Matching line driver output impedance over wideband
The input capacitance of the JFET(s) will require charge form the line you are tapping.
You need to also define how much upset that line will tolerate.
A single pF at 1GigaHertz is -j 159 ohms.
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If the ADC driver uses negative feedback, and has bipolar (silicon or SiGe) technology, the R_out should be 0.026/Iemitter_amps. Thus operating a class AB push_pull output at 5milliAmps should give an R_out of 0.026 / 0.005 = 5 ohms.
Perhaps provide link to your favorite ADC driver datasheet, and we can think further.