I used this online tool to play around and design a Sallen-Key low pass filter with a cutoff frequency of 0.3Hz. When I now put the resulting values for R1, R2, C1 and C2 in the formula I get exactly my desired 0.3Hz.
But when simulating the circuit with LTspice, the cutoff frequency was unexpectedly at 0.46Hz. That is more than 50% off!
Can anybody tell me where the difference is coming from? Am I missing something obvious? The only thing I can think of is that Spice is using numerical solutions, but the error of the calculation should never be that high! And the OpAmp shouldn't make any difference at these low frequencies either…
What frequency would I probably get when acutally building the circuit?
Best Answer
There's no difference in the simulation and calculation, at least none that I can see. Here's what the calculator gives:
The "problem" is that in the calculator the cut-off frequency seems to mean the frequency where there is a pole. And with this given topology at that frequency there is 6 dB of attenuation. You need to find the -3 dB cut-off frequency from the graph.