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?
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.