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

Although this article focuses on circuits related to high-speed operational amplifiers, the issues and methods discussed in this article 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 can only get ordinary performance if it is affected by careless and careless wiring. Therefore, pre-consideration and attention to important details during the entire wiring process will help ensure the expected circuit performance. Schematic Although a good schematic does not guarantee a good wiring, a good wiring starts with a good schematic. When drawing the schematic diagram, we must think carefully, and we must consider the signal direction 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. In this way, even if some problems cannot be solved by the circuit design engineer, customers can also seek other channels to help solve the circuit problems. In addition to the common reference identifiers, power consumption, and error tolerance, what other information should be given in the schematic? The following will provide some suggestions to turn ordinary schematics into the best schematics. Add waveforms, mechanical information about the casing, length of printed lines, and 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 other information, etc. Don’t believe that if you don’t design the wiring yourself, you must allow ample time to carefully check the design of the wiring person. A small prevention can be worth a hundred times the remedy. Don’t expect the wiring person to understand the designer’s ideas. Early opinions and guidance in the wiring design process are the most important. The more information that can be provided, and the more involved in the entire wiring process, the better the resulting PCB will be. Set a tentative completion point for the wiring design engineer, and quickly check according to the desired wiring progress report. This closed loop method can prevent the wiring from going astray, thereby minimizing the possibility of redesign. 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 about each signal layer and ground plane: power consumption , Ground wire, analog signal, digital signal and RF signal, etc.); which signals are required for each layer; the placement of important components is required; the exact location of bypass components; those 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 PCB Above, which ones are placed below. Wiring design engineers can never complain about too much information that needs to be given. There is never too much information. Next, I will share a learning experience: about 10 years ago, I carried out a design project of a multi-layer surface mount circuit board with components on both sides of the circuit board. Use a lot of screws to fix the board in a gold-plated aluminum housing (because there are very strict standards for shock resistance). 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 the engineer has clearly defined the location of these components. Where are these components installed? Just below the board. When product engineers and technicians have to disassemble the entire device and reassemble them after completing the settings, this procedure becomes very complicated. Therefore, such errors must be minimized as much as possible. Position is just like in the PCB, position 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 be creative. This is why paying attention to wiring details will have a significant impact on subsequent manufacturing. Start with the location of key components and consider the specific circuit and the entire PCB. Specifying the location of key components and the path of the signal from the beginning helps ensure that the design achieves the expected work goals. Getting the right