I did something ugly with a 3.3v cmos 7406 varient and a feedback
resistor, which works well enough to get an accurate reading on one
version of the device under test, but not on the other (both have been
verified with real test equipment) It also tends to self-oscillate
with no input...

7406 is open collector. Did you mean 7404?

What size feedback resistor? What sort of oscillations?

I've had reasonable luck with that sort of hacking. Not great.

What's the output of your gate look like? Is it cleanly switching
or struggling to switch at that speed?

You might want to skip the external gate and use an inverter
in the FPGA out to a feedback pin. That gets the feedback
covering the input pin that you are really interested in.


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