AD8001 input resistance

Dear engineers. I tried to use ad8001 as an input buffer after a resistive voltage divider. I found that the divisor coefficient does not match the calculated one. It seems that the input resistance of the ad8001is tens of kiloohms instead of 10 Mom. Could this be due to the fact that I used ad8001 with single supply 5v? What other problems may arise with the 5v power? Can you recommend an inexpensive amplifier with a high input impedance (CMOS)  for 500 MHz. Thank you.