Interesting thread!
A while back (2+ years as it turns out) I did some work on a "general purpose" low pass filter routine which, like many projects, never really made it to the finish line.
I based it on one of Tracy Allens filter routines and it's basically that 15/16 type filter disussed in later posts of the thread - with some adjustments. With my version you're able to select one out of 9 different filter constants and it handles negative values (two's complement). Seeing the dissusions here on that type of filter not converging properly, I tool a look at the old plots I did while testing and the output of my filter does seem to converge to the actual input value:

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It's a bit hard to see in the plot but all "traces" does end up at the actual input value when given enough time, here's a detailed view of the first step of signal:

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Finally here's a plot that shows an "ideal" sinewave and that same sine with another sine (f=10x) on top. Then it shows what the filter output looks like at a couple of different filter constants:

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I should probably note that all of the above tests was performed with "simulated data" ie the waveform fed to the filter was stored in the PIC so no ADC was involved. I don't think that actually makes any difference though.

If there's any interest in it I can clean it up a bit and post it. It will not be the smallest and most likely not the fastest but it may have other benefits.

/Henrik.