You can use USB sniffer software to determine how the UPS software communicates with the device. There are plenty of free/free-trial versions that you come up if you google "USB Sniffer Software". I've personally used Free USB Analyzer before and it worked well.

You should look up the basics of op-amps, you're not using them correctly. This task would be easier accomplished with a comparator or (maybe) a differential amplifier. I don't really understand why you would need to do it though or why the amplitude would be changing

Can you elaborate on what your goal is?

@Blueyes There are concrete methods to find SRF like the one I cited in the answer. However, they might depend on what equipment you have to test with. The capacitor critical frequency is typically given on the data sheet though.