Detailed Explanation of the Advantages of Differential Signals

The differential signals

Differential signals are increasingly used in high-speed circuit design, and the most critical signals in the circuit often have to be designed with differential structures. What is a Differential Signal? In layman's terms, the driver sends two equal and inverting signals, and the receiver compares the difference between the two voltages to determine whether the logic state is "0" or "1". The pair of traces that carry the differential signal is called the differential trace.

The advantages of differential signals

Compared with ordinary single-ended signal traces, the most obvious advantages of differential signals are reflected in the following three aspects:

1. Strong anti-interference ability

Because the coupling between the two differential traces is very good, when there is noise interference in the outside world, they are almost coupled to the two lines at the same time, and the receiver is only concerned about the difference between the two signals, so the external common-mode noise can be completely canceled.

2. Effectively Suppress EMI

In the same way, due to the opposite polarity of the two signals, the electromagnetic field they radiate to the outside can be canceled out each other, and the tighter the coupling, the less electromagnetic energy is discharged to the outside.

3. Accurate Temporal Localization

Because the switching change of the differential signal is located at the intersection of the two signals, unlike the ordinary single-ended signal that relies on the high and low threshold voltages, so it is less affected by the process and temperature, which can reduce the temporal error, and is also more suitable for the circuit of low amplitude signal.

The general requirements for differential routing

1. Keep the length equal

In order to ensure that the two differential signals remain of opposite polarity at all times and to reduce the common-mode component, the distance between the two differential lines must be constant over the entire length.

2. Equidistant

Equidistant is mainly to ensure that the differential impedance of the two is consistent and reduce reflection. The differential impedance changes with the degree of coupling, and the degree of coupling is related to the spacing of the traces.

3. The lines need to be as close as possible

The closer the differential lines are to each other, the smaller the loop area, the smaller the loop area of the induced current below the traces, and the better the EMI suppression. In any case, the main return path of the differential trace is still at ground level.

4. The impedance calculation for differential lines

The impedance calculation of the differential line is important for the differential signal and differential trace.

5. Try to be on the same layer as much as possible

Differential traces are required to be on the same board layer, because the difference in impedance and vias between different layers will reduce the effect of differential mode transmission and produce common mode noise.

6. Try to reduce the number of vias as much as possible

Too many vias can be a factor that causes discontinuity in the circuit.

7. Avoid the 90-degree line

Because 90-degree line can cause discontinuity in the resistance value, you can use a circular arc or a 45-degree polyline to connect the wiring.