I needed an active low pass filter for my project that can produce an output voltage within the required settling time and ripple voltage ranges. Although I managed to get a good enough output voltage with the shown resistance/capacitance values, if you calculate the cut-off frequency of this Sallen-Key low pass filter it's around ~4250 Hz. The input signal however, is 20 kHz.
The Sallen-Key low pass filter:
The output waveform:
My questions are:
How does a low pass filter with a cut-off frequency of 4250 Hz let a 20 kHz signal through?
Are cut-off frequencies, and therefore, stopbands of filters ignored in LTspice for some reason?
Additional info:
Op amp used in the circuit is the ideal single pole amplifier model of LTspice named "opamp" in op amps directive.
Input signal is 20 kHz 0-5 V pulse waveform with 50% duty cycle.
If the op amp is switched for different models such as LT1001, the output voltage level is around a few pV. This still happens when the resistance and capacitance values are changed and the cut-off frequency is more reasonable. Example: 5k resistors, 1n capacitors, ~32 kHz cut-off.