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

Ian Smith taciturn_unquiet at hotmail.com
Sun May 24 07:00:32 CEST 2009


Hey Jason,

I could be completely misinterpreting this but... the analog inputs read between 0-5V and output a number between 0 and 1023. If you're bumping the centerline up to 2.5V that's a returned number of 511. The fact that it's only 10-bit reolution could account for the not quite precise reading. *shrug* Just my US$0.02.

-Ian

> -----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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