In the process of electronic system whose clock frequency increasingly rises, signal integrity issues come out gradually such as incorrect timing sequence and incorrect reflection of transmission lines, badly influencing the normal running of circuit system. Furthermore, traces on PCB become so compact that crosstalk noise will be generated and signal transmission obtains bad effect. For high-speed digital-analog mixed circuitry, according to the practical situation of signal running, PCB design should be reasonably implemented to solve signal integrity issues, to constantly increase the quality of signal transmission and to provide important information sources for the development of different industries and fields.
Signal Integrity of High-Speed Digital-Analog Mixed Circuitry
Signal integrity refers to the quality of signals on signal lines. To ensure the integrity of signal, certain requirements have to be met including space integrity insurance and corresponding requirements of circuitry. For example, the requirement of low level has to be met for maximized input. Plus, time integrity has to be obtained and minimum maintenance time of circuitry has to be left.
• Elements influencing signal integrity of circuitry
Generally, signal transmission depends on leads on PCB and transmission delay may be caused in the process of transmission. Once delay takes place on transmitted signals, timing sequence of circuitry system will be influenced, which further affects integrity of signals. Transmission delay derives from some elements including length of leads and dielectric constant of adjacent medium.
2). Reflection and crosstalk noise
During the running of circuitry system, if through-hole vias and bending problems take place on signal networks, reflection noise will be produced. And if electromagnetic coupling takes place between circuitry networks and power distribution systems, crosstalk noise will be generated so that signals will be interfered with signal transmission influenced.
• Problems to be solved for circuitry signal integrity
1). Power distribution
In the process of high-speed digital-analog mixed circuit board design, power distribution network has to be analyzed from head to toe. It needs to provide necessary power to circuitry with low noise, containing VCC and grounding. Furthermore, it has to offer corresponding signal circuit with signals that are generated and received on PCB as its main object.
2). Crosstalk issue and application of EMC
Crosstalk refers to redundant signal coupling between traces, with properties of capacitance and inductance. Capacitive crosstalk is capacitive coupling between signal lines and once different lines come close to each other, crosstalk issues will be generated. Inductive crosstalk is signal coupling between coils of redundant transformers and crosstalk issues are generated under the effect of current loop. With the help of EMC (electromagnetic compatibility), all kinds of electric devices and systems can exist in the environment of electromagnetic. From some perspectives, circuit system signals won't be influenced as a result of the effect of EMC and available performance and function won't be destroyed, leading to massive electromagnetic amount in surrounding environment, which influences the normal running of adjacent devices.
High-Speed Digital-Analog Mixed Circuit PCB Design
Based on the complete understanding of EMC, rules have to be followed. In the process of PCB design, area that has been captured by current loop is required to be as small as possible in order to ensure that circuitry signals are capable of passing smoothly and large-scale loop antenna can be avoided. Furthermore, multiple reference planes can't be applied in the process of design, with the formation of dipole antenna avoided in case signal transmission will be influenced.
• Layout and routing
In the process of component layout, analog circuitry and digital circuitry should be isolated from each other. Take digital signals as an example, routing is implemented inside digital circuitry. As a result, digital signals won't enter analog signal area in case they will interfere with analog signals and influence the normal transmission of signals. If traces have relatively high frequency, manual routing is required. Therefore, positions at which input and output connectors are placed should be noticed and routing of analog circuitry and digital circuitry has to be well dealt with to avoid mutual influence. Power and ground network with low impedance should be applied in order to avoid relatively large inductive reactance suffered by digital circuitry leads and capacitive coupling on analog lines. What's more, if digital circuitry has a relatively high frequency and analog lines have relatively strong sensitivity, certain distance has to be mutually maintained.
• Power and ground lines
In the process of design, ground lines have to be reasonably routed and processed in order to increase circuitry performance. When optimizing high-speed digital-analog mixed circuit design, the method has to be totally understood in terms of circuitry back to ground. If ground plane lines are needed to be divided, spacing routing has to be crossed. Single-point connection is required to connect divided ground and establish a connection bridge. Based on routing optimization through the connection bridge, direct circuitry backflow path has to be arranged under each signal line. Of course, optical isolation devices can be applied to divide signal spacing across fields. In the process of PCB design, digital circuit and analog circuit should be comprehensively applied with attention on circuit signal routing to effectively deal with the practical problems. High-speed digital-analog mixed PCB test results should be totally analyzed to optimize design scheme and EMC should be flexibly applied with PCB reasonably designed. Plus, in terms of mixed signal PCBs, independent digital and analog power have to be obtained and power surface has to be controlled with the help of divided power surface.
• Processing of hybrid devices
Generally speaking, hybrid devices all have crystal oscillation and the inside of devices is composed by digital circuit and analog circuit. In the process of design, pins of DGND and AGND should be connected to the same low impedance and leads should be as short as possible in order to ensure all the DGND is capable of passing. Although digital current inside converter will enter the analog ground plane, relatively large interference won't be produced to signals and the normal transmission of information can be ensured. Based on that, pins of digital and analog circuitry need to be connected to the analog power plane and be close to bypass capacitor.