Input impedance of low pass filter on LTspice must be constant in it's pass band?
Do you think the AC analysis extracted the correct impedance?


Yes. it looks likely that the graph starting at about 30 kohm and falling to 3 kohm is precisely the input impedance of the filter design. The only problem is that your filter design is unrealizable because the bare-bones input capacitance of the AD8029 is 2 pF (typically) and your model is attempting to use a capacitor from the non-inverting input to ground of 0.5 pF i.e. your design relies almost entirely on the internal parasitic capacitance of the op-amp being 2 pF (+ 0.5 pF in parallel) and stable. That is a poor way to design a circuit and will lead to disappointment.

You should also rethink having a 120 ohm resistor as the output load because you will get poor results from this op-amp with a load this low in value. If you need to drive a load of 120 ohm, use a suitable line driver circuit added to the output.

But if you are going to do this you might just as well dispense with the AD8029 altogether and use a passive filter approach rather that an op-amp sallen key filter because op-amps used in this configuration never really deliver the goods when operated close to their full bandwidth and it seems like you need that bandwidth.

Use and RLC low pass filter followed by a buffer amp is my opinion.