Differential Pairs in High Frequency PCB Layouts
In high-speed PCBs, differential pairs provide a novel way to route bitstreams with fast edge transitions that reduce electromagnetic interference (EMI) from the traces themselves. However, differential signaling also introduces a number of additional considerations when compared to single-ended signals. These include matching lengths, skew correction, and signal-to-ground capacitance. How do you handle these challenges when routing differential pairs in your high frequency pcb?
A differential pair comprises two transmission lines that are equal and opposite in polarity. They must also be closely timed to one another. These characteristics enable differential pairs to carry signals with very low noise levels because the magnetic fields generated by the pairs cancel each other out. The signals can also be transmitted over longer distances without degradation because the pair’s differential impedance is lower than that of a single-ended line.
The disadvantage of differential pairs is that they generate electromagnetic field noise that can affect nearby single-ended traces. To minimize this, you should place the differential pairs well away from each other. You should also make sure that you have enough space between the signals on each side of the pair to prevent cross-talk from occurring. You can also use a wider differential ground plane to improve this situation.
How to Handle Differential Pairs in High Frequency PCB Layouts
For example, when routing a clock signal, you should place it in the middle of the board to ensure that there is sufficient separation between the signals on each side. This can help reduce the parasitic inductance and shorten the trace length, thereby improving performance.
You must also pay special attention to the differential impedance when determining how long you should make the signal traces. The maximum net length for differential pairs may differ from the limit for single-ended traces depending on the signaling standard you are working with. This is because the differential pair impedance and delay-per-unit-length values are different from those of a single-ended signal. Therefore, you should add length to the differential pair traces only when necessary, but be careful not to add too much because it can lead to an undesirable increase in skew.
You can encode many of the standard high-speed design rules that apply to single-ended signals into differential pair specific net classes and routing constraints in your CAD software. This enables you to automatically correct traces for skew and length while routing the pair to meet your signaling standards and performance requirements. Using these features will help you get your differential pair routes right the first time, so that they are ready for fabrication and assembly.
Then you can focus on other important considerations, like component placement and stackup, routing, and layout. Start your free trial of Altium Designer + Altium 365 today to find out how easy it is to create a top-performing high-speed PCB design with differential pairs. The complete suite of design, layout, and simulation tools in this leading PCB software will give you everything you need to deliver a high-speed circuit that meets your performance and manufacturability goals.
