[sdiy] Differential filter query

[Tom Wiltshire]

Hi All,

I'm currently looking at differential filters (for the DAC output on the dsPIC 33FJ128GP802 uP). Doing some searches found this page:

	http://freeschematiccircuit.com/multiple-feedback-filter-using-differential-low-pass-filter-circuit/

The diagram here looks to be taken from a TI application note, which also turned up in my searches:

	http://focus.tij.co.jp/jp/lit/an/sloa064/sloa064.pdf

Anyway, my question is this;

There are two capacitors on the LHS marked C2A and C2B. These are given the value "4C" for a Butterworth filter. Fair enough so far.
But the two capacitors are in series, and therefore have an effective value which is half the given value. So why wasn't the diagram drawn with only one capacitor C2 with a value given as 2C?

I can see that there's something to be said for keeping the symmetry of the design, but I don't think merging those two caps spoils that. It'd work just as well for Bessel and Chebyshev, with the appropriate adjustments of the factor for C.

Thanks,
Tom