analogdigital-logicground
I am considering using a chip, but the datasheet is confusing me as it has several distinct power pins.
It has:
How do I hook up each? Do I connect AVDD straight to DVDD, and what about PLL_VDD, and the ground pins? The suggested schematic seems to show them all connected to the same potentials (with different symbols for both types of grounds); from a cursory glance it doesn't mention anything about PCB layout. So how should I wire it up?
What you are looking for is called a "USB Power Injector".
Many commercial (cheap) offerings exist. Here's an example. They are available in all USB connector types: A, B, mini-B, micro-B, etc.
Your assumption in the second bullet point is correct: that answer does not apply to your situation. The linked question is discussing separate but electrically equal analog and digital ground planes, which is a technique used to reduce electrical noise, not to provide galvanic isolation.
To answer your more general question, you should either use multiple different symbols (you may be able to rename the net for the 'gnd' symbol in Eagle to create different nets; it's a while since I used Eagle), or use named nets for your separate grounds.
Your schematic as shown won't work as intended, however. The isolated side of your optoisolators is powered from a power supply that's (presumably) referenced to the ground of the Arduino side of the circuit.
If you want each of the four circuits to be isolated from each other as well as from the Arduino, you will need a separate power supply for each, and four grounds (plus the Arduino's ground). If you only want to have isolation between the CCFL and Arduino sections of the circuit, you will need a single isolated power supply for the CCFL side - and a single ground in addition to the Arduino's ground.
In every case, remember that you can only use power supplies and signals referenced to the same ground as that part of your circuit.
Best Answer
Divide your board into those discrete sections: PLL, Analog, Digital, and IO. Try to keep all of the copper (signal & power) for each section separate except where they require interconnect. Join the power to each sub-circuit only at near the power supply, within the sub-circuit and at chip when possible. Bypass each at the chip grounds with a cap or two (you might leave an extra footprint here for test).
You may find that the layout doesn't permit total separation of sub-circuits. If so, at least try to use a wider trace to supply the split and filter where necessary.