Separate Your Analog and Digital Power Rails

You need to have separate analog and digital grounds, and separate analog and digital power rails throughout your design. Not only that, but you need to appreciate why this is important, and how a common power plane couples noise from digital circuitry directly into your analog circuits.

The short answer is that every power rail in your system is really a multi-tap resistor, and changes in current drain on the rail induce changes in voltage drop along the power rail that couples noise directly into the analog circuitry attached to that rail. Few analog circuits can reject noise on their power rails, unless they're specially designed that way. Just because your CAD package shows power and ground planes as single contact points doesn't mean that's what you get when the PCB layout is done, and power planes don't have zero resistance back to the power supply.

If the PCB layout is done by a different engineer to the schematics, they also need to appreciate why the power planes have been separated, and all the other design features included to reject noise; sometimes these can be lost when the schematics are converted into PCB layouts or optimised away inadvertantly.

This is really the absolute minimum you must do to avoid noise problems; but it's always the first thing I look for.

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