Understand PCB layout differential signal
Time:2022-07-02
Views:1968
With the continuous development of semiconductor technology and deep pressure micron process, the switching speed of IC has increased from tens of MHz to hundreds of MHz, even to several GHz. In high-speed PCB design, engineers often encounter signal integrity problems such as false triggering, damping oscillation, overshoot, undershoot, crosstalk, etc. This paper will discuss their formation reasons, calculation methods and how to solve these problems by using Ibis simulation method in allegro. 1 definition of signal integrity signal integrity (SI) refers to the signal quality on the signal line. Poor signal integrity is not caused by a single factor, but by a variety of factors in board level design. The causes of signal integrity damage include reflection, ringing, ground bounce, crosstalk, etc. With the continuous improvement of signal frequency, signal integrity has become the focus of high-speed PCB engineers. 2 reflection 2.1 the formation and calculation of reflection the discontinuity of impedance on the transmission line will lead to signal reflection. When the impedance of the source end and the load end does not match, the load will reflect part of the voltage back to the source end. The differential transmission line solves many problems.
What is a differential signal? Generally speaking, the driver sends two equivalent and inverse signals, and the receiver judges whether the logic state is "0" or "1" by comparing the difference between the two voltages. The pair of lines carrying the differential signal is called the differential line. How to calculate the differential line impedance? The impedance of various differential signals is different. For example, the USB d+ d-, the differential line impedance is 90ohm, and the 1394 differential line impedance is 110ohm. It is best to see the specifications or relevant data first. Now there are many impedance calculation tools, such as polar‘s si9000. The factors that affect the differential impedance include linewidth, differential line spacing, dielectric permittivity, and dielectric thickness (the dielectric thickness between the differential line and the reference plane). Generally, the differential impedance is controlled by adjusting the differential line spacing and linewidth. When making boards, it is also necessary to explain to the manufacturer which lines should control the impedance. A differential signal uses a numerical value to represent the difference between two physical quantities. Strictly speaking, all voltage signals are differential, because one voltage can only be relative to another voltage. In some systems, the system ‘ground‘ is used as a voltage reference point. When ‘ground‘ is used as voltage measurement reference, this signal planning is called single ended. We use this term because the signal is represented by the voltage across a single conductor.
The first advantage of differential signal is that you can easily identify small signals because you are controlling the ‘reference‘ voltage. In a system with ground as the benchmark and single ended signal scheme, the precise value of the measured signal depends on the consistency of ‘ground‘ in the system. The farther the distance between the signal source and the signal receiver is, the more likely there is a difference between their local voltage values. The signal value recovered from the differential signal is largely independent of the precise value of ‘ground‘, but within a certain range.
The second advantage of differential signal is that it is highly immune to external electromagnetic interference (EMI). An interference source affects each end of the differential signal pair almost equally. Since the padlogic voltage difference in pads determines the signal value, any similar interference on the two conductors will be ignored. In addition to being insensitive to interference, differential signals generate less EMI than single ended signals.
The third advantage of the differential signal is that the timing positioning is accurate. Since the switching change of the differential signal is located at the intersection of the two signals, unlike the common single ended signal, which depends on the high and low threshold voltages, it is less affected by the process and temperature, which can reduce the timing error. At the same time, it is more suitable for circuits with low amplitude signals. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signal technology.
The difference may not consider crosstalk, because their crosstalk results will be offset at the final acceptance In addition, the difference should be balanced. Parallelism is only part of the balance
I think the coupling of differential pairs is still necessary. For single line matching, although the theory is very mature, the actual PCB circuit still has an error of about 5% (I have not made a copy of the material myself). On the other hand, the differential line can be regarded as a self loop system, or the signals on its two signal lines are correlated. Loose coupling may cause interference from different sources. For some interface circuits, the equal length of Allegro training differential pairs is an important factor in controlling line delay. Therefore, I think the difference line should be tightly coupled.
For most high-speed PCB boards at present, it is beneficial to maintain good coupling
But I hope you don‘t mistakenly think that coupling is a necessary condition for differential pairs, which sometimes limits the design idea.
