Original question:
The capacitance is normally the most important, the resistive part is so high as to not matter normally,
Hi Lidong,
As Michael pointed out, the input bias current in OPA320 is extremely low, which is 0.9pA (max) at room temperature. The input impedance is predominately determined by the input capacitance, which it is determined by the part's operating frequency in 1/(2*pi*f*9pf) in input capacitance of approx. 9pf, which is very low.
Even considering a static input impedance, say input voltage of 2.5V, the OPA320's input impedance will be at least decade higher than 2.5/0.9pA =2.8TΩ . The ultra low input bias current of 0.2pA (typical) is caused by the leakage current of ESD protection cells at the input. Of course, the ESD leakage current tends to increase with increasing in the operating temperature. Thus, OPA320 is a very good high impedance sensor interface part as described in 8.1.3, see the image below.
If you have additional questions, please let us know.
Best,
Raymond
