Electronic Design - PCB mixed-signal circuit board design guidelines

For modern board design, the concept of mixed-signal PCB is relatively vague, because even in purely "digital" devices, there are still analog circuits and analog effects. Therefore, in the early stages of design, in order to reliably achieve strict timing allocation, simulation effects must be simulated.

Another difficulty in modern mixed-signal PCB design is that there are more and more different digital logic devices, such as GTL, LVTTL, LVCMOS and LVDS logic. The logic threshold and voltage swing of each logic circuit are different, but these different logic thresholds The circuit with the voltage swing must be designed together on a PCB. Here, through thorough analysis of the layout and wiring design of high-density, high-performance, mixed-signal PCBs, successful strategies and technologies can be mastered.

Basics of mixed-signal circuit wiring

When digital and analog circuits share the same components on the same board, the layout and wiring of the circuit must be methodical. The matrix shown in Figure 1 is helpful to the design and planning of mixed-signal PCBs. Only by revealing the characteristics of digital and analog circuits can the required PCB design goals be achieved in actual layout and wiring.

In mixed-signal PCB design, there are special requirements for power supply wiring, and analog noise and digital circuit noise are required to be isolated from each other to avoid noise coupling. As a result, the complexity of layout and wiring increases. The special requirements for power transmission lines and the requirement to isolate noise coupling between analog and digital circuits have further increased the complexity of the layout and wiring of mixed-signal PCBs.

Layout and routing of modern mixed-signal PCB

The following will illustrate the technology of mixed-signal PCB layout and routing through the design of the OC48 interface card. OC48 stands for Optical Carrier Standard 48, which is basically oriented to 2.5Gb serial optical communication. It is one of the high-capacity optical communication standards in modern communication equipment. The OC48 interface card contains several typical mixed-signal PCB layout and wiring problems. The layout and wiring process will specify the sequence and steps to solve the mixed-signal PCB layout scheme.

The OC48 card contains an optical transceiver that realizes the bidirectional conversion of optical signals and analog electrical signals. Analog signal input or output digital signal processor, DSP converts these analog signals into digital logic levels, which can be connected with microprocessor, programmable gate array, DSP and microprocessor system interface circuit on OC48 card . The independent phase-locked loop, power filter and local reference voltage source are also integrated.

Among them, the microprocessor is a multi-power device, the main power is 2V, and the 3.3V I/O signal power is shared by other digital devices on the board. Independent digital clock source provides clock for OC48 I/O, microprocessor and system I/O.

OC48 card layout

The high-speed analog signal between the optical transceiver and the DSP is very sensitive to external noise. Similarly, all special power supply and reference voltage circuits also cause a lot of coupling between the analog and digital power transmission circuits of the card. Sometimes, limited by the shape of the chassis, a high-density board has to be designed. Due to the high position of the external optical cable access card and the relatively high component size of the optical transceiver, the position of the transceiver in the card is largely fixed. The system I/O connector position and signal distribution are also fixed. This is the basic work that must be completed before layout.

Like most successful high-density analog layout and routing schemes, the layout must meet the routing requirements, and the layout and routing requirements must be balanced. For the analog part of a mixed-signal PCB and a local CPU core with a 2V operating voltage, it is not recommended to use the "layout before wiring" method. For the OC48 card, the DSP analog circuit part including the analog reference voltage and the analog power supply bypass capacitor should be interactively wired first. After completing the wiring, the entire DSP with analog components and wiring should be placed close enough to the optical transceiver to fully ensure the shortest wiring length, bending and vias from the high-speed analog differential signal to the DSP. The symmetry of the differential layout and routing will reduce the impact of common mode noise. However, it is difficult to predict the best plan for the layout before routing.

Consult the chip distributor for design guidelines for PCB layout. Before designing in accordance with the guidelines, it is necessary to fully communicate with the distributor's application engineer. Many chip distributors have strict time constraints on providing high-quality layout recommendations. Sometimes, the solutions they provide are feasible for "first-level customers" who use the device. In the field of signal integrity (SI) design, the signal integrity design of new devices is particularly important. According to the distributor’s basic guidelines and combined with the specific requirements of each power and ground pin in the package, you can begin to lay out and route the OC48 card with integrated DSP and microprocessor.

After the location and wiring of the high-frequency analog part are determined, the remaining digital circuits can be placed in accordance with the grouping method shown in the block diagram. Pay attention to carefully design the following circuits: the location of the PLL power filter circuit in the CPU with high sensitivity to analog signals; the local CPU core voltage regulator; the reference voltage circuit for the "digital" microprocessor.

The electrical and manufacturing guidelines for digital wiring can be properly applied to the design at this time. The aforementioned design of the signal integrity of the high-speed digital bus and clock signal reveals some special wiring topology requirements for the processor bus, balanced Ts, and time delay matching of certain clock signal wiring. But you may not know, some people have also put forward an updated suggestion, that is, to increase a number of termination resistors.

In the process of solving the problem, it is natural to make some adjustments in the layout stage. However, before starting wiring, a very important step is to verify the timing of the digital part in accordance with the layout plan. At this moment, a complete DFM/DFT layout review of the board will help ensure that the card meets the needs of customers.

Digital wiring of OC48 card

For the power lines of digital devices and the digital part of the mixed-signal DSP, the digital wiring should start with SMD escape patterns. Use the shortest and widest printed line allowed by the assembly process. For high-frequency devices, the printed lines of the power supply are equivalent to small inductances, which will worsen power supply noise and cause undesired coupling between analog and digital circuits. The longer the power trace, the greater the inductance.

Using digital bypass capacitors can get the best layout and routing scheme. In short, fine-tune the position of the bypass capacitor as needed to make it easy to install and distribute it around the digital components and mixed-signal components. Use the same "shortest and widest trace" method to route the bypass capacitor.

Summarize

After the completion of the OC48 card layout board, signal integrity verification and timing simulation are required. The simulation proves that the wiring guidance meets the expected requirements and improves the timing indicators of the second-layer bus. Finally, the design rule check, final manufacturing review, photomask and review are carried out and issued to the PCB circuit board factory manufacturer, then the board layout task is officially ended.