If there's one thing that PCB and EMC design have in common, it's that these designs are often left until the very end of the product design cycle. As with EMC design, waiting until the last moment to consider PCB design can be costly, time-consuming, and stressful, especially when dealing with high-speed PCB design.
High-speed PCB design is any design in which signal integrity is affected by the circuit board's physical characteristics. For these PCBs, designers face various issues, such as managing signal timing and integrity as well as mitigating noise. EMI can occur when currents within the PCB travel in an open loop rather than a closed loop. In order to mitigate EMI and design a robust high-speed PCB, follow the tips below.
First, it's important to design a schematic for your PCB, giving appropriate attention to signal flow. If you are designing the PCB but will not be engineering it, remember to provide essential information to reduce any EMI or other issues. For instance, you should outline which components, circuits, and traces need to be kept away from each other in order to minimize interference.
Because high-speed PCB design relies heavily on the board's physical characteristics, it is vital to pay attention to where components are located. To ensure your design works exactly as intended, specify critical component locations and signal routing paths, as these are the most important elements in the design.
In order to minimize noise, you can strategically place your traces further apart, as close proximity can lead to interference. Additionally, you can ensure that unwanted noise does not make contact with the operational amplifier by paralleling capacitor values, which compensate for the amplifier's power supply rejection (PSR). If you are having difficulty placing all four capacitors in the circuit, you can tie one or more bypass capacitors between the operational amplifier's negative and positive supply rails.
Minimize Interference Because high-speed PCBs contain various signals that could possibly interfere with one another, it is vital to reduce the ways in which these signals interact. Follow the guidelines below for reducing signal interference:
To reduce inductive coupling, minimize long parallel runs of signal traces
To prevent capacitive coupling, minimize long traces on adjacent layers and use orthogonal routing
To minimize signal sensitivity to noise, provide the proper impedance between the transmitter and receiver
To minimize radiation from traces that are crossing plane splits, stitch the planes together near the trace crossing using decoupling capacitors
Signal traces that require isolation should be routed on separate layers.
You can prevent EMI by considering it early on in the PCB design process. To prevent EMI, you should use a four-layered PCB (two layers for signals and two layers for power and ground); use solid ground planes; avoid vias with transmission lines; minimize current loops; and keep signals within an LVDS pair closely coupled.
While high-speed PCB design can be a headache, there are various tricks for reducing EMI early on, saving you time, money, and stress. At Rhein Tech Laboratories, our engineers and designers are equipped to review your existing schematic design, analyze complex printed circuit boards for potential compliance problems, and provide recommendations to alleviate problems discovered through EMI testing. Skip the headache and contact us today for a quote!
Sources: http://www.ti.com/lit/an/snla302/snla302.pdf, https://www.analog.com/en/analog-dialogue/articles/high-speed-printed-circuit-board-layout.html#, https://www.autodesk.com/products/eagle/blog/high-speed-pcb-design/