connect these to
two separate ground planes on my board?
No. At least, not unless the two ground planes are connected together very near
the chip. In all
other situations, it is best to keep the MicroConverter on a single ground
plane. If you have two
separate ground planes on your board, then place the MicroConverter on the
quieter of the two
(usually the analog plane) to obtain optimum ADC and DAC performance.