Gabriel,

I would recommend  using an analog filter in front of something like this.
 Saying this, I can think of a number of projects that I didn't use an
analog front end due to cost constraints.   I assume accuracy is more
important than cost in your appliacation?

You'll need an anti-aliasing filter.  This will be less true if you are
just reading values and storing them, more true if you are doing any kind
of digital filtering like averaging or median.  All this of course depneds
on the bandwidth you intend to log and what kind of noise you expect to
see.

As far as what kind of filter, butterworth or sallen-key etc.  it is
probably not critical.  Each of these has slightly different pass band
characteristics.  If you want a flat pass band, vs steep cut-off, and so
on.   Often a simple one pole low pass filter to attenuate frequencies
above the nyquist limit is all you need and the rest can be done
digitally.

Generally you also need a filter in front of the PIC to achieve some gain,
if you are working with small signals.


-- Lawrence Lile
x




Gabriel Caffese <gcaffese@INTERLAP.COM.AR>
Sent by: pic microcontroller discussion list <PICLIST@MITVMA.MIT.EDU>
10/13/2003 07:20 PM
Please respond to pic microcontroller discussion list


        To:     PICLIST@MITVMA.MIT.EDU
        cc:
        Subject:        [PIC]: AGAIN, About A/D conversion using internal A/D


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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