In order to transfer data faster, we can think of a way to increase the data transmission rate of a single channel in addition to transferring several bits of data at a time (increasing the number of parallel buses), but with the increase of single channel rate The problem of signal integrity will become more and more prominent, especially crosstalk and loss. In order to solve these problems, a new way of data transmission has emerged, as shown in Figure 1, he is ---- differential (differential line, differential interconnection).
figure 1
The main feature of differential (differential line, differential interconnect) is that the sampling voltage takes the difference between the two voltages, as shown in the following figure:
figure 2
We know that V1 and V2 have a single-ended voltage relative to their common ground. The signal receiving end is concerned with the voltage difference between the two lines. So what are the advantages of using this type of transmission?
Differential Line Advantage 1: Differential drive total dI/dt is significantly lower than single-ended signal lines, reducing potential electromagnetic interference (EMI). As shown below:
image 3
The red and blue colors in the above figure are single line voltages. Purple is the differential voltage. We can see that with differential transmission, the voltage peak-to-peak value of the signal is doubled, but the current on a single line remains the same. If the traditional single-wire transmission method is adopted, the dI/dt of a single line will also be doubled, which is more likely to cause EMI problems.
Differential Line Advantage 2: The value of the differential signal is largely independent of the exact value of the "ground" and is well resistant to power supply interference. Like the picture below, the differential signal is concerned with the voltage difference between the two lines, which has little to do with the height of the sea level.
Figure 4
We still verify by simulation, as shown in Figure 5 below:
Figure 5
As shown in the figure below, the left side is the single-ended and differential signal waveforms at ideal GND, and the right side is the single-ended signal and differential signal when the ground plane is fluctuating.
Figure 6
It can be clearly seen that the single-ended signal is greatly disturbed by the "ground". But these disturbances are offset by the difference. The principle of differential against external electromagnetic interference is the same, which is what we often say: differential form propagation, which is more robust against crosstalk and mutations in the return path.
Differential Line Advantage 3: Each signal in the differential pair has its own return path, which can mitigate the effects of signal cross-segmentation. What happens to a single line and a differential line? Let's verify it. In the simulation, we make the single-line and differential pair lengths 1inch, using ordinary Fr4 plates to see the effect of cross-segmentation on the signal.
Figure 7
Figure 8
Seen from the figure, a single line is divided across a great influence on the transmission line, the difference is not so sensitive to the line across the split, the main reason is, two wire differential pair may refer to each other, two lines may be used as a return path from each other.
The above three points are the most obvious advantages of the differential line that I understand. These advantages lay the foundation for the stable transmission of serial signals. Of course, differential signals also have some drawbacks. The biggest drawback is that differential interconnects require twice the number of signal lines compared to single-ended signal transmission. Furthermore, if the differential signal is asymmetrical, potential EMI is generated.
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