I would normally implement such a filter as:

Total =3D Total + New - Filt
Filt =3D (Total+1) / Factor
And I usually  make Factor =3D 2^N so I can divide by right shifting.
This keeps everything as integers, and if you want the extra resolution,=20
you can use Total.

Kerry


Olin Lathrop wrote:
> RussellMc wrote:
>  =20
>> Simplistic example. If random noise is added to a signal and it reads
>> 145 mV 60% of the time and 146 mV 40 % of the time then actual voltage
>> MAY be about 145.4 (not 145.6) mV. Take care when arttempting to apply
>> this magic. It does work, but ...
>>    =20
>
> It's pretty easy to apply on the firmware end.  Just low pass filter
> multiple A/D readings.
>
> When you have the cycles and real time available, it's a good idea to
> oversample with the A/D, then low pass filter in firmware anyway.  It get=
s
> you a little extra resolution if the right kind of input noise is present=
 as
> you said, but it also reduces quantization noise and relaxes anti-aliasin=
g
> filtering requirements on the external hardware.
>
> Fortunately a single pole low pass filter is easy to compute:
>
>   FILT <-- FILT + FF(NEW - FILT)
>
> where FILT is the low pass filter being updated with input value NEW each
> iteration.  FF is the "filter fraction".  FF =3D 1 is just a pass thru fi=
lter,
> and FF =3D 0 is a infinitely heavy filter.  Useful values are obviously i=
n
> between.  In a small system with limited math capabilities, you often
> arrange FF =3D 1/(2**N), so that the multiply by FF becomes a right shift=
 by
> N.
>
>
> ********************************************************************
> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
> (978) 742-9014.  Gold level PIC consultants since 2000.
>  =20



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