I have been reading this application note from National and I have a question about this phrase:
… traces carrying switching currents/voltages must also
be kept away from ’quieter’ traces to avoid crosscoupling. …
How many mils should they be away? Does this distance change with the current or voltage? Does the flow of current, voltage affects 'Quieter' traces?
Best Answer
There is no exact answer for how far is far enough. It is better to understand the mechanisms so that you can decide for yourself how to deal with them. There are two independent mechanisms at work that cause cross coupling.
Capacitive coupling. Adjacent traces have finite capacitance between them. Three things drive coupling from a power trace to a signal trace:
Inductive coupling. Current in the power trace causes a circular magnetic field around it. This can induce voltages in nearby parallel traces. Essentially the two parallel traces act like a transformer. Two things drive this coupling:
The impedance of the signal trace does not matter for inductive coupling because it causes a offset voltage in line with the trace. It is a additional voltage source in series with the trace.
So, what to do? It should be obvious that moving the traces apart will help for either type of coupling. To reduce capacitive coupling a separating trace or guard trace can be used. Other things that help are limiting the rate of voltage and current changes, avoiding long runs, avoiding parallel runs, and making the signal nets as low impedance as possible. Whether these are possible, under your control, or reasonable considering other tradeoffs like size and cost, are issues with your specific design you have to decide for yourself.