AVR-Chat

At 10:49 AM 5/11/2004 -0600, Craig Limber wrote:


>Hi there;
>
>I am having a wee bit of trouble with a noisy analog to digital conversion
>setup and was wondering if anyone out there could suggest some things
>to try to clean it up.
>
>The processor is a Mega32 running at 8 MHz with a crystal.  Power is
>regulated 5 volts from a battery.  The circuit has a very generous
>supply of bypass and filter capacitors.  A 'scope on the power and ADC
>lines doesn't show any noise that it can detect.

AC couple, at about 1mV/Div. Your ADC is 10 bit, so that's 1mV per volt on AREF, per ADC count.

>  I am using the LC
>circuit the manual recommends for the AVCC and it's voltage measures
>the same as that found on the chip's VCC lines.  I have a .1uf disk cap
>going from AREF to ground 


Which ground?

> As soon as I try to multiplex them I
>run into trouble.  The values keep jumping about plus or minus about
>half a volt around the reading I expect to get. 

Key point. IIRC you're supposed to toss the conversion immediately after a channel change.
I've done this by using an int to roll through the channels, changing the channel before starting the next conversion.

I seem to remember there's a finite time that you have to wait after an ADMUX selection, for the mux output amplifier to settle, as well.