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Don't treat this drawing as a bible .. it is just an example to give you some ideas ..
Diodes are not required and if you imagine shielded wires you will have so many wires between DGND and GND. In practice, on a PCB these wires don't exist ..
ian has the right idea, let me try to explain it another way.
splitting the ground lines is called star ground or kelvin ground. when something switches, it takes a spike of current that causes a little voltage spike in the ground line. even though the trace resistance is low, it is resistive and WILL create a voltage spike for every current spike. for things like switching converters these spikes can be 10A or so as a big mosfet gate is pulled to ground in 20ns. you cannot have your analog circuitry "upstream" of these noise spikes, or analog side will get bounced around, giving unpredictable operation.. (some circuits are slower than others, so the noise is not really common mode as you might think - it depends on the actual circuit getting bounced.)
the easiest way to avoid this junk like clock noise in digital circuits, or switching noise in power circuits is to run the grounds seperately back to the real ground connector on the PCB. now if one ground is bounced all over the place, it does not pollute the other ground, because there is no reason for the spike to travel up the other ground. (assuming the connector is NOT too resistive) The best implementation would use two ground connectors but that may be overkill.
The back to back diodes are in case digital ground gets bounced more than +0.4 or -0.4v (schottky) from the other one. you have to keep them relatively close, or an analog logic low may not be a digital logic low anymore. (and you can risk latchup, but you need severe bounce to do something like that)
In my designs I cut a ground plane of PCB onto analog and digital areas in order to prevent digital currents (return currents, etc.) to flow through the analog part of design. Those parts were connected in a single point with zero Ohm resistor.
The bunch of wires in the drawing tells us that analog and digital ground must be routed in "star-shape" manner. It's a common practice. Any daisy-chains aren't alowed in the ground because they create some voltage dropout and whole device becomes too sensitive to an electric noise.
Considering the traces inductance, IMHO this would be a better approach. I apologize for the poor quality of the corrections. The original example is good from the point of view of digital/analog separation, but the decoupling is wrong, both in analog and digital sections.
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