Hi.
If anyone would like to see BUF802-based active probe - here are my preliminary results.
If anyone has his own experiences - You are welcome to share.
I've used circuit published in application note, with minor changes of the input part (including additional protection with PIN diodes, biased by zener diodes).
Results looks promising, IMO.
I attached plot from spectrum analyzer. Tracking generator was connected directly to the high impedance probe's input, so some impedance mismatch effects are obvious.
Best.
Probe loading effect may be quite high using the stand circuit in the BUF802 datasheet. Specified input capacitance of the chip alone is 2.4 pF, which turn to be 66 ohm @ 1 GHz, not to mention the PCB parasitic. That will make the probe useless in real world HF circuit.
A common practice for HF active probe is adding a high impedance passive attenuator in front of the buffer to isolate the capacitance.
Like nctnico has mentioned, using a matched impedance test fixture is also a good practice.
Also, measure S11 of a known good termination and then probe it to see how the S11 degrade. If don't have a VNA, try measure S21 and see how the probe influence transmission. The goal is to design a probe have minimum effect on the DUT.