As Oliver already mentioned, these capacitive humidity sensors are usually part of an oscillator circuit, where the change in humidity, means the change in sensor capacitance results in a change of frequency. A suited circuit can be found here:

http://www.hygrosens.de/fileadmin...or_dbd.pdf

Take a CMOS555, like TLC555, though.


The reason is, that the input current of an CMOS OPamp is only due to leakage! A bipolar OPamp is different in this issue: There is a NPN or PNP at input, providing a certain current gain, so you can calculate a certain input impedance. Not so with a CMOS OPamp. The input NMOS- or PMOS-FETs are extremely high ohmic due to the insulating poly-silicon gate barrier.
The leakage is mainly determined by eventual reverse biased protection diodes or else reverse biased parasitic pn-junctions at inputs. With CMOS OPamps only these leakage currents are of interest. And these highly depend on die temperature (doubling every 10K, or so) by the way.

Another fact is, that in a real OPamp circuit the loop gain (open loop gain / closed loop gain) increases the input impedance of an OPamp. So, this would result in an input impedance of 10,000 times 10^12Ohms for your CMOS OPamp, which is only of academic interest (if at all), because the leakage currents tell the true story.

Kai