Mon 5 Nov 2007
Here is a short application note (AN-404) from Analog Devices that deals with high performance analog and digital layout on the same printed circuit board. The specific example deals with AD1845 and CS4231 codecs and demonstrates some ideas for clean power and ground plane separation, among others. The application note provides some handy numbers, such as a “ballpark” estimate inductance of a PCB trace of 1nH/mm. Another helpful hint is that the note helps prioritize the various pins of the codec on page six to optimize noise management.
While on the subject of PCB design, here is a nice tutorial covering various dielectric materials used in printed circuit board fabrication. Its main goal is to give an overview of the various properties of the materials so the designer has a better judgement of which to use for higher performance RF boards and which is most economic for medium-speed digital designs.
( an-404.pdf )
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November 16th, 2007 at 5:33 am
In the practice, till now the best layout I have found in all of the cases is to keep only one solid ground plane (if you can afford 4 layers PCB). It is very difficult to have ground plane splitting working correctly. You have to take into acount all the noise and power signal returns, and to avoid signals crossing the line between the analog and digital planes. That considerations use to led to less efficient layouts.
My advice is to try first the simpler, one ground plane, layout.
November 17th, 2007 at 4:32 pm
I agree that keeping the design simple will get the job done in most cases. Another design trick that sometimes reduces noise in low frequency circuits is adding grounded copper pours. I think the best part of the application note is laying out the steps for determining how to split the ground planes according to the pin sensitivity. That is, it gets people to start thinking about ICs as circuit with different characteristics at each pin instead of a uniform device.