I have created a bandpass filter with an LMC660CN chip, as shown in the image below.
However, in regards to just the high pass filter section, the filter's AC response is set so that at Vout a high pass filter is created with 1 kHz as the filter cutoff point, and it then has unity gain set by RF and RG. I also coupled RG to a 2.5 V voltage reference, in hopes that the output would oscillate about 2.5 V. That has not happened.
V1 in the schematic is a simple piezo disc in a black plastic case. It is AC coupled to the + input of the LMC660.
When the piezo is "at rest", the filter gives no output. When the piezo is slightly stimulated by the water pipe it is resting on, and vibrates at about 2.5 kHz, the filter begins to oscillate at about 20 Hz, and on each oscillation it reaches the positive power rail, 5 V. Also, this filter oscillation is also detected in the signal line of the piezo, probably from the + input interacting with the op amp's output.
Can anyone suggest why the filter begins to self-oscillate when the piezo creates a 100 mVpp at the + input? I am wondering now if it is connected to RG being coupled to VREF.