Re: Analog GND <-> Digital GND = how to place both GN
The grounding requirements for analog and digital are quite different, and therefore one often has to use for instance that "single point" or "star" or "zero-ohm-resistor" concepts to keep track what is grounded and where.
However, the essence in my experience cooks down to some very basic rules:
1) Low level analog signals are easily disturbed and collect unwanted noise
2) Logic signals are noisy in their nature, and generate also noise over the impedance of ground coductor.
Therefore, you have to keep the digital side ground return currents from generating (ground) noise to those low level analog signals. But also:
3) Digital ground returns are part of the (usually quite high speed) digital signal channel. Too much impedance there may cause digital side ground bounce, even in degree that disturbs the digital side.
Therefore, the digital ground should always follow a low-impedance path towards the other end of the digital signal path.
By separating the analog ground one can solve the issues 1 and 2, but care has to be taken to not to weaken the digital side return path. Therefore, if possible, the joining point of these two grounds should generally be under or close to the mixed mode chip (for instace an A/D converter or such), while the digital side is directly grounded to the "main" ground plane or similar.
Personally I prefer the ground connection in the PCB itself, and not by a zero-ohm resistor component. In most PCB layout systems you may solve the issue in several ways. For instance by having only one "GND" net for both, and manually tracking the different "sides" of it, or having a dummy component, which has a built-in short circuit (such as "2D line" in PADS), and placing it between "Analog GND" and "GND".
Good Luck,
Ted