You could use an op amp to scale your input to the full range of your A/D.
I googled around a bit and came up with this article (no association):

http://www.electrotap.com/blog/495

DougM

On Tue, Nov 23, 2010 at 7:23 AM, Olin Lathrop <olin_piclist@embedinc.com>wr=
ote:

> Kerry Wentworth wrote:
> >>   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 filter, and FF =3D 0 is a infinitely heavy filter.  Useful values
> >> are obviously in 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.
> >
> > I would normally implement such a filter as:
> >
> > Total =3D Total + New - Filt
> > Filt =3D (Total+1) / Factor
>
> I'm not sure what exactly that results in, but it's not a single pole low
> pass filter, otherwise known as a exponential decay.  The simple case of =
a
> unit step input with N=3D1 proves that easily enough.  Your filter jumps =
to 1
> in the first iteration, then stays there.  It also requires more
> computation
> and more persistant state, so it's not clear what the advantage is.
>
>
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