Interfacing the ~3MHz variable frequency output of a capacitive sensor to a 5i25

Howdy folks. I've been working to create a workable fiber laser control system using LinuxCNC and eventually plan to use PlasmaC. I have a base machine configuration working, and am at the point where I am ready to integrate a capacitive sensor for torch height control (the on to PlasmaC). The sensor I am integrating is a BCL-AMP style connector that is outputting a variable frequency signal at around 3MHz and around 5V. There seems to be a complete lack of documentation for this capacitive amp, but I've managed to reverse engineer it enough so that it is basically working. But I don't trust it that much unfortunately.

Which leads me to want to ensure I don't inadvertently send more that 5V to the Mesa card. Opto isolators like the PC817 or even H11L1 are too slow. Would a zener diode voltage clamp be fast enough? Any other ideas?

I believe that at 3MHz I'll probably need to set the filer attribute of the hostmot2 encoder to false. I'll also need the encoder in counter mode to provide a velocity value that can basically act like a THCAD for PlasmaC. Any other advice on integrating a signal like this?

Thanks,

Scott

If you are connecting directly to the 5I25 input the most likely
cause of damage would be  negative inputs from high frequency ground
bouncing causing excessive currents to flow in the input clamp diodes.

You can prevent this with a  220 Ohm series resistor.

If you are connecting to a normal Mesa daughtercard encoder input,
no extra protection is needed.