You mean a software filter?  Got any slick ways of performing this =
trick?


  ----- Original Message -----=20
  From: Olin Lathrop=20
  To: Lawrence Lile=20
  Sent: Tuesday, November 07, 2000 9:43 AM
  Subject: Re: [PIC]: The tintinnabulation of the noise


  >>
  Now, I am not real sure if I will have 60 hz noise (picked up from the =
air)
  or 120 hz noise (from a full wave rectified power supply) as the major =
noise
  component.  It may actually be both, at various times.

  Does Old Nyquist rule in this situation, should I sample at twice 120 =
hz or
  more to cancel out the effects of noise?   I'm not interested in the
  frequency of the signal at all, just in an accurate DC level.

  Let's see: 120 hz =3D 8.33 milliseconds per oscillation.  If I sample =
at twice
  this, that's 4.33 milliseconds.  sampling at exactly twice is cutting =
it
  fine, so say we sample at four times that's ~2 milliseconds.

  I've got enough memory to increase the number of samples, maybe that =
will
  help as well?
  <<

  I would recommend a different approach.  Sample as fast as you =
possibly can,
  then run the result thru a one or two pole low pass filter.  You only =
need
  to store one number per pole, not one number per sample in the =
average.
  This type of filter (gaussian) is better at squashing high frequencies =
than
  the box filter you described, requires less state, and is generally =
easier
  to implement.  The main drawback is the slower response, which sounds =
like
  can be much slower than 60Hz from your description.  Make the time =
constant
  as long as you can tolerate to maximize the random noise reduction.  =
It
  seems I end up doing at least a single pole filter in 90% of the =
projects
  that measure an analog voltage.  The trick is to make the filter =
fraction a
  power of 2 so that it can be done with bit shifting, and the rest is =
real
  easy.


  *****************************************************************
  Olin Lathrop, embedded systems consultant in Devens Massachusetts
  (978) 772-3129, olin@cognivis.com, http://www.cognivis.com

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