When doing high-speed design or analysis, we should not only know how most people do it, but also understand why others do it, and then understand and improve it on the basis of others‘ experience, so as to constantly exercise our creative thinking ability
Matching is necessary, but the reason for matching is not reflection, but to reduce the interference degree of series winding. If the reduction is related to the use of matching method, if the series resistance is used, it will have no effect, but if the termination matching method of grounding or power supply is used, the series winding will be reduced because the line impedance of the two lines is reduced
For PCB layout engineers, the most important thing is to ensure that these advantages of differential routing can be fully played in the actual routing. Perhaps anyone who has been in contact with layout will understand the general requirements of differential wiring. PCB design is "equal length and equal distance". Equal length is to ensure that two differential signals always maintain opposite polarity and reduce common mode components; Equidistance is mainly to ensure that the differential impedance of the two is consistent and reduce reflection. "As close as possible" is sometimes one of the requirements of differential routing. Differential routing can also be carried out in different signal layers, but it is generally not recommended because the differences in impedance and via generated by different layers will destroy the effect of differential mode transmission and introduce common mode noise. In addition, if the coupling between the two adjacent layers is not close enough, the ability of differential routing to resist noise will be reduced, but if the appropriate spacing with the surrounding routing can be maintained, crosstalk is not a problem. At the general frequency (below GHz), EMI will not be a very serious problem. The experiment shows that the radiation energy attenuation of the differential line at a distance of 500mils beyond 3M has reached 60dB, which is enough to meet the electromagnetic radiation standard of FCC. Therefore, the designer need not worry too much about the electromagnetic incompatibility caused by insufficient differential line coupling. But all these rules are not used to copy mechanically. Many engineers seem to have no understanding of the essence of high-speed differential signal transmission. The following focuses on several common misunderstandings in PCB differential signal design.
I think the differential routing must be very close. The purpose of making the differential routing close is to enhance their coupling, which can not only improve their immunity to noise, but also make full use of the opposite polarity of the magnetic field to offset the electromagnetic interference to the outside world. Although this approach is very beneficial in most cases, it is not absolute. If we can ensure that they are fully shielded from external interference, we do not need to achieve the purpose of anti-interference and EMI suppression through strong coupling. How can we ensure that the differential wiring has good isolation and shielding? Increasing the distance from other signal lines is one of the most basic ways. The electromagnetic field energy decreases with the distance in a square relationship. Generally, when the line spacing exceeds 4 times the line width, the interference between them is extremely weak and can be basically ignored. In addition, the isolation through the ground plane can also play a good role in shielding. This structure is often used in the design of high-frequency (above 10g) IC package PCB. It is called CPW structure, which can ensure strict differential impedance control (2z0)
It is considered that the differential signal does not need the ground plane as the return path, or that the differential routing provides the return path for each other. The reason for this misunderstanding is that they are confused by the surface phenomenon or do not have a deep understanding of the mechanism of high-speed signal transmission. The differential circuit is insensitive to similar ground bombs and other noise signals that may exist on the power supply and ground plane. The partial backflow cancellation of the ground plane does not mean that the differential circuit does not take the reference plane as the signal return path. In fact, in the signal backflow analysis, the mechanism of the differential routing is consistent with that of the ordinary single ended routing, that is, the high-frequency signal always returns along the circuit with the smallest inductance. The biggest difference is that the differential lines are coupled with each other in addition to the ground. Which coupling is strong, That one becomes the main return path In PCB circuit design, the coupling between differential routing is generally small, often only accounting for 10~20% of the coupling degree, and more is the coupling to the ground. Therefore, the main return path of differential routing still exists in the ground plane. In case of discontinuity in the local plane, the coupling between the differential routing will provide the main return path in the area without reference plane. Although the discontinuity of the reference plane has less serious impact on the differential routing than on the ordinary single ended routing, it will still reduce the quality of the differential signal and increase EMI, which should be avoided as much as possible. Some designers also believe that the reference plane below the differential routing can be removed to suppress some common mode signals in differential transmission, but this is not desirable in theory. How to control the impedance? Failure to provide a ground impedance loop for common mode signals is bound to cause EMI radiation, which does more harm than good.
