Understanding Sallen-Key LPF

Thread Starter


Joined Aug 8, 2014
RLT - just for clarification: I have used two resistors 162.3k instead of 102.3 k.
That is the reason for the discreapancy between our results. My fault.
As I have suspected, OKAWAs "oscillation ffrequency" is the imag. part of the pole pair (wn) - and it can be observes as a damped oacillation for larger Q values only (several periods...).

One final remark (real opamps): If you are going to realize this filter you will see that the damping properties of the circuit are not as good as shown in OKAWAs simulation. The reason is that each opamp has a finite output resistance - and for larger frequencies (far above the pole) there will be a rising portion of the input signal which arrives DIRECTLY via the feedback capacitor at the opamp output and will produce an unwanted output signal across the output resistance (which, unfortunately, will even rise with frequency). Hence, the maximum possible attenuation in the stop band is perhaps only 30...40 dB.
This is one of the drawbacks of this filter configuration.
Thanks LvW!!! I absolutely agree, ideal opamps like most things in life (including myself) are not perfect. One way to help reduce the rise at high frequencies is to put a single pole R/C filter on the output of the S-K LPF. This delays the adverse effect of the unwanted decrease in attenuation at the higher frequencies.