The general concensous these days is to have only one ground, not only as pointed out above, multiple ADC's or DAC's make it impossible to have 1 star point, but historicly when the grounds were split, logic was 5V and analogue was quite often 12-16V, so a 16 bit convertor had steps of 0.75-1V. Nowadays convertors work anywhere between 1.2V to 5V, so for 16Bits you have steps of 75mV to 0.3V, so if you have two grounds connected by a high impedance star point, it dosen't take a lot to have the grounds at different levels, wiping out the conversion accuracy. This is even worse when you gey to 24 bits.