On my board, I can't get the same low jitter numbers that are shown in the datasheet. Am I doing something wrong?

Many factors contribute to the overall jitter performance. This includes: the jitter of a reference source, the clock input, the design of the PLL loop, the VCO/VCXO choice, noise on the power supplies and bypassing, slew rate of input signals, output termination configurations, and signal routing. Differences in any of these can affect jitter performance. Also, the method of jitter measurement is critical because jitter at these low levels is very difficult to measure.

See AN-501 and AN-756 for more information.