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TECH ARTICLE

T

he term “ground” is a fine concept at DC voltages, but it just does not exist at the frequencies running on today’s typical boards. All metal has some amount of resistance, and even if that resistance was near zero ohms, the current flowing through a conductor in a loop creates inductance. Current through that inductance results in a voltage drop. This means that the metal ground plane/wire/bar/etc. has a voltage drop across it, which is in direct contradiction with the intention and definition of ground. The important point is that for EMI/EMC we need to consider the current, not the voltage, in our signal paths. Since current must always flow in a loop back to its source, the return current path must be considered as well as the intended signal path along a PCB trace. Any interruptions to the return current path can have serious negative effects to the EMI/EMC performance of a PCB. A very slight deviation in return current path can result in enough inductance to dramatically increase emissions. The return current path is also very important when considering mother/daughter board configurations. Figure 1 shows a simple fourlayer board example of a mother/daughter board configuration and a signal path from the mother board to the daughter card through a connector. If we consider how the return current will flow from this configuration, we should expect that the return current will spread out to include displacement current through the dielectric between GND and PWR, as well as local decoupling capacitors (depending on their distance and the plane separation). Figure 2 shows the return current for this configuration. The added return current path length results in additional inductance in the total path, resulting in a ‘noise’ voltage between the two GND planes (across the connector). This noise voltage will drive the wide, thin, monopole-like antenna, resulting in increased emissions. However, if we had simply considered the return current path and routed the signal trace so that it was referenced to the same plane (PWR or GND), the return currents are able to stay close to the signal trace (Figure 3), and emissions are greatly reduced.

Connector

Signal Path

GND PWR Signal Layers

Figure 1: Initial Two Board Configuration

Connector

Signal Path

GND PWR Return Current

Signal Layers

Figure 2: Return Current Paths for Initial Configuration

Decoupling Capacitors

Displacement Current Connector Signal Path

GND PWR Signal Layers

Figure 3: Improved Return Current Design

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