[sdiy] while we're talking about scaling & biasing etc

[Jason Proctor]

i have a funky problem with the analogue inputs on my Arduino module 
that i'd like to run past the experts.

from the outside world, the incoming signal goes through a unity gain 
inverting stage, and then gets chopped in half and biased up 2.5v by 
another inverting stage compress a +/-5v range into 0-5v. whatever's 
left then hits the uproc pin. i've confirmed that this works fine, ie 
the correct voltage hits the pin.

however, reading the pin on the uproc side gives me a number which 
has no correlation whatsoever to the voltage. it has a 10 bit range 
and hovers semi-randomly between 550 and 850. almost as though it was 
floating.

i've checked the hardware by swapping the connection between the 
scale/bias input and a pot from 5v, works fine. all good there.

the only contributing factor i can think of is that the uproc is 
powered off a different 5v regulator than the scale/bias circuits. i 
hadn't thought this was an issue as there is only one ground on the 
board. but before i jump into the rats' nest and check this 
supposition, can anyone comment on this possibility?

thanks much!
j