How to address gain error issues with AD817?

I am using AD817 with a Burr Brown BUF634 in the feedback loop (boosts current to 250mA) as an instrumentation buffer. Specifically I am using AD817 for it's ability to drive capacitive loads because my buffer goes to a 50 ohm coaxial cable that eventually goes to a digitizer in a PC. I noticed an offset that is reasonable as per the AD817 datasheet, but I also see a gain error that is up to 15% for voltage readings captured by the digitizer. I quantified this by forcing a series of DC voltages and digitizing them. At lower voltages (0 to .5V) there is little error, but by the time it is 1.5 to 2V I am seeing 80 - 100mV error. It seems to change slope around 1V, making the issue worse.

Here is my circuit:

I have voltages higher than 16V present at the input, so I have a HV mosfet with 5V on the gate in series with the BufIn and the signal I want to capture as a voltage blocker. But the measurements were all taken with scope probes on BufIn and BufOut and the oscilloscope agrees with the digitizer. I also have clamp diodes to ground and 12V as additional protection for the AD817. The 1K resistor to ground is do not load because it will allow current to flow to ground through my voltage blocking mosfet and it can become damaged.

Any ideas how to improve the gain error in my circuit? I could measure voltage very accurately with the digitizer but for this problem. Thanks a lot!

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