Differential signal VS single-ended signal
Single-ended signal
Single-ended signal is a simple and common way to transmit electrical signals from a transmitter to a receiver. Electrical signals are transmitted through voltage (usually a changing voltage) that is based on a fixed potential, usually a 0V node called “ground”.
Differential signal
Differential transmission is a signal transmission technology. Different from the traditional practice of one signal line and one ground line, differential transmission transmits signals on both lines. The two signals have the same amplitude and opposite phase. The signal transmitted on these two lines is a differential signal.
Compared with ordinary single-ended signal traces, the most obvious advantages of differential signals are reflected in the following three aspects:
a. Strong anti-interference ability, because the coupling between the two differential traces is very good, when there is noise interference from the outside world, it is almost coupled to the two lines at the same time, and the receiving end only cares about the difference between the two signals, so the common mode noise from the outside world can be completely offset.
b. It can effectively suppress EMI. For the same reason, since the polarities of the two signals are opposite, the electromagnetic fields they radiate can cancel each other out. The tighter the coupling, the less electromagnetic energy is released to the outside world.
c. Accurate timing positioning. Since the switching change of the differential signal is located at the intersection of the two signals, unlike the ordinary single-ended signal, which relies on the high and low threshold voltages, it is less affected by the process and temperature, which can reduce the timing error and is more suitable for circuits with low amplitude signals.
Disadvantages: The main disadvantage of differential signal lines is that they increase the area of PCB. If the area of the circuit board is very tight, the single-ended signal can have only one signal line, and the ground line runs on the ground plane, while the differential signal must run two lines of equal length, equal width, close proximity, and on the same plane. This situation often occurs when the pin spacing of the chip is so small that only one line can be passed through.
Requirements for PCB wiring of differential signals
Usually (of course there are some exceptions) differential signals are also high-speed signals, so high-speed PCB design rules are usually applicable to the wiring of differential signals, especially when designing signal lines such as transmission lines. This means that we must be very careful in designing the routing of the signal lines to ensure that the characteristic impedance of the signal lines is continuous and constant everywhere along the signal lines.
On the circuit board, the differential traces must be two lines of equal length, equal width, close proximity, and on the same layer.
1. Equal length: Equal length means that the lengths of the two lines should be as equal as possible to ensure that the two differential signals always maintain opposite polarity. Reduce common mode components.
2. Equal width and equal distance: Equal width means that the trace widths of the two signals need to be consistent, and equal distance means that the spacing between the two lines should remain unchanged and parallel.
3. Minimum impedance change: When designing a PCB with differential signals, one of the most important things is to find out the target impedance of the application and then plan the differential pairs accordingly. In addition, keep the impedance change as small as possible. The impedance of the differential line depends on factors such as trace width, trace coupling, copper thickness, and PCB material and stackup. Consider each of these when you try to avoid anything that changes the impedance of the differential pair.