> I have to get the *exact* timing (30ns resolution) of some low frequency =
(<250Hz) pulses
> via an input capture module, however I'd like to filter the signal coming=
 in, as it will
> contain a lot of high frequency noise, but filtering via a capacitor intr=
oduces a phase
> delay, which I can't even compensate (by frequency) as the phase delay ch=
anges with input
> signal amplitude.
>
> What is the way You'd approach this problem? I thought about using a zene=
r to clip the
> signal and thus work on a known amplitude, but I suspect it will introduc=
e errors too.

The classic "Bessel" transfer function produces a filter with
maximally flat group delay / maximally linear phase delay (just as
Butterworth =3D maximally flat and Chebychev is maximum initial cuttoff
rate ).

        http://en.wikipedia.org/wiki/Bessel_filter
        http://en.wikipedia.org/wiki/Group_delay_and_phase_delay
        http://www.radiolab.com.au/DesignFile/DN004.pdf                mayb=
e

You can implement Bessel filters using building block ICs or "roll
your own" with opamps + RC or implement them in software.

Digikey all Bessel building blocks here    http://bit.ly/DigikeyBessel

Or see eg http://datasheets.maxim-ic.com/en/ds/MAX7401-MAX7405.pdf



 Russell McMahon
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