Andy Kunz says:
[We should separately bypass/isolate the AVcc and AGND]It came about as a result of using a 17C756 (in which they are labelled as AVcc/AGND) while having poor results with a '74. One day, a lightbulb went off, and all of a sudden it worked with the new power layout. Ever since then I've used a fixed part (separated the two power/ground systems) in the schematic package and have not had such problems any more.
I think it's an oversight on their part NOT to tell us this. What I can't figure is how they get a '73/76 to work well, since it's the same die as the 40-pin part just a different bonding.
I never thought of it until just now, but I wonder how much resistance is between the two grounds and two power supply pins.
Dan Michaels says:
This is most unfortunate. Exactly the other side of the "AGnd" are the oscillator pins, so this gnd pin makes a really great place to return the little gnd-trace-island that you draw around the xtal/cap/cap area on your pcb to help isolate and confine the hi-freq stuff.So, instead of reducing overall noise, you end up directing it straight into your AGnd. Gakkk. Wrost of all possible worlds.
Interested: