I have input data from a pressure sensor:https://physicsforums-bernhardtmediall.netdna-ssl.com/data/attachments/93/93464-7e84f120a9323e7d16b7dfa59c069739.jpg [Broken] We run this in to an FFT and found that the max point is at 3.3378Hz. So we did a Digital band pass Filter for 3.3Hz - 3.6Hz. and then we got this : and this did a fft on that and we got back the same value of 3.3378Hz and that is good. But the problem is that this wave form has negative numbers and the pressure sensor does not out put negative volts : like you can see in the 1st pic. We want to know the voltage at 3.3378Hz so we can get the pressure at that hz for troubleshooting a flow loop. can we did the real voltage back at the hz? are is that not something we can get back ? Why does the waveform in the second pic have negative numbers?