Sallen Key

Started by RichD December 6, 2018
Reviewing some old stuff, I crossed the Sallen-Key filter.  
Analysis of this topology is relatively straightforward, 
but what I never sussed is the why - what makes it so 
important?   What was the great advance, at that time?  
And is it still considered so consequential, today?


--
Rich
On 12/6/18 10:25 AM, RichD wrote:
> Reviewing some old stuff, I crossed the Sallen-Key filter. > Analysis of this topology is relatively straightforward, > but what I never sussed is the why - what makes it so > important? What was the great advance, at that time? > And is it still considered so consequential, today? > > > -- > Rich >
More an s.e.d question than comp.dsp, but I'm an analog guy so sure. Sallen-Key gives you 2 poles (lowpass) per op-amp when you care about op-amp costs, with fewer parts than an MFB and no inversion to worry about (which can add yet more complication). Plus, with no explicit gain, you get a gain of exactly 1 (less open-loop error), whereas with an MFB you have to buy expensive precision low tempco resistors to even approximate the same behavior. Sallen-Key filters are my go-to workhorse filter topology. They're what I use barring an active reason to use something else. -- Rob Gaddi, Highland Technology -- www.highlandtechnology.com Email address domain is currently out of order. See above to fix.
On December 6, Rob Gaddi wrote:
>> Reviewing some old stuff, I crossed the Sallen-Key filter. >> Analysis of this topology is relatively straightforward, >> but what I never sussed is the why - what makes it so >> important? What was the great advance, at that time? >> And is it still considered so consequential, today? > > More an s.e.d question than comp.dsp, but I'm an analog guy so sure.
There's no digital equivalent?
> Sallen-Key gives you 2 poles (lowpass) per op-amp when you care about > op-amp costs, with fewer parts than an MFB and no inversion to worry > about
? Don't know what MFB or inversion refers to. -- Rich
On 7.12.18 18:52, RichD wrote:
> On December 6, Rob Gaddi wrote: >>> Reviewing some old stuff, I crossed the Sallen-Key filter. >>> Analysis of this topology is relatively straightforward, >>> but what I never sussed is the why - what makes it so >>> important? What was the great advance, at that time? >>> And is it still considered so consequential, today? >> >> More an s.e.d question than comp.dsp, but I'm an analog guy so sure. > > There's no digital equivalent? > >> Sallen-Key gives you 2 poles (lowpass) per op-amp when you care about >> op-amp costs, with fewer parts than an MFB and no inversion to worry >> about > > ? > Don't know what MFB or inversion refers to. > > -- > Rich
MFB: Multiple feedback. Inversion: the output signal is inverted from the input. There is no sense to simulate a Sallen-Key in the digital domain. There are better ways to implement the IIR filter. -- -TV
Tauno Voipio  <tauno.voipio@notused.fi.invalid> wrote:

>There is no sense to simulate a Sallen-Key in the digital >domain. There are better ways to implement the IIR filter.
Generally a digital filter with positive feedback would look sort-of like a Sallen-Key, in some cases close to the same topology. It's not something I've ever chosen to do, however. Steve