The Best Design Tips for Differential Pair Routing

January 17, 2019 , in Blog

I love going to sporting events. There’s so much excitement in the air, and there’s just something special about being in the same place with thousands of other people for the same reason. The only thing I don’t like about attending games is the excessive noise or interference from other conversations. While this interference is simply an unpleasant side-effect of a sporting event, it is more than a mere nuisance when it comes to PCBs. Electromagnetic Interference or (EMI) can significantly impact the signal integrity on your boards; however, there are routing options that can help minimize this impact. The use differential pair routing can help protect sensitive signals on your board since they’re mostly immune to common mode noise and reduce the current flowing through the ground plane. However, differential pairs come with their own set of disadvantages, so you’ll have to manage trace widths, copper weights, impedances and current flows to avoid spreading EMI on the PCB and causing timing mismatches in the pairs themselves.

Differential pair with meander on a PCB

Pros and Cons of Differential Pairs

If differential pairs are so difficult to route, then why invest the time and effort to use them? The answer is that differential pairs are naturally resistant to common mode noise, have little return through the grounding plane, minimize signal loss and increase the signal-to-noise ratio (SNR). They are also routed close together, meaning that outside noise shifts the signals in the pair by approximately the same amount, thereby increasing their noise rejection. Furthermore, because current returns through one of the traces, very little actually flows through the ground plane to elsewhere in your system. This means that when routed correctly, differential pairs can cross analog/digital planes and power boundaries without causing too much EMI.

On the negative side, differential pairs obviously take up more real estate and require careful routing and impedance matching. If differential pairs aren’t routed correctly, they can inject a significant amount of noise into your other circuits, possibly causing more headaches than they solve.

Pros and Cons of Differential Pair Routing

Pros

Cons

Mostly immune to common mode noise

Take up more space than single traces

Little return through ground: easier power routing

Impedance matching may be required

More precise reference for switching signals

Can generate significant noise if routed incorrectly

Effectively increase the signal to noise ratio (SNR)

Top Routing Tips

There are three main differential pair routing rules or tips you’ll need to follow in order to maintain signal integrity and keep your differential pairs from inducing noise into your PCB signals. The most important thing to remember is to keep trace lengths as close as possible while adhering to your CM’s layout  tolerances and ensuring that impedances match throughout the length of the traces.

Tip #1: Match Trace Lengths

The primary advantage of differential pairs is how the positive and negative signals arrive at their destinations at the same time. Of course, this only applies if the traces carrying the signals are of the same length. Different communication protocols have their own skew requirements, which come with length tolerances. However, even if you are within the margin of error, it’s best to aim for perfection. When differential pair traces are different lengths, tiny currents will actually be induced in the ground plane, rendering void one of their primary advantages. If you’re routing a high-speed signal, this can pose a real problem and may cause significant EMI. This can be avoided by making good use of meanders, added trace segments to restore length matching.

Tip #2: Reduce Separation Distance Between Traces

Keeping your differential pair traces close together will not only save you space but will also reduce the likelihood of future troubles. Even if you’ve matched the lengths of your traces perfectly, they will still induce a current directly on the plane beneath them. Theoretically, these currents should be equal and opposite, but their flow will still make up a current loop, the same kind of loop that causes EMI. This noise may be minimal in low-speed applications, but if you’re transmitting a high-speed signal, it can grow. Keeping a pair close together will decrease the loop area and thus minimize EMI.

Tip #3: Maintain Matched Impedances

When differential pairs are routed close together, they will induce currents in each other. This mutually-induced crosstalk will mostly be rejected if the pairs are the same distance apart for as much of the run as possible. When the distance between the traces changes, so does the differential impedance, causing a mismatch. Impedance differences can also occur because of a variety of other reasons, including layer changes through vias. It may not be necessary to exactly calculate the impedance of each trace; however, to reduce induced EMI from the differential pair, you should match their impedances as closely as possible.

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Differential pair routing can be a pain, but when done correctly, it can give you the same thrill of victory as when your team brings home a win.  By following the three design tips above you can minimize the negative impact of noise on your signals.

Here at Tempo Automation, we aim to help you optimize every step of that process by furnishing information for your DFM and enabling you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on differential pair routing and how you can minimize EMI, contact us.

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