OPA2322: input parasitic capacitance

Yamamoto-san,

The input parasitic capacitances are listed in the Electrical Characteristics table of the OPA2322 datasheet. There are two input capacitance, the differential input capacitance (CD) which is listed as 5 pF (typical), and the common-mode input capacitance (CCM) which is listed as 4 pF (typical). If you need information about these capacitances mean please see the following TI blog:

The input capacitance is only characterized at room temperature (25 C). Likely because of the difficulty of accurately measuring very small capacitances in a temperature chamber.

The OPA2322 has CMOS input transistors and the capacitances of the junctions are very small. This makes up part of the op amp input capacitance. Based on information that I found about the MOSFET junction capacitance it is nearly constant across temperature. However, there is another circuit element at each input pin internal to the op amp; an ESD cell.

The OPA2322 inputs have an ESD current steering diode to each power supply rail. These are low leakage current diodes that are reverse-biased and turned off when the op amp is mounted into a circuit and operating under normal conditions. They are not only the main contributor to the op amp input bias current, but also a contributor to the input capacitance.

I searched for information about zero and reverse biased silicon diode junctions and found that  their capacitance have a positive temperature coefficient. No specifications are provided for the temperature coefficient, but it tells us the junction capacitance increases with increased temperature. We would expect the OPA2322 input capacitances to increase with temperature because of this diode characteristic.

Regards, Thomas

PA - Linear Applications Engineering