Hi! I guess this thread (could) bear similarities to the discussion in the "Low noise low pass" https://www.physicsforums.com/showthread.php?t=332819" thread, but I didn't want to hijack that :) so beginning a fresh one. I find that the electronic noise gets aggravated, when I connect the output of an amplification stage to a BNC connector. To elaborate, I need to have the (opamp's) output high-pass filtered first (a simple RC filter, built using a SMD capacitor and resistor, with fCutoff being ~ 100 kHz) and then it reaches to the BNC connector with a ~2.5 cm long shielded cable. The ends of the cable are soldered to the PCB and the connector, so that portion (< 0.5 cm on both ends) is unshielded, of course. I've attached the noise spectrum for these two measurements. The top figure is using a probe at the output pin of the opamp and the bottom being the output from the BNC connector cable (i.e. after the signal has passed thru the HPF and shielded cable) - you can observe the gain peaking in 0-120 MHz frequency band and the radio-frequency peak (around 910 MHz) becoming quite worse in the bottom figure. The circuit as such is placed inside a steel box, with its chassis sharing the power-supply ground. I've attached a schematic of it, most of the details are in this thread: https://www.physicsforums.com/showthread.php?t=327510&highlight=cascading" In essence, it serves to amplify the difference of photocurrents (Idiff) using two successive opamp stages. The idea is to be able to do this fast enough (i.e. have a high bandwidth of operation) so that, if the input to the photodiodes is an optical pulse train (with a repetition rate in MHz, typical to most pulsed lasers these days), then the statistics of Idiff can be collected quite fast & efficiently. Coming back to the noisy troubles, my question is now, is it really something like bad impedance matching and/or poor shielding, that causes the noise to aggravate so nastily? Or could it be that a normal BNC cable is the culprit, because the probe-cable (that I use for measuring the opamp output) would be pretty well shielded. Lastly and most relevantly, how should I try to fix this? :) Thanks aprior!