At 07:11 PM 1/21/03 -0800, you wrote:
>I am wondering what the best method is to average an
>a/d input in C.

Just skimming responses and adding a bit, here are
some options:-

1)      Do a boxcar filter by implementing something like
         a circular buffer and averaging the results.
         This requires enough RAM to store each result.

2)      IIR filter, the single pole version of which is

         Dn = Dn-1 + K(x - Dn-1)

         Where K = Ts/(Ts + Tf) (or Tf = (1/K-1)* Ts

         Tf is the desired filter time constant
         Ts is the periodic sample time.

         By making K a 1/(2^n) you can make the calculations
         faster.  Watch that there's still enough bits
         left after you do the division or your filter output
         can have a persistent error.

         Eg.     Ts = 0.01 second
                 K = 1/256

         you get a time constant of 2.55 seconds.

         This is about the simplest possible filter, this
         subject is HUGE.

3)      If your output rate is low and you can
         oversample a lot, just accumulate a number n readings
         then divide the result by n. n can be made a power
         of two or some other number to control the reading
         rate (effectively an integration period).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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