> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Olin Lathrop
> Sent: 23 November 2010 15:23
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Accurate measurement of battery voltage
>=20
> 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
>=20
> 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.


Factor=3D2 causes the output to immediately jump to full scale with a unit
input, but after the input falls below 1 it does actually start working
as a filter.  It's better behaved with high values of 'Factor', though
it still has a glitch if the filter starts with an input of 0. =20

As you point out, it's difficult to see what the extra complexity buys
you.

Regards

Mike

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--=20
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