It is considered that maintaining equal spacing is more important than matching line length. In the actual PCB wiring, it can not meet the requirements of differential design at the same time. Due to the existence of pin distribution, vias, and wiring space, the purpose of wire length matching must be achieved through appropriate winding, but the result must be that some areas of the differential pair cannot be parallel The most important rule in the design of PCB differential routing is to match the line length. Other rules can be handled flexibly according to the design requirements and practical applications.
What is a differential signal? Generally speaking, the driver sends two equivalent and inverse signals, and the receiver judges whether the logic state is "0" or "1" by comparing the difference between the two voltages. The pair of lines carrying the differential signal is called the differential line. How to calculate the differential line impedance? The impedance of various differential signals is different. For example, the USB d+ d-, the differential line impedance is 90ohm, and the 1394 differential line impedance is 110ohm. It is best to see the specifications or relevant data first. Now there are many impedance calculation tools, such as polar‘s si9000. The factors that affect the differential impedance include linewidth, differential line spacing, dielectric permittivity, and dielectric thickness (the dielectric thickness between the differential line and the reference plane). Generally, the differential impedance is controlled by adjusting the differential line spacing and linewidth. When making boards, it is also necessary to explain to the manufacturer which lines should control the impedance. A differential signal uses a numerical value to represent the difference between two physical quantities. Strictly speaking, all voltage signals are differential, because one voltage can only be relative to another voltage. In some systems, the system ‘ground‘ is used as a voltage reference point. When ‘ground‘ is used as voltage measurement reference, this signal planning is called single ended. We use this term because the signal is represented by the voltage across a single conductor.
The first advantage of differential signal is that you can easily identify small signals because you are controlling the ‘reference‘ voltage. In a system with ground as the benchmark and single ended signal scheme, the precise value of the measured signal depends on the consistency of ‘ground‘ in the system. The farther the distance between the signal source and the signal receiver is, the more likely there is a difference between their local voltage values. The signal value recovered from the differential signal is largely independent of the precise value of ‘ground‘, but within a certain range.
The second advantage of differential signal is that it is highly immune to external electromagnetic interference (EMI). An interference source affects each end of the differential signal pair almost equally. Since the padlogic voltage difference in pads determines the signal value, any similar interference on the two conductors will be ignored. In addition to being insensitive to interference, differential signals generate less EMI than single ended signals.
The third advantage of the differential signal is that the timing positioning is accurate. Since the switching change of the differential signal is located at the intersection of the two signals, unlike the common single ended signal, which depends on the high and low threshold voltages, it is less affected by the process and temperature, which can reduce the timing error. At the same time, it is more suitable for circuits with low amplitude signals. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signal technology.
The difference may not consider crosstalk, because their crosstalk results will be offset at the final acceptance In addition, the difference should be balanced. Parallelism is only part of the balance
I think the coupling of differential pairs is still necessary. For single line matching, although the theory is very mature, the actual PCB circuit still has an error of about 5% (I have not made a copy of the material myself). On the other hand, the differential line can be regarded as a self loop system, or the signals on its two signal lines are correlated. Loose coupling may cause interference from different sources. For some interface circuits, the equal length of Allegro training differential pairs is an important factor in controlling line delay. Therefore, I think the difference line should be tightly coupled.
For most high-speed PCB boards at present, it is beneficial to maintain good coupling
But I hope you don‘t mistakenly think that coupling is a necessary condition for differential pairs, which sometimes limits the design idea.
