Re: gEDA-user: General ground/power plane design questions

[norwoods <norwoods@gbronline.com>]

Randall Nortman wrote:

> The good news is that I'm measuring what ought to be very
> low-frequency signals on the analog inputs (I don't care about
> anything over 10Hz, really), so I'm going to fake a low-pass filter by
> averaging multiple samples together.  (I want to keep the number of
> discrete components down, so I think this is better than an actual RC
> low-pass filter, though perhaps I'm missing something important.) 
>  
due to sampling, signals get heterodyned down by multiples of the 
sampling frequency; this is called aliasing.  for example, with a 1000Hz 
sampling frequency and a 1,000,005 Hz  noise in the signal, the 1000 
multiple of the sampling frequency will heterodyne the noise down to 5 
Hz.   therefore, you should have  a filter on the analog signal.  in 
your case, you should have a low-pass filter with a cutoff higher than 
10 Hz to pass your signal and lower than 1/2 the sampling frequency to 
eliminate aliasing.

norwood sisson