Below is the transfer function of the two cascaded first-order high-pass filters (each with 20.6 Hz cut-off frequency) and their circuit:
And then there is the transfer function of the second-order high-pass filter (with 20.6 Hz cut-off frequency) and its circuit:
Both transfer functions (cascaded and sallen-key) show the same response in Matlab.
The values for the resistors were computed using this info:
In LTspice I get this response:
Why are they different?
Note: The op amps used are ideal ones, that's why they dont have any supply connected
I was told here that i need to calculate the resistors and capacitors values based at the highpass filter cut frequency ( at 20.5 Hz) with Q=0.5. That's what i did. In the second image i put here i used Q=0.5, i just didn't put the values there.
Even if i follow a different method to obtain the transfer function, like this one taken from wikipedia:
This is what i did:
With fc = 20.6Hz and Q = 0.5 i get the same transfer function.
I used C = 10nF and got 772594 ohms for the resistors (ignore the , there ). If i put that in LTspice i get the same response as the cascaded one.
So perhaps the problem is in the attribuition of the R1 and R2 value from that first document i have shown here.