Re: [avr-gcc-list] ADC anomaly

[Richard Urwin]

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