I want to make an isolator so that I can use a logic analyzer with 24VDC and 120VAC signals. For the 120VAC I don't need a very fast optocoupler, and in fact it will be a pretty slow signal anyway since I will use a cap on the opto output to maintain a constant "digital high" output as the AC wave crosses zero. But when monitoring 24VDC I want to be able to switch the cap out of the circuit and allow the isolator be as fast as the analyzer. I would like to do this with a minimum of components, so I would prefer to use the same opto for AC & DC. All the AC input optocouplers I can find are pretty slow. The fastest one I can find (Avago ACPL-244) a combined tON+tOFF of 6μS to 42μS, which if my math is right, means a probable max frequency of 42-167kHz. What's the point of a several-hundred-MHz test device if it's limited by a 42kHz isolator? I'd like to know if there's such a thing as an AC version of the high speed DC-only optos, whose tON and tOFF are measured in nS.