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Common misunderstandings in PCB differential signal design.
PCB News
Common misunderstandings in PCB differential signal design.

Common misunderstandings in PCB differential signal design.

2021-11-09
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Author:Kavie

        Misunderstanding 1: It is believed that the differential signal does not need a ground plane as a return path, or that the differential traces provide a return path for each other. The reason for this misunderstanding is that they are confused by superficial phenomena, or the mechanism of high-speed signal transmission is not deep enough. From the structure of the receiving end in Figure 1-8-15, it can be seen that the emitter currents of transistors Q3 and Q4 are equal and opposite, and their currents at the ground exactly cancel each other (I1=0), so the differential circuit is Similar bounces and other noise signals that may exist on the power and ground planes are insensitive. The partial return cancellation of the ground plane does not mean that the differential circuit does not use the reference plane as the signal return path. In fact, in the signal return analysis, the mechanism of differential wiring and ordinary single-ended wiring is the same, that is, high-frequency signals are always Reflow along the loop with the smallest inductance, the biggest difference is that in addition to the coupling to the ground, the differential line also has mutual coupling. Which kind of coupling is strong, which one becomes the main return path. Figure 1-8-16 is a schematic diagram of the geomagnetic field distribution of single-ended signals and differential signals.
 

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        In PCB circuit design, the coupling between differential traces is generally small, often only accounting for 10 to 20% of the coupling degree, and more is the coupling to the ground, so the main return path of the differential trace still exists on the ground plane . When the ground plane is discontinuous, the coupling between the differential traces will provide the main return path in the area without a reference plane, as shown in Figure 1-8-17. Although the influence of the discontinuity of the reference plane on the differential trace is not as serious as that of the ordinary single-ended trace, it will still reduce the quality of the differential signal and increase EMI, which should be avoided as much as possible. Some designers believe that the reference plane under the differential trace can be removed to suppress some common mode signals in differential transmission. However, this approach is not desirable in theory. How to control the impedance? Not providing a ground impedance loop for the common-mode signal will inevitably cause EMI radiation. This approach does more harm than good.
 
Misunderstanding 2: It is believed that keeping equal spacing is more important than matching line length. In actual PCB layout, it is often not possible to meet the requirements of differential design at the same time. Due to the existence of pin distribution, vias, and wiring space, the purpose of line length matching must be achieved through proper winding, but the result must be that some areas of the differential pair cannot be parallel. What should we do at this time? Which choice? Before drawing conclusions, let's take a look at the following simulation results.
 

        From the above simulation results, the waveforms of Scheme 1 and Scheme 2 are almost coincident, that is to say, the impact caused by the unequal spacing is minimal. In comparison, the line length mismatch has a much greater impact on the timing. (Scheme 3). From the theoretical analysis, although the inconsistent spacing will cause the differential impedance to change, because the coupling between the differential pair itself is not significant, the impedance change range is also very small, usually within 10%, which is only equivalent to one pass. The reflection caused by the hole will not have a significant impact on signal transmission. Once the line length does not match, in addition to the timing offset, common mode components are introduced into the differential signal, which reduces the quality of the signal and increases EMI.
 
       It can be said that the most important rule in the design of PCB differential traces is the matching line length, and other rules can be flexibly handled according to design requirements and actual applications.
 
Misunderstanding 3: Think that the differential wiring must be very close. Keeping the differential traces close is nothing more than to enhance their coupling, which can not only improve immunity to noise, but also make full use of the opposite polarity of the magnetic field to offset electromagnetic interference to the outside world. Although this approach is very beneficial in most cases, it is not absolute. If we can ensure that they are fully shielded from external interference, then we do not need to use strong coupling to achieve anti-interference. And the purpose of suppressing EMI. How can we ensure good isolation and shielding of differential traces? Increasing the spacing with other signal traces is one of the most basic ways. The electromagnetic field energy decreases with the square of the distance. Generally, when the line spacing exceeds 4 times the line width, the interference between them is extremely weak. Can be ignored. In addition, isolation by the ground plane can also play a good shielding effect. This structure is often used in high-frequency (above 10G) IC package PCB design. It is called a CPW structure, which can ensure strict differential impedance. Control (2Z0).
  
         Differential traces can also run in different signal layers, but this method is generally not recommended, because the differences in impedance and vias produced by different layers will destroy the effect of differential mode transmission and introduce common mode noise. In addition, if the adjacent two layers are not tightly coupled, it will reduce the ability of the differential trace to resist noise, but if you can maintain a proper distance from the surrounding traces, crosstalk is not a problem. At general frequencies (below GHz), EMI will not be a serious problem. Experiments have shown that the attenuation of radiated energy at a distance of 500 mils away from the differential trace has reached 60dB at a distance of 3 meters, which is enough to meet the FCC electromagnetic radiation standard, so The designer does not have to worry too much about the electromagnetic incompatibility caused by insufficient differential line coupling.

The above is an introduction to common misunderstandings in PCB differential signal design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.