[sdiy] Designing 4-pole filters with identical 2-pole stages - why not?

[Tom Wiltshire]

Yeah, I have looked at it, and I was aware of the improved performance. I was still avoiding it because of that one extra resistor, but I suppose I should get over it.

Does the rising output impedance really make much odds unless the frequencies are up into MHz though? There's a bit about it in that TI paper I linked (page 7 and 8) with a little graph showing the frequency response starting to climb back up after a certain point - but it doesn't turn the corner until 7MHz. I'd probably hope to flatten frequencies up there with a smattering of small caps appropriately placed in my circuit, rather than having to expect my Sallen-Key filter to remove them.

Thanks,
Tom


On 22 Dec 2015, at 23:09, Richie Burnett <rburnett at richieburnett.co.uk> wrote:

> Tom, since we're taking about anti-alias lowpass filters, have you looked at the MFB multiple-feedback lowpass filter?
> 
> For the added complexity of another resistor it gives you more flexibility in the combinations of achievable gain, Q and cutoff frequency. The performance of the MFB lowpass is also better than Salen-key filter deep into the stop band when used with real op-amps that have rising output impedance at high frequencies. 
> 
> -Richie,
> 
> Sent from my Xperia SP on O2