About the input impedance of the non-inverting end of the op amp LM258 chip

[RachelAnne2020]

Regarding the input impedance of the non-inverting end of the op-amp LM258 chip, you need to use LM258 as a voltage follower. As shown in the figure, a voltage of 390KΩ is connected in series at the non-inverting end. A 500mV, 200KHz sine signal is given at point A, and no voltage can be measured at the output end and point B., Change the resistance to 33KΩ, the voltage at point B is about one-fifth of point A (input signals of 1V, 500mV, and 100mV respectively), so that the input impedance is about 8, 9KΩ, and the resistance is changed to 9.1 KΩ is measured again, the voltage at point B is about half of that at point A. Shouldn’t the input impedance of the op-amp be very large? Is it usually a few megohms? Why is the input impedance that I measured like this so small? (The reason for the design of the circuit is that a large resistor must be connected in series at the input) Who can help me?
Reference: https://www.utmel.com/components/lm258- ... eet?id=448
Thanks in advance.

[Soumitrab]

You haven't mentioned the voltage rails but assuming they're sufficiently high, your signal source may lack the drive current. Also since you're operating with ac signals, it may be a good thing to use a dc blocking capacitor as well.
Is there a reason why you would need a high value resistor in series ?

[Thomas.Pahl@t-online.de]

The input impedance should be very high. There must be a fault in your hardware.