I performed the AC analysis of the following circuit:
This is a simple second order low pass filter satisfying the Butterworth filter requirements. According to me, after the break point there will be a continuous decrease in magnitude by -40 dB/decade but this was not the case as seen below:
The pictures shows that the response decreased by almost -40 dB per decade but it then started to increase until a certain point, which according to me indicates the presence of a zero. But there is no zero in the transfer function of the Sallen-Key topology. Why does this happen?