Electronic Design - How to design a mixed-signal PCB?

The operation of PCB circuit board analog circuits depends on continuously varying current and voltage. The operation of digital circuits relies on the detection of high or low levels at the receiver based on a predefined voltage level or threshold, which is equivalent to "true" or "false" judging the logical state. Between high and low levels of a digital circuit, there is a "gray" area in which the digital circuit sometimes exhibits analog effects, such as overshoot and ringback reflection when the digital signal jumps fast enough from a low level to a high level (state). The concept of mixed-signal PCB circuit boards is ambiguous for modern plate design because there are analog circuits and analog effects even in pure "digital" devices. Therefore, at the beginning of the design, in order to reliably achieve strict time series allocation, the simulation effect must be simulated. In fact, in addition to the reliability that communication products must work without failures for several years, simulation effects are particularly needed in mass-produced low-cost/high-performance consumer products.

Another difficulty in modern mixed-signal PCB circuit board design is that there are more and more devices with different digital logic, such as GTL, LVTTL, LVCMOS and LVDS logic. The logical thresholds and voltage swings of each logic circuit are different. However, these different logic thresholds and voltage swings must be designed together on a PCB circuit board. Here, you can master successful strategies and techniques by thoroughly analyzing the layout and wiring design of high density, high performance, mixed signal PCB circuit boards.

Hybrid Signal Circuit Wiring Base

When digital and analog circuits share the same components on the same board, the layout and wiring of the circuit must be methodical.

In the mixed-signal PCB circuit board design, there are special requirements for power wiring and the separation of analog noise and digital circuit noise to avoid noise coupling, which increases the complexity of layout and wiring. The special requirements for power transmission lines and the requirement to isolate noise coupling between analog and digital circuits further complicate the layout and wiring of mixed-signal PCB circuit boards.

If the power supply of the analog amplifier in the A/D converter is connected with the digital power supply of the A/D converter, the interaction between the analog part and the digital part of the circuit is likely to occur. Perhaps the layout scheme must mix the wiring of digital and analog circuits due to the location of the input/output connector.

Before layouts and wiring, engineers need to understand the basic weaknesses of the layout and wiring scheme. Even with false judgments, most engineers tend to use layout and wiring information to identify potential electrical impacts.

Layout and wiring of modern mixed-signal PCB circuit boards

The following describes the layout and wiring technology of mixed-signal PCB circuit boards through the design of OC48 interface cards. 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. OC48 interface card contains several typical layout and wiring problems for mixed-signal PCB circuit boards. The layout and wiring process will indicate the order and steps to solve the layout scheme for mixed-signal PCB circuit boards.

The OC48 card contains a transceiver that converts optical and analog signals in both directions. An analog signal input or output digital signal processor that is converted by a DSP to a digital logic level to connect to a microprocessor, a programmable gate array, and a system interface circuit of a DSP and a microprocessor on an OC48 card. Separate phase-locked loops, power filters, and local reference voltage sources are also integrated.

Among them, the microprocessor is a multi-power device, the main power supply is 2V, 3.3V I/O signal power is shared by other digital devices on the board. A stand-alone digital clock source provides clocks for OC48I/O, microprocessors, and system I/O.

After checking the layout and wiring requirements of different functional circuit blocks, it is preliminarily recommended that 12-layer boards be used, as shown in Figure 3. Configuration of microstrip and stripline layers can safely reduce coupling of adjacent layers and improve impedance control. Grounding between Layer 1 and Layer 2 isolates the wiring of sensitive analog reference sources, CPU cores, and PLL filter power supplies from the microprocessor and DSP devices in Layer 1. Power and connection layers always appear in pairs, just as they do on the OC48 card for sharing the 3.3V power layer. This will reduce the impedance between the power supply and the ground, thereby reducing the noise on the power signal.

To avoid digital clock lines and high frequency analog signal lines near the power layer, otherwise, the noise of the power signal will be coupled to the sensitive analog signal.

To meet the requirements of digital signal wiring, careful consideration should be given to the use of power supplies and split openings in analog grounding layers, especially at the input and output ends of hybrid signal devices. Traveling through an opening in the adjacent signal layer can result in discontinuous impedance and poor transmission line loops. These all cause signal quality, timing and EMI problems.

Sometimes adding several grounding layers or using several peripheral layers for the local power or grounding layer under one device can remove the openings and avoid these problems. Multiple grounding layers are used on the OC48 interface card. Keeping the opening and wiring layers stacked symmetrically avoids jamming and simplifies the fabrication process. Because 1 ounce of copper clad sheet can withstand high current, 1 ounce of copper clad sheet is used for 3.3V power supply layer and corresponding grounding layer, and 0.5 ounce of copper clad sheet can be used for other layers, which can reduce the voltage fluctuation caused by transient high current or during peak period.

If you are designing a complex system from the ground floor up, you should use cards of 0.093 inch and 0.100 inch thickness to support the wiring layer and the ground isolation layer. The card thickness must also be adjusted according to the wiring characteristic dimensions of the through-hole pads and holes so that the ratio of the bore diameter to the finished card thickness does not exceed the width-height ratio of the metallized holes provided by the manufacturer.

If you want to design a low-cost, high-yield commercial product with a minimum number of layers of wiring, carefully consider the wiring details of all the special power sources on the mixed-signal PCB circuit board before laying out or wiring. Have the target manufacturer review the initial layered plan before starting the layout and wiring. Basically layers are based on the thickness of the finished product, the number of layers, the weight of copper, the impedance (with tolerance) and the minimum size of through-hole pads and holes, and the manufacturer should make a written recommendation for layering.

It is recommended that all configurations of controlled impedance striplines and microstrip lines be included. Consider combining your impedance predictions with those of the PCB circuit board manufacturer. These impedance predictions can then be used to validate the signal wiring characteristics in the simulation tool used to develop CAD wiring rules.