When doing high-speed design or analysis, we should not only know how most people do it, but also understand why others do it, and then understand and improve it on the basis of others‘ experience, so as to constantly exercise our creative thinking ability
Matching is necessary, but the reason for matching is not reflection, but to reduce the interference degree of series winding. If the reduction is related to the use of matching method, if the series resistance is used, it will have no effect, but if the termination matching method of grounding or power supply is used, the series winding will be reduced because the line impedance of the two lines is reduced
For PCB layout engineers, the most important thing is to ensure that these advantages of differential routing can be fully played in the actual routing. Perhaps anyone who has been in contact with layout will understand the general requirements of differential wiring. PCB design is "equal length and equal distance". Equal length is to ensure that two differential signals always maintain opposite polarity and reduce common mode components; Equidistance is mainly to ensure that the differential impedance of the two is consistent and reduce reflection. "As close as possible" is sometimes one of the requirements of differential routing. Differential routing can also be carried out in different signal layers, but it is generally not recommended because the differences in impedance and via generated by different layers will destroy the effect of differential mode transmission and introduce common mode noise. In addition, if the coupling between the two adjacent layers is not close enough, the ability of differential routing to resist noise will be reduced, but if the appropriate spacing with the surrounding routing can be maintained, crosstalk is not a problem. At the general frequency (below GHz), EMI will not be a very serious problem. The experiment shows that the radiation energy attenuation of the differential line at a distance of 500mils beyond 3M has reached 60dB, which is enough to meet the electromagnetic radiation standard of FCC. Therefore, the designer need not worry too much about the electromagnetic incompatibility caused by insufficient differential line coupling. But all these rules are not used to copy mechanically. Many engineers seem to have no understanding of the essence of high-speed differential signal transmission. The following focuses on several common misunderstandings in PCB differential signal design.
I think the differential routing must be very close. The purpose of making the differential routing close is to enhance their coupling, which can not only improve their immunity to noise, but also make full use of the opposite polarity of the magnetic field to offset the electromagnetic interference to the outside world. Although this approach is very beneficial in most cases, it is not absolute. If we can ensure that they are fully shielded from external interference, we do not need to achieve the purpose of anti-interference and EMI suppression through strong coupling. How can we ensure that the differential wiring has good isolation and shielding? Increasing the distance from other signal lines is one of the most basic ways. The electromagnetic field energy decreases with the distance in a square relationship. Generally, when the line spacing exceeds 4 times the line width, the interference between them is extremely weak and can be basically ignored. In addition, the isolation through the ground plane can also play a good role in shielding. This structure is often used in the design of high-frequency (above 10g) IC package PCB. It is called CPW structure, which can ensure strict differential impedance control (2z0)
It is considered that the differential signal does not need the ground plane as the return path, or that the differential routing provides the return path for each other. The reason for this misunderstanding is that they are confused by the surface phenomenon or do not have a deep understanding of the mechanism of high-speed signal transmission. The differential circuit is insensitive to similar ground bombs and other noise signals that may exist on the power supply and ground plane. The partial backflow cancellation of the ground plane does not mean that the differential circuit does not take the reference plane as the signal return path. In fact, in the signal backflow analysis, the mechanism of the differential routing is consistent with that of the ordinary single ended routing, that is, the high-frequency signal always returns along the circuit with the smallest inductance. The biggest difference is that the differential lines are coupled with each other in addition to the ground. Which coupling is strong, That one becomes the main return path In PCB circuit design, the coupling between differential routing is generally small, often only accounting for 10~20% of the coupling degree, and more is the coupling to the ground. Therefore, the main return path of differential routing still exists in the ground plane. In case of discontinuity in the local plane, the coupling between the differential routing will provide the main return path in the area without reference plane. Although the discontinuity of the reference plane has less serious impact on the differential routing than on the ordinary single ended routing, it will still reduce the quality of the differential signal and increase EMI, which should be avoided as much as possible. Some designers also believe that the reference plane below the differential routing can be removed to suppress some common mode signals in differential transmission, but this is not desirable in theory. How to control the impedance? Failure to provide a ground impedance loop for common mode signals is bound to cause EMI radiation, which does more harm than good.
It is considered that maintaining equal spacing is more important than matching line length. In the actual PCB wiring, it can not meet the requirements of differential design at the same time. Due to the existence of pin distribution, vias, and wiring space, the purpose of wire length matching must be achieved through appropriate winding, but the result must be that some areas of the differential pair cannot be parallel The most important rule in the design of PCB differential routing is to match the line length. Other rules can be handled flexibly according to the design requirements and practical applications.
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