Okay this is quite a weird effect I'm getting here and I'm interested in seeing if anyone can explain it for me...
I'm building the quick and dirty oscilloscope from Practical Arduino and have hooked it up to the ArduinoScope processing sketch. At the moment I'm using the Analog version.
Relevant links: ArduinoScope (http://code.google.com/p/arduinoscope/wiki/Usage
Practical Arduino Analog Code: (https://github.com/practicalarduino/ScopeAnalog
What I'm building: (http://www.practicalarduino.com/projects/scope-logic-analyzer
My test circuit is simply a 1.5v battery and I'm measuring the amount of voltage from it (1.37v on my DMM).
I've got the system all up and running and it seems to work okay *except* for one really odd effect which is this:
I've simplified my code to eliminate the actual oscilloscope body altogether and gone back down to my arduino only. Basically I've got GND from arduino to +1.5v going to Analog pin 0.
Running the code given at the github link above I'm getting 1.37v (281 raw read) but not just on pin 0 but on the other pins as well (although it drops off a tiny bit as I go through each one. (pin1: 279, pin 2: 276 etc all the way to pin 6: 240).
If I connect pin 1 to the +ive terminal then it stabilises exactly as pin 0 does and I get a nice solid +1.37v.
This is replicable across all the pins and I've tried it with a couple of other simple circuits as well (eg lighting an LED etc).
Part of me says I shouldn't care what the other unconnected analog pins are doing however it's something I noticed so I wanted to see if there was something wrong with what I'm doing or else someone has an explanation for what's going on?