I'm studying the KHN biquadratic section and its outputs to obtain a lowpass filter, a highpass filter and a bandpass filter. I simulated the filter using Mathematica and obtained the following plots, which match the theoretical.
Now, then I decided to simulate the circuit using LTSPICE.
Obtaining the following plots:
Now, comparing both plots everything looks fine with the Low pass output and with the High pass output, both in terms of magnitude and phase. My question is with the band pass output. While the magnitude plot looks fine, the phase plot has nothing to do with the one I plotted with Mathematica. Why does this happen? What is the difference between the simulation and the theoretical plot? I didn't expect such an abrupt phase change as the one obtained with Mathematica, but the phase plot obtained has nothing to do with it: the limits are different, the variation is the exact opposite... What am I missing here?