I’ve encountered some interesting surprises while renovating my 1950’s farmhouse. When we moved in, we soon located a ground fault that periodically tripped a breaker on multiple appliances and the adjacent bedroom. Apparently, the electrician that installed the original wiring was an amateur. We got it fixed, but the experience reminded me how important good grounding is in electronics.
Grounding techniques in PCBs encompasses many design aspects that are critical in designing boards for modern applications. As PCBs run at higher speeds, process more data, become smaller, and communicate with other devices wirelessly, they require more layers to handle routing requirements. This then begs the question of how ground planes should be arranged and designed in multilayer PCBs.
The goal in arranging multilayer stacks in high performance PCBs is to minimize signal integrity problems like impedance, ground loops or ground bounce, or improperly routed traces. Routing signals on inner layers between ground planes helps suppress EMI between traces on different layers. Crosstalk between traces in the same layer can still be suppressed with differential routing.
Ground planes almost never extend throughout an entire layer. Instead, the shape of a ground plane can be very complex. Even with designing a complex ground plane, the ground plane in a single layer should remain as a single conductive layer. Since protecting against EMI requires judicious ground plane placement, designers must take care to ensure that routing traces in multilayer boards meets these basic design requirements.
Ground Planes in RF Devices
In PCB transmission lines carrying RF signals, placing a ground plane below the signal layer is not enough to suppress induced currents in the top signal layer. Routing these signal lines as coplanar waveguides provides much better EMI suppression. Via fences should be placed in the surrounding coplanar traces, and the via fences then need to be shorted to the ground layer. This shields nearby components from RF interference and shorts any induced currents directly to the ground plane.
In multilayer boards that use RF signals, RF routing on inner layers using striplines requires that striplines be placed between a pair of ground planes. Sandwiching striplines between ground planes enhances EMI immunity. The drawback to this arrangement is that signals will have slower propagation speed because the conductor is completely surrounded by dielectric material.
Ground planes provides natural EMI shielding
Since ground planes are large flat conductors that span a large area within a single layer, they provide natural shielding against EMI. In devices using RF antennas for communication, this actually works against you as the ground plane can block the RF signal emitted from the antenna. An improperly placed antenna can mean unnecessary impedance and frequency disruption in the PCB. Therefore, trace and chip antennas should not be placed above a ground plane.
Instead, place the antenna at the edge of the board and let the ground plane run close to the edge of the antenna. While a monopole antenna is different, in a dipole antenna, for example, the radiation pattern will have a zero intensity line that runs parallel to the length of the antenna. The emitted radiation pattern will be orthogonal to the ground plane, and emission at wider angles will not be blocked by the ground conductor.
Grounding in Mixed Signal and PCBs
Most RF devices are really mixed signal devices, so it helps to discuss mixed signal grounding techniques. With so many systems operating with mixed signals, proper grounding techniques should aim to segregate digital signals from analog signals. In mixed signal PCBs, partitioning the ground plane into digital and analog sections will prevent digital signals from interfering with your analog signals.
This is preferable to placing two different ground planes (one for digital and one for analog) that are unconnected. Using an analog and digital ground plane in a star-like topology sets the ground reference for both signal types to the same level. The start point used to connect the two ground planes is also a convenient point to connect to the ground lead of your power supply.
Electronic components on the blue printed circuit board
Just like ground planes themselves need to be well defined and largely separated, the use of a buss line to deliver analog and digital signals to their respective ground planes is not recommended. The idea behind the use of a buss line follows the logic that since the ground planes are really a single large conductor and act similar to a buss, it shouldn’t matter if my digital and analog signals use a buss for a return path. This is simply not correct.
Sharing digital and analog signals on the same buss line creates a low impedance connection between digital and analog components the system. Signals have an opportunity to mix by flowing backwards through the ground connection, similar to a ground loop.
If you are sourcing clock signals in your PCB, some designers will contend that it is appropriate to extend the ground plane beneath your clock IC. This is actually a bad idea, as placing the ground plane beneath the clock creates a center-fed patch antenna. If your clock frequency matches the ground plane resonance, your clock will induce a strong current in the ground plane. This can create additional EMI that interferes with nearby signals.
Since proper grounding is paramount to ensuring reliability, you need a PCB design software package that gives you the best design and analysis tools. A CAD system that can help by providing smart and intuitive design-rule checking on top of easily translated CAD drawings is ideal. This way your mechanical team doesn’t have to worry about trying to completely decipher your board designs.
The CAD tools in Altium Designer 18.1 lets you customize the geometry of your ground plane, and the analysis tools you need to analyze signal integrity. If you are interested in learning more, talk to an Altium expert today.
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