Getting back to ground planes… I’m certainly no expert, I just learned how to do a ground plane in Eagle about a month ago. But I’ve found that they make routing your ground nearly effortless. Just do a polygon covering the entire bottom layer and name it GND. Then anytime you need something connected to ground, just pop in a via, click ratsnest, and you’ve got it. You just need to avoid putting so many non-ground traces on that layer that they chop the plane into multiple segments.
But do be sure and read your datasheets in case you have something that says not to put it over a ground plane. I’m working with a pressure sensor that gives analog output and the manufacturer notes to use ground planes selectively. Actually, that’s not in the datasheet, it’s in a application note referenced from the datasheet. It’s got a section on PCB layout considerations for digital and analog circuits that looks pretty useful. The App note is Freescale’s application note AN1646, [“Noise Considerations for Integrated Pressure Sensors”, but take a look starting on page 5. It looks like pretty useful info for more than just pressure sensors.
Keith](http://www.freescale.com/files/sensors/doc/app_note/AN1646.pdf)