### Q

would like to know more about the AD822’s input impedance. In the datasheet,the AD822’s input impedance is a typical spec, do you have any more information

regarding an estimated variation of this impedance? What's the best way to

measure this parameter?

### A

We do not measure the input impedance directly. An indirect way of inferringthe common mode input impedance is as follows:

. Input Impedance = Change in applied voltage / change in input current.

So you can estimate the input impedance from looking at Ibias versus VCM curve.

Of course, this simplistic model may not apply in all cases. Sometimes the

typical datasheet spec is also based on detailed knowledge of the process

corners (simulation results).

The input bias current and input resistance are DC specs. A simple model of the

op amp input can be something like attached document.

Some of the bias current you see is fixed, and some of it will vary with the op

amp’s input impedance. Needless to say there is no physical resistance and what

you see is mainly due to leakage currents through the oxide/base current of

transistors, ESD leakage current etc.

From our understanding, many applications do not need to know the exact input

impedance. For example, in a transimpedance amplifier circuit, the designer

would be more concerned on keeping the input bias current negligible vis-a-vis

the photodiode current rather than knowing the absolute value of input

impedance. Furthermore, if the photodiode is operated with zero bias or a fixed

bias then the input common mode also does not vary so a fixed base current

model works perfectly okay. Likewise in a non-inverting amplifier the gain

resistors would be designed to carry say at least 100X times the worst case

input bias current to guarantee an accurate amplification.