When you start working on simple embedded interfaces, you see single signal lines carrying data from source to destination. Like for example, think of a single GPIO, where, from a micro controller pin this goes to a base of a transistor. This is a single ended signal, where the signal will take the reference of a nearest ground plane to that particular signal. the ground plane carries the return current for that signal. We can say that a single ended signal is unbalanced by design. When you start working on more complex interfaces and high speed interfaces, you need special signalling mechanism which helps you to give best performance even in the presence of noise. One such signalling mechanism is differential signalling.
When you say differential signalling, there are two signal lines involved unlike the single ended where there is one signal and hidden ground. The two signal lines carry complementary signals (one HIGH and one LOW and vice versa) and the resulting output of this particular signalling mechanism is the difference of voltage between the two lines. Differential signalling is used to transmit data between source and destination reliably. The main advantage of differential signalling is that both the signals will be affected equally by the noise present around those signals and as the output is difference between two signals, the noise cancels out.
It is not just like that a differential signalling reduces noise. There is a need for symmetry of routing between these signals on a PCB and on a physical media, these are to be transmitted over a twisted pair cable. The major PCB design guideline that need to be considered is that the differential signals are to be routed with same length, which we term "length matching". Also, closer the differential signals are routed higher the coupling between them. Based on this, terms like loose coupling, strong coupling are defined and as per the interface requirements coupling mechanism is to be followed.
For any signal routed on the PCB, it's impedance is very important. Impedance helps the signal to maintain it's integrity and hence the reliable communication of the data in the channel. For example, Ethernet signals have to maintain 100 ohm differential impedance and USB signals have to maintain 90 ohm differential impedance. for any interface, to maintain that impedance, the following parameters are to be taken into consideration.
- Trace width of each signal in the differential pair
- Distance between the two differential signals
- PCB substrate used
- Layer in which differential signal is routed
- PCB stack up
There are several tools available in the embedded to calculate the above parameters. Most of the layout tools like PADS, Allegro, Xpedition can help calculating these parameters. some other tools like AppCAD, Saturn also help to calculate the width and spacing of differential pair.
The most important consideration in differential signalling is that signals must be routed with proper reference plane. Here reference plane meant that there must be a reference path on the PCB for the differential signal. Any break in the return paths will cause impedance discontinuity to occur and this causes signal integrity issues on the board.
Generally, when we route differential signals in inner layers, vias are to placed on the board to route them to inner layer. During this transition period to inner layer, the reference plane will be lost which causes signal integrity issue. To avoid this issue, ground vias are to be placed near to this vias for achieving good reference plane.
Interfaces that use differential signalling:
USB, Ethernet, PCIe, CAN, RS485, SATA, DDR
Advantages of differential signalling:
- Good noise immunity
- Common mode noise rejection
- As the embedded community are going for lower voltages day by day, considering the power savings, differential signalling is the automatic choice for high speed signals
- With lower voltages, comes the lower emissions and hence lesser EMI issues
]]>
0 Comments