[sdiy] Differential filter query

[Simon Brouwer]

Op 28-7-2010 15:20, Tom Wiltshire schreef:
> 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 think it has to do with the sentence "component ratios are shown for 
common unity gain filters". It clarifies the design of such a 
differential filter as the mirrored combination of two such 
non-differential filters, and has nothing to do with practical 
implementation.

-- 
Vriendelijke groet,
Simon Brouwer.