> >No, only the frequencies that cause aliasing at your sample rate.  There
is
> >nothing wrong with filtering a signal digitally as long as it is not
> >aliased.  Digital filters are more accurate and can often be free by
using
> >cycles and memory that were available anyway.
> >
>
> Any frequency above 1/2 your sample rate is going to cause aliasing.

I was separating A/D sample rate from the rate at which the values get used
internally.  If your highest meaningful input frequency is 10Hz, then you
need to sample at least at 20Hz.  If you sample close to the 20Hz rate, you
may need an analog filter to avoid aliasing, depending on what you expect
the noise content of the signal to be.  However, it is perfectly legitimate
to sample at 1000Hz, for example, and then apply a digital filter before
using the values internally.  This protects you from noise frequencies up to
500Hz, which may very well allow you to drop the analog input filter.  I
have used this latter approach successfully a number of times.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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