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GROUNDING IN HIGH SPEED SYSTEMS
Walt Kester, James Bryant
The importance of maintaining a low impedance large area ground plane is critical
to practically all analog circuits today, especially at high speeds. The ground plane
not only acts as a low impedance return path for high frequency currents but also
minimizes EMI/RFI emissions. Because of the shielding action of the ground plane,
the circuits susceptibility to external EMI/RFI is also reduced.
All IC ground pins should be soldered directly to the ground plane to minimize series
inductance. Power supply pins should be decoupled to the ground plane using low
inductance ceramic surface mount capacitors. If through-hole mounted ceramic
capacitors must be used, their leads should be less than 1mm. Ferrite beads may be
also required.
The ground plane allows the impedance of PCB traces to be controlled, and high
frequency signals can be terminated in the characteristic impedance of the trace to
minimize reflections when necessary.
Each PCB in the system should have at least one complete layer dedicated to the
ground plane. Ideally, a double-sided board should have one side dedicated to
ground and the other side for interconnections. In practice, this is not possible, since
some of the ground plane will certainly have to be removed to allow for signal and
power crossovers and vias. Nevertheless, as much area as possible should be
preserved, and at least 75% should remain. After completing an initial layout, the
ground layer should be checked carefully to make sure there are no isolated ground
"islands." IC ground pins located in a ground "island" have no current return path to
the ground plane.
The best way of minimizing ground impedance in a multicard system is to use
another PCB as a backplane for interconnections between cards, thus providing a
continuous ground plane to the mother card. The PCB connector should have at
least 30-40% of its pins devoted to ground, and these pins should be connected to the
ground plane on the backplane mother card. To complete the overall system
grounding scheme there are two possibilities: (1) The backplane ground plane can be
connected to chassis ground at numerous points, thereby diffusing the various
ground current return paths. (2) The ground plane can be connected to a single
system "star ground" point (generally at the power supply).
The first approach is often used at very high frequencies and where the return
currents are relatively constant. The low ground impedance is maintained all the
way through the PC boards, the backplane, and ultimately the chassis. It is critical
that good electrical contact be made where the grounds are connected to the sheet
metal chassis. This requires self-tapping sheet metal screws or "biting" washers.
Special care must be taken where anodized aluminum is used for the chassis
material, since its surface acts as an insulator.
In other systems, especially high speed ones with large amounts of digital circuitry,
it is highly desirable to physically separate sensitive analog components from noisy
digital components. It is usually desirable to use separate ground planes for the
analog and the digital circuitry. On PCBs which have both analog and digital
