I'm having trouble with a cascaded butterworth notch filter I'm designing and was hoping to get some insight on the summing amplifier stage at the end. I'm still a student, so my question may be a bit basic. I'm trying to get a unity gain response for the filter, but the summing amp pushes the response more positive rather than overlaying the responses.
I tried an inverting summing amplifier with 1kohm resistors to achieve a unity gain, and I've generally fiddled with the resistors as much as I can to get the response I want. That didn't work, so I tried switching to a non-inverting summing amp, thinking my response was pushed upward due to the inverting nature of the original design (seen below).
simulate this circuit – Schematic created using CircuitLab The non-inverting amp didn't work for me either. It could be because I was using bad values for the resistors in the non-inverting summing amp, so I'm looking for any insight as to what I might be doing that is causing this response to behave the way it is.
EDIT: I should add for clarity that the filter is a notch filter in the range of 77.5KHz with a B of 15KHz using a 4th-order butterworth design.