Confusion with a measurement method suggested for an opamp input impedance
So my question is, is it possible to measure the input impedance here by using the methods mentioned in the text?


I'm not convinced you can measure it accurately, as it maybe be ridiculously small depending on your op-amp of choice, but I believe you should at least be able to approximate the -3 dB point.

A quick simulation in LTSpice seems to confirm this: sweep the value of C1 (from, say 1fF to 100pF), use AC analysis, and see how the response changes.

On the bench, you should first determine where your -3 dB corner is, without the op-amp connected. That is, connect the scope to the 1M resistor (which is presumably connected to a waveform generator), and sweep the frequency. You should be able to see the corner at fairly low frequencies due to the high source impedance (my LTSpice simulation shows a corner at ~2.5 kHz without the "op-amp" connected, but that will depend a lot on the rest of your environment and equipment).

Connecting the op-amp (if it has a high-ish input capacitance, if it it's got a input capacitance of 100 fF or something, it's probably not a good candidate) should move the -3 dB corner a bit, although you may need to look at the -6 dB point to better find the trend.

In my simulation, it seems that changing the probe capacitance or scope capacitance doesn't actually change this by a wide margin.

So, is it a good way to measure the input capacitance of an op-amp? No, and you should always be aware that measuring high-impedance (or fast) signals with a passive scope probe can lead to trouble. However, with a sufficiently capacitive op-amp, I think you should at least be able to observe the difference between op-amp and no amp-amp.

Oh, and for actually measuring input capacitance, definitely look at Tony's answer, it's great.