Re: [avr-gcc-list] ADC anomaly

[Douglas Dotson]

I'll give that a shot. My input is slow moving DC
so I wasn't thinking about noise being aliased.
Strange that the other 2 inputs do not show this
effect though. And that it always settles down to the
same point that is 3 or 4 counts low.

Doug

--- Richard Urwin <address@hidden> wrote:
> On Sunday 15 Feb 2004 4:44 pm, Douglas Dotson wrote:
> > I'll give it a shot. I haven't brought many of the
> > other on-chip peripherals online other than the
> > USART. I am filtering the readings through a
> > simple digital filter which is working well. This
> > problem is very slow moving. I have 3 channels
> > of the ADC operational and the other channels
> > are not showing any similar behavior. Strange.
> 
> Make sure you have hardware filtering on the input
> at least two octaves 
> below your sampling period. No amount of software
> filtering can remove 
> aliased noise from the signal.
> 
> For example, if you have a 1kHz sampling rate, and
> there is 999Hz noise 
> being added to that, you will end up with a 1Hz
> signal on your data. It 
> would not be unlikely for the signal or a harmonic
> of it to be even 
> closer to your sampling frequency. Mains hum is very
> accurate, maybe 
> only a few micro hertz out; dimmer switches create
> lots of harmonics; 
> fluorescent lights often work in the kHz range.
> 
> -- 
> Richard Urwin