So testing a bunch of photo diode TIA's today. (opa124, not recommended for new designs.) The low gain (332 ohm) step response had a ~0.5 us delay, and then a sharper edge than expected. (higher gains had similar weird stuff.) (I'm trying to upload dropbox files from home... but it's not working.) The circuit is TIA, no bias, N-I input grounded, opa124, Z_f is
332 ohms and parallel C ~470 pF, (maybe less.)Here's the normal response (50 ohm termination.)
ab-normal, 50 ohm term.
and ab-normal with no 50 ohm load on the long (~6') thin (RG-172) coax output... which is how I test it. Typically the 50 ohms makes little difference.
I'll replace the opamp, and hopefully that will fix it... But it was interesting to me.
George H.