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

[Richie Burnett]

It's just another weapon to have in your signal processing arsenal. I like to use it for anti-aliasing, and particularly anti-imaging low-pass filters because the Hf stopband slope just keeps on going.

The 2nd order stopband slope of the Salen-key LPF shallows out due to a zero resulting from the op-amp output resistance and the capacitor that goes from input to output in the schematic. Sometimes you really don't want fast slewing edges from sampled signals to nip past the filter in the form of little glitches.

I also like the "decoupling" of the gain and Q parameters, which can be very useful, but you can achieve similar with the SK too with a bit of modification. 

-Richie, 

Sent from my Xperia SP on O2

---- Tom Wiltshire wrote ----

>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
>
>_______________________________________________
>Synth-diy mailing list
>Synth-diy at dropmix.xs4all.nl
>http://dropmix.xs4all.nl/mailman/listinfo/synth-diy