Well first off, I'm afraid this is a lazy post, 'cause I should be able to find the answer in my opamp books at work tomorrow.
Anyway for my 60 Hz pick up problem, I've slowed down the opamp driving the fet by hanging a bigger compensating cap on it. There's a pic of the schematic here. Standard opamp driving a fet V->I. (feedback R is 10 k not 1 k shown.)
Driving home tonight, I could only think of it as some ratio to whatever cap is inside the opamp, and set's the slew rate/ GBW. This particular opamp is the opa134 GBW = 8MHz, 20 V/us slew rate.. I need some current to turn that into a capacitance.
Or am I thinking about this wrong? TIA
George H.