In Part 1: Principles of Current Flow, we began with the basics. We learned that high-frequency signals flow not in the path of least resistance, but in the path of least impedance. We also discussed some fundamental principles of current flow in PCBs with ground planes.
In Part 2: Design to Minimize Signal-Path Crosstalk, we applied those principles to real-world circuits and to the PCB layout of these circuits. We learned how to place components and route signal traces to minimize problems with crosstalk.
In this final Part 3 we consider the power source currents and how to apply what we have learned to circuits with multiple mixed-signal ICs. We finish with an example where a ground plane cut is useful.
What About the Power?
At the end of Part 2 in our series we decided to eliminate the ground cuts in our example layout because there are no signal return currents that "want" to cross the cuts. We do, however, have to consider the power connections. If both analog and digital power is from the exact same supply, then the source and its return must
be on one side of the cut or the other (Figure 1
Figure 1: AC signal currents with proper routing. See Figure 8 in Part 2 of this series.
In this case all the DC return currents (and frequencies low enough that significant current comes from the supply and not the bypass capacitors) from the other side of the cut must funnel through the narrow ground bridge rather than going straight to the power return connection. This makes their path longer, the resistance that they encounter larger, and thus the voltage drops greater.
This layout is no problem for return ground currents where the pins on the ADC sink the signal current, because these currents return from the ground pins which are both at the bridge. However, currents from ground pins on other components have to take an indirect route. Figure 2 illustrates these currents.
Figure 2: DC ground currents with cuts.
Removing the Cuts
If we remove the cuts, the DC return currents can flow more directly, with lower resistance and thus lower voltage drops. Figure 3 shows the same ground currents but with the cuts removed.
Figure 3: Circuit of Figure 2 with the ground cuts removed.
The same thinking can be extended to the situation where there are multiple rails. We just have to remember where the return currents will flow and take the multiple rails into account, just as we have done with the single rail.