I am trying to look for a high input impedance op-amp to use as a buffer to prevent source loading.

When I started checking the input impedance value for different op-amps, I noticed that some of the amplifiers mention it and others don't.

For example, the AD8597 datasheet doesn't mention the input impedance value.

In this case, how do I determine the input impedance?

  • \$\begingroup\$ The datasheet mentions the op-amp's input bias current -- is there a reason why this parameter is unsuitable for comparison for your application? \$\endgroup\$
    – nanofarad
    Aug 12, 2020 at 1:00

3 Answers 3


As a bipolar-input amplifier, The input impedance of that part is relatively low, less than 1G ohm, typically as you can see from the change in input bias current with CM voltage. Eg. Fig.15.

Typically, to get the benefit of the front end which is run at relatively high current with super beta transistors, you’d want a source impedance less than 1K so that’s of little concern.

The input bias current is typically only 40nA (still quite high) but can be as high as 340nA over temperature, which, along with the large offset voltage, are probably bigger concerns.

It’s optimal for low impedance AC sources where noise is a primary concern.

If high input resistance and bias current is your main concern, you can get CMOS-input amplifiers with bias currents in the double digit fA range, which is more than 7 orders of magnitude less.


The data sheet says this: -

The AD8597/AD8599 are the optimum choice for low noise performance if the source resistance is kept < 1 kΩ

Clearly, this amplifier should only really be used when the source impedance is less than or equal to 1 kohm. Reason: it's a very low noise amplifier and this means that having a source impedance higher than 1 kohm is missing the whole point of using this amplifier.

The data sheet also says this: -

Input Capacitance

Differential Capacitance 12.1 pF

Common-Mode Capacitance 5.1 pF

And that's all you need to know.

I am trying to look for a high input impedance op-amp to use as a buffer to prevent source loading

Then you're looking at the wrong device.


The input impedance of AD8597 ca. 45MΩ @ 1kHz and 2MΩ @ 20kHz.



Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.