May I ask how to find the input impedance for the output of VG1 in the calculation and in simulation tests? Should I consider the op-amp as a resistor with the ideal input impedance when calculating?
From the datasheet, the input impedance for component U8 is 10^13 ohm for both Common-mode Input and Differential Input. The input frequency should range from 1 Hz to 100 Hz
I am trying to find the input impedance for 3 different circuits. All of them should have similar ideas, I think if I can understand the simplest one, I can solve the other two.

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  • \$\begingroup\$ It is \$\frac{dVG1}{dI(VG1)}\$ \$\endgroup\$
    – tobalt
    Jul 3 at 6:40
  • 1
    \$\begingroup\$ Input impedance for what output? The signal generator? If so, what components you think affect the input impedance and how? Can you think if frequency affects the impedance? \$\endgroup\$
    – Justme
    Jul 3 at 6:43

2 Answers 2


Should I consider the op-amp as a resistor with the ideal input impedance when calculating?

The answer is "no" if the input frequency range from 1 Hz to 100 Hz for the whole circuit.

If only the "real" input of the op-amp is of interest, it is another thing.
Just remember that there are "parasitic" capacitors at inputs and between inputs ...
1 pF @ 1 kHz -> 159 MOhm << 10^13 Ohm.

As @tobalt pointed out in his comment, for one input :
\$Zin= dVg / dIg = dVg/i(R5)\$ (my labels).

From this simulation, EE&O, one can see that AC input impedance is variable with frequency (RED curve).
It is "high" for low frequencies, but not so high as one could think.
Something as 400 MegOhm @ 1mHz, but only 400 kOhm at 1 Hz... until 11 kOhm (f > 10kHz).

enter image description here


As you noted the input impedances of U8 for both signal modes are really huge, so I think in this problem we can assume that they are infinite and no current goes through + and - pins of the U8.(In real world this amount is fairly negligible)

Now we can say that the input impedance that VG1 sees is just a resistor (R23) series with two parallel capacitors (C22, C23).

(Note 1: remember that almost no current passes through + pin of U8 so we can assume any connection to the + pin, open circuit)

(Note 2: for calculating input impedance of an arbitrary circuit, we should short-circuit every independent voltage source inside the circuit and we should make all independent current sources inside circuit, an open-circuit. So in this case we short-circuited VREF, and it became 0 volts in our calculations for input impedance)

In conclusion, the answer for this circuit is:

$$ R_{23} - \frac{j}{2\pi f(C_{23} + C_{22})} $$

where \$f\$ denotes VG1's frequency.

If VG1 is a dc source, then the input impedance is infinite.


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