Altium, mixed signal board, convey to PCB designer whether net must stay over analog region of return plane or digital region of return plane

Question

Some oversimplification is necessary to explain my question--please let's not debate PCB layout techniques--I just need help with a simple Altium question.

Preface:

One solution to making a good mixed signal PCB design (without high currents that can create a voltage drop across a plane) is -to make a single ground plane -to place the digital parts together -to place the analog parts together, separated from the digital parts. -to connect the returns of the analog and digital parts to the single ground plane -to ensure that currents flowing to/from analog parts do not cross the paths of current flowing to/from digital parts until as close as possible to the power supplies. (No digital power plane over an analog power plane)

Things get messy when you have and ADC and DAC in the board. The ADC and DAC have separate analog and digital return pins. However, to get specified performance these pins must be joined to the single plane very close together at the "border" of the analog region and digital region of the plane.

There are 40-50 signals going to the 60 MHz 14 bit ADC and DAC including LVDS, differential clocks, SPI's, and differential analog inputs and outputs.

To the PCB designer, it is not clear whether these signals must be classified as analog or digital so he can locate the traces appropriately.

Using Altium, what is the best way to add the "keep over analog region" or "keep over digital region" meta-data to each net so that the PCB designer can use rules?

Answer 1

The normal way to do this sort of thing (communicate metadata from schematic to PCB) is with Directives. In this case, you could design with separate AGND and DGND nets, connected together by a net tie and a short circuit exception. Then you would label each of your nets (of import) with a net class, either "ReferencedToAGND" or "ReferencedToDgnd". Alternatively, you could set a ReferenceNet parameter that is either DGND or AGND.

The PCB designer can then develop DRC based on those directives (along with some explanatory comments on the schematic).