Before describing the importance of differential pairing, first understand the factors that affect the transmission line impedance: the line width, line length, line thickness, side wall shape, solder mask coverage and transmission line media is a significant influencing factor. The dielectric constant and the thickness of the media will also affect the transmission line impedance accuracy, the specific calculation formula, please refer to the signal integrity analysis related books. Knowing these factors can help us to design good traces, especially when we need to clear the trace impedance, through the relevant software to adjust the alignment width and thickness, etc. can complete the fixed impedance of the alignment.

The difference pair is called a differential pair because the signal it conveys is equal to the difference between two complementary and mutually referenced signals, so that interference with the outside can be greatly reduced. The wiring of the differential pair should be properly close and parallel. The so-called proper proximity is because this spacing will affect the value of differential impedance, which is an important parameter for designing differential pairs. Need to be parallel to the line. In the circuit design, if all the signal lines are single-ended line, do the impedance design, under normal circumstances the system can work properly, once the signal at both ends of the ground potential is different, even the gap is relatively large, it will cause the system can not Normal work, and the use of differential pairs of wiring or alignment is an effective solution. Because the differential line has an equal length, equal impedance, after similar to the same environment, which in the stability of the signal has a congenital advantage.

In the schematic design, the differential signal is usually marked with "_N" and "_P" as a stop, and the differential line can effectively solve the problem that there is no good reference between the signal source and the load, which can suppress the interference and reduction of the electronic product Electromagnetic interference (EMI) generated by the signal line.

So, why is the differential line effective in eliminating noise? First, we look at the bus differential line when the conventional requirements: differential line in the design to ensure that the length of the two lines are equal, usually within 5%. There are 3 w distance between the two differential lines and the differential line around the package is a good design experience. In this way, on the one hand, the differential lines of the two signal lines generated by the magnetic field offset each other to reduce EMI; the other hand, the differential signal line if the introduction of external noise interference signal, because of its differential results, this can be very good Eliminate the noise, which with the classic three op amp amplifier has the same purpose.

In the mapping of the PCB differential pair, as far as possible in the same layer of wiring, differential pair of wire transfer layer will increase due to the hole, will introduce the impedance of the discontinuity. Secondly, if the layer will make the loop current does not have a good low-impedance loop, there will be RF circuit, if the difference between the longer, then the common mode RF energy will have an impact. There is also a reason for the difference between the different board layers have different signal transmission speed, signal integrity analysis can be seen in the relevant data on the microstrip line transmission faster than the strip line, which will cause a certain The time delay. In the connection, but also pay attention to the differential connection problem, if the load is not a direct load but a capacitive load, then may introduce EMI. In the circuit design also need to pay attention to the terminal impedance matching, to prevent the transmission of reflection and the introduction of EMI problems.

On the terminal impedance matching, differential signal transmission we usually use differential mode, there is a transmission mode is common mode. The terminal resistance pairing design is shown below
Draw a well-behaved method of differential pairing
The size of the matching resistor is usually measured and calibrated with an impedance analyzer.

For our commonly used differential mode terminal matching resistor, usually selected 50 ohms or 100 ohm, the actual need for further matching. Since the signals transmitted by the differential mode are independent of each other with reference to each other, there is no common mode RF energy. It can also be used in the design of the common mode of differential mode, in the latter part of the debugging process using a different model for comparison, which is a good way.