A single ground plane is sufficient to give good performance, provided the analog, digital, and clock sections of the board are cleanly partitioned. See the ADS414x, ADS412x EVM User Guide for details on layout and grounding.
Because the ADS412x and ADS414x already include internal decoupling, minimal external decoupling can be used without loss in performance. Note that decoupling capacitors can help filter external power-supply noise, so the optimum number of capacitors depends on the actual application. The decoupling capacitors must be placed very close to the converter supply pins.
In addition to providing a path for heat dissipation, the thermal pad is also electrically internally connected to the digital ground. Therefore, solder the exposed pad to the ground plane for best thermal and electrical performance. For detailed information, see application notes, QFN Layout Guidelines (SLOA122) and QFN/SON PCB Attachment (SLUA271) that are both available for download at www.ti.com.