SN74ALVC244PWR

Hi Patrick, to get a worst-case estimate of input impedance, look at the input leakage spec, II.

3.6 V / 5 uA = 720k ohms.

In reality, 5 uA is much higher than typical. In the lab, I typically see more in the range of nA.

So in worst case, expect 720k ohms, but typically tens (or even hundreds) of Megaohms.