Hi,
This can sometime be caused by noise on the digital line, transitioning from low to high the voltage level can spike above the TTL threshold dip below it and then back to steady state as the system finds equilibrium. Your high frequency measurement could be cause by this ringing action when it is large enough to trip the TTL thresholds multiple times. Can you scope you input signal to observe this phenomena or apply external digital filtering?
Matt
Applications Engineer
National Instruments