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Rauch vs Sallen Key filters

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benzi11

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Hello,
I am currently working in class with a microcontroller. As we are using a microphone we needed to design a low pass filter. I know that there are two different types of filters I can use (Rauch or Sallen Key). Our teacher recomanded using a Rauch filter, however, he didn't explain why.

Could anyone explain the main differences between the two types of filters?
What is the upside of using a rauch filter instead of a Sallen Key filter ?

Thanks for your help.
 

The name "Rauch"-Filter is rather uncommon - and you hardly will find some information using this term.
It is a one-opamp filter (infinite gain) with two feedback pathes - better known as "multi-feedback"-filter structure (MFB).
In comparison - the Sallen-Key structure is based on a finite-gain amplifier and a single feedback path.
Comparison:
MFB-filters have low (passive) sensitivities to parts tolerances - but rather high active sensitivities (to opamp gain non-idealities).
Exactly the opposite behaviour can be observed for sallen-Key filters (higher passive and lower active sensitivities)
 

Continuing with the excellent explanation that LvW has provided,
nowadays opamps have improved so much as compared to the late 1970s, when a NE5532 was considered a high performance audio opamp. Also high quality passives were available at reasonable cost, such that the Sallen-Key topology was used everywhere.

Nowadays there are literally dozens and dozens of very high performance opamps. And the high quality film capacitors, which are an extremely important and crucial components in filters, are becoming scarcer and relatively more expensive, it makes sense to re-think the common wisdom and utilize the MFB topology.
 

schmitt trigger said:
............such that the Sallen-Key topology was used everywhere.
Click to expand...
Everywhere?
There is another disadvantage for the S&K topology : The damping in the stopband is limited due to DIRECT feedthrough through the feedback capacitor - because an unwanted voltage is developped across the finite output impedance which even increases for rising frequencies.
 

My wrong choice of words.
"Very common" would describe it better.
 

Just adding something more.

*LPF with MFB inverts the signal <> Sallen Key does not.
*MFB Noise gain = 1 + actual gain <> Sallen Key Noise gain = actual gain
 

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