M.Adam Davis,

        Thank you very much for the information.

        Gabriel.-


-----Mensaje original-----
De: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]En nombre de M. Adam Davis
Enviado el: Lunes, 13 de Octubre de 2003 22:53
Para: PICLIST@MITVMA.MIT.EDU
Asunto: Re: [PIC]: AGAIN, About A/D conversion using internal A/D


Simplified:
You always implement some form of analog filtering if any component of
the signal being converted is at a frequency of larger than 1/2 the
conversion rate.

Reality:
Every electrical conductor is also an antenna.,  Even if your ideal
signal has no components above 1/2 the sampling rate, it will still pick
up some interference locally.  If the interference is small compared to
your signal then it won't appreciably alter your readings.  If it's
significant (such as next to a motor, a monitor, or AC line) then you
may get aliasing - the high frequency components show up as low
frequency signals in your samples.

So the upshot is that if you want this to be a do-all data logger, IMHO
you ought to design in some traces which will allow the placement of at
least a passive filter, ideally an opamp and active filter.

Digital filtering will only help on signals which are 1/2 the sampling
rate or lower.  This is really well explained in a good DSP tutorial and
book.  Some website resources on DSP were posted recently - you might
look into the first few lessons of those.

-Adam

Gabriel Caffese wrote:

>Hi everyone,
>
>        I am working on a portable datalogger. IT SHOULD BE "UNIVERSAL", in
>a way that I will use the same PCB always (no matter if used for logging
>low-speed
>or mid-speed signals).
>        Next week (I suppose) will start working on how to implement A/Ds
>low pass filtering.
>        And here comes the question:
>
>      Do you always use analog filtering (such us Butterworth or Sallen
>Key), or implement it digitally ? OR DO NOT IMPLEMENT IT AT ALL ?????
>
>        Thank you very much,
>
>                                Gabriel Caffese.-
>
>Gabriel
>ICQ#: 154107558
>More ways to contact me: http://wwp.icq.com/154107558
>_________________________________________
>
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