PCB Tech - Some practical PCB wiring guidelines

High-integrity circuit boards need to be very careful in PCB layout. A little carelessness may bring about electromagnetic compatibility and interference problems. Now I will introduce some practical wiring guidelines.

Printed circuit board (PCB) wiring plays a key role in high-speed circuits, but it is often one of the last few steps in the circuit design process. There are many problems with high-speed PCB wiring, and a lot of literature has been written on this topic. This article mainly discusses the wiring of high-speed circuits from a practical perspective. The main purpose is to help new users pay attention to many different issues that need to be considered when designing high-speed circuit PCB layouts. Another purpose is to provide a review material for customers who have not touched PCB wiring for a while. Due to the limited layout, this article cannot discuss all the issues in detail, but we will discuss the key parts that have the greatest effect on improving circuit performance, shortening design time, and saving modification time.

Although this is mainly for circuits related to high-speed operational amplifiers, the problems and methods discussed here are generally applicable to wiring used in most other high-speed analog circuits. When the operational amplifier works in a very high radio frequency (RF) frequency band, the performance of the circuit largely depends on the PCB layout. The high-performance circuit design that looks good on the "drawing" will only get ordinary performance if it is affected by carelessness and carelessness in wiring. Pre-consideration and attention to important details during the entire wiring process will help ensure the expected circuit performance.

Schematic diagram

Although a good schematic cannot guarantee a good wiring, a good wiring starts with a good schematic. Think carefully when drawing the schematic, and you must consider the signal flow of the entire circuit. If there is a normal and stable signal flow from left to right in the schematic, then there should be an equally good signal flow on the PCB. Give as much useful information as possible on the schematic. Because sometimes the circuit design engineer is not there, customers will ask us to help solve the circuit problem, the designers, technicians and engineers engaged in this work will be very grateful, including us.

In addition to common reference identifiers, power consumption, and error tolerance, what other information should be given in the schematic? Here are some suggestions for turning ordinary schematics into first-class schematics. Add waveforms, mechanical information about the casing, length of printed lines, blank areas; indicate which components need to be placed on the PCB; give adjustment information, component value ranges, heat dissipation information, control impedance printed lines, comments, and brief circuits Action description... (and others).

Don't believe anyone

If you are not designing the wiring yourself, be sure to allow plenty of time to carefully check the wiring person’s design. At this point, a small prevention is worth a hundred times the remedy. Don't expect the wiring person to understand what you think. Your opinion and guidance are the most important in the early stages of the wiring design process. The more information you can provide, and the more you intervene in the entire wiring process, the better the resulting PCB will be. Set a tentative completion point for the wiring design engineer-quick check according to the wiring progress report you want. This "closed loop" method can prevent the wiring from going astray, thereby minimizing the possibility of rework.

The instructions that need to be given to the wiring engineer include: a short description of the circuit function, a schematic diagram of the PCB indicating the input and output locations, PCB stacking information (for example, how thick the board is, how many layers there are, detailed information of each signal layer and ground plane-function Power consumption, ground wire, analog signal, digital signal and RF signal); which signals are required for each layer; the placement of important components is required; the exact location of bypass components; which printed lines are important; which lines need to control impedance printed lines ; Which lines need to match the length; the size of the components; which printed lines need to be far away from each other (or close to); which lines need to be far away from each other (or close to); which components need to be far away from each other (or close to); which components need to be placed On the top of the PCB, which ones are placed on the bottom. Never complain that there is too much information for others-too little? Is it too much? Do not.

A learning experience: About 10 years ago, I designed a multilayer surface mount circuit board-there are components on both sides of the board. Use a lot of screws to fix the board in a gold-plated aluminum shell (because there are very strict anti-vibration indicators). The pins that provide bias feedthrough pass through the board. This pin is connected to the PCB by soldering wires. This is a very complicated device. Some components on the board are used for test setting (SAT). But I have clearly defined the location of these components. Can you guess where these components are installed? By the way, under the board. When product engineers and technicians had to disassemble the entire device and reassemble them after completing the settings, they seemed very unhappy. I haven't made this mistake again since then.

Location

Just like in a PCB, location is everything. Where to put a circuit on the PCB, where to install its specific circuit components, and what other adjacent circuits are, all of which are very important.

Usually, the positions of input, output, and power supply are predetermined, but the circuits between them need to "play their own creativity." This is why paying attention to wiring details will yield huge rewards. Start with the location of key components and consider the specific circuit and the entire PCB. Specifying the location of key components and signal paths from the beginning helps to ensure that the design meets the expected work goals. Getting the right design the first time can reduce costs and pressure-and shorten the development cycle.

Bypass power

Bypassing the power supply at the power supply side of the amplifier in order to reduce noise is a very important aspect in the PCB design process-including high-speed operational amplifiers or other high-speed circuits. There are two common configuration methods for bypassing high-speed operational amplifiers.

Grounding the power supply terminal: This method is the most effective in most cases, using multiple parallel capacitors to directly ground the power supply pin of the operational amplifier. Generally speaking, two parallel capacitors are sufficient-but adding parallel capacitors may bring benefits to some circuits.

Parallel connection of capacitors with different capacitance values helps to ensure that only low alternating current (AC) impedance can be seen on the power supply pin over a wide frequency band. This is especially important at the attenuation frequency of the operational amplifier power supply rejection ratio (PSR). This capacitor helps compensate for the reduced PSR of the amplifier. Maintaining a low impedance ground path in many ten-octave ranges will help ensure that harmful noise cannot enter the op amp. Figure 1 shows the advantages of using multiple capacitors in parallel. At low frequencies, large capacitors provide a low impedance ground path. But once the frequency reaches their own resonant frequency, the capacitance of the capacitor will weaken and gradually appear inductive. This is why it is important to use multiple capacitors: when the frequency response of one capacitor starts to drop, the frequency response of the other capacitor starts to work, so it can maintain a very low AC impedance in many ten-octave ranges.