Best way to read 18kHz PWM signal with wide V_pk-pk (5.5-18V)
I would be inclined to use an NPN transistor. The gate capacitance of a MOSFET, combined with the limited current you can draw, is going to present some serious problems WRT edge timing.







simulate this circuit &ndash; Schematic created using CircuitLab

27k base resistor limits current to about 200 to 600 &micro;A. Schottky diode between base and emitter clamps any negative-going input transients. Second one from base to collector, just like with Schottky logic, serves to keep the transistor from fully saturating, keeping the switching action fast.