a thunderstorm or a nylon carpet, where much more than 5V could accumulate on an open pin. Then it's best to ground unused inputs.

Martin


> Date: Thu, 12 Nov 2009 18:02:05 +0100
> From: altern2@gmail.com
> To: pd-list@iem.at
> Subject: [PD] arduino problem with analog sensors

>
> hi
>
> not a PD question I guess... i dont know much of electrical stuff. i am
> connecting two analog sensors to the arduino. One is a pressure sensor
> and the second a potentiometer, they are both from an old icube.
>
> i noticed that when i raise the potentiometer (analog pin 5) the
> pressure sensor (analog pin 2) reports values ranging 0 to ~0.16
> increasing together as the potenciometer goes up. It is like some kind
> of residual noise comming from pin 5 that goes into pin 0.
>
> the power cables of both sensors go together to 5v inlet of the arduino
> and the ground cables go to ground pin, each of the data cable goes to
> its separate analog pin.
>
> I am using arduino from Pure Data extended using latest Firmata, so i am
> just filtering out any value smaller than 0.16 coming from analog pin 0.
>
> is there anything I am doing wrong? or is this normal?
>
> thanks
>
> enrike
>
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