Hi,


that's a Sallen-key low-pass filter. I'm guessing you're talking about the in-band Zin. In that case, you can treat the capacitors as open circuit. Your input impedance is just the parallel combination of the two biasing resistors (looks like R1 and R3 but its difficult to make out). Nearer to the 3dB frequency, your input impedance will reduce.


cheers,
Aaron