PCB News - Layout requirements for differential signals

In the design of high-speed PCB, differential signal is more and more widely used, mainly because compared with ordinary single ended signal routing, differential signal has the advantages of strong anti-interference ability, effective EMI suppression and timing positioning. As a (quasi) PCB Design Engineer, we certainly need to fully understand the differential signal!

About differential signals, Strictly speaking, all voltage signals are "differential" because one voltage is always relative to another voltage. But in most cases, we will use "ground" as the voltage reference point to measure another voltage value. This signal is called single ended signal. Because it is compared with "ground", the performance of single ended signal on PCB usually has only one track.

What is a differential signal? Different from the traditional practice of one signal line and one ground line, differential transmission transmits signals on both lines. The amplitudes of the two signals are equal, the phase difference is 180 degrees, and the polarity is opposite. The signals transmitted on these two lines are differential signals.

Advantages and disadvantages of differential signal

advantage

Strong anti-interference ability. Generally, the interference noise will be loaded on two signal lines equally and simultaneously, and the difference is 0, that is, the noise will not affect the logical meaning of the signal.

. it can effectively suppress electromagnetic interference (EMI). Because the two wires are close together and the signal amplitude is equal, the amplitude of the coupled electromagnetic field between the two wires and the ground wire is also equal. At the same time, their signal polarity is opposite, and their electromagnetic fields will cancel each other. Therefore, the electromagnetic interference to the outside world is also small.

The timing positioning is accurate. The receiving end of the differential signal is the point where the difference between the signal amplitudes on the two lines jumps positive and negative, which is used as the point for judging the logic 0 / 1 jump. The common single ended signal takes the threshold voltage as the jump point of signal logic 0 / 1, which is greatly affected by the ratio of threshold voltage to signal amplitude voltage, so it is not suitable for low amplitude signals.

shortcoming

If the area of the circuit board is very tight, the single ended signal can only have one signal line, and the ground wire goes through the ground plane, while the differential signal must go through two lines with the same length, width, close proximity and at the same level. This often happens when the pin spacing of the chip is so small that it can only pass through one wiring.

Differential signal wiring requirements

On the PCBcircuit board, the differential routing must be two wires of equal length, equal width, close proximity and on the same level.

Equal length: equal length means that the length of two lines should be as long as possible to ensure that the two differential signals maintain opposite polarity at all times. Reduce common mode components.

Equal width and equal distance: equal width means that the routing width of two signals needs to be consistent, and equal distance means that the spacing between two lines should remain unchanged and parallel.

The closer the differential lines are to each other, the smaller the loop area, and the smaller the loop area of induced current under the routing. It is also good for EMI control.

Differential routing is required to be on the same board layer, because the impedance, vias and other differences between different layers will reduce the effect of differential mode transmission and introduce common mode noise.

Reminder: in the PCB wiring rules, there is a principle of "key signal line first", that is, key signals such as power supply, simulation signal, high-speed signal, clock signal, differential signal and synchronization signal are wired first.