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

[Jerry Gray-Eskue]

A few of possibilities occur to me off hand,

1 The Micro you are using has an analog input mux and it is looking at
different pin.
2 The D2A is not setup correctly and so is not reading the voltage on the
pin. This setup can be tricky with a number of registers and bit flags
involved.
3 The Port Pin you are using has not been configured as an analog input.
4 You are setup for an External Reference and this reference voltage is
missing.

With only one ground you should be reading the voltage value on the pin,
check the voltage on the Input Pin to the Ground Pin of the uprocessor, this
is what the converter will see.

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Jason Proctor
Sent: Saturday, May 23, 2009 8:28 PM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] while we're talking about scaling & biasing etc


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
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