TMS320F280049: Layout guide and analog/digital ground separation

Hi Wayne,

For best possible performance you'd ideally separate VSSA and VSS planes. However, this can be expensive (additional layers) and has some pitfalls where separating the grounds could actually make things worse, not better.

For transient signals, the ground return path goes directly under or over the trace in the nearest plane that that signal is referenced to (see the diagrams here: electronics.stackexchange.com/.../real-current-return-path). The 'loop area' of this signal, which is where the signal E+M fields exist and thus the signal energy exists is in that area between the trace and it's ground-return. If the signal is running directly adjacent to the appropriate ground plane, the loop area will be small. However, if the signal crosses a cut in the planes, the signal will have to find some circuitous way around. This results in a large signal loop-area, so the energy occupies a large space which is very likely to overlap with the signal energy of other signals on the board. Crosstalk thus occurs between signals if your signals cross a separated ground plane (or run adjacent to the wrong ground plane).

As noted above, since most of the signal energy is near the signal trace (not in the whole plane) you can prevent coupling between analog and digital signals by just physically separating the signals from each other. One easy method is to separate the planes, route the signals, and then remove the separation between planes. This is usually good enough.

Overall it is very difficult for us to provide definitive guidance on this topic (and other board layout topics) because the trade-offs between board size vs board cost vs board complexity vs performance vs other application requirements have to take into account all the ICs and components on the board, not just C2000.

(But we are trying to get more HW design type collateral for customers, either directly in the datasheets or potentially in a HW design guide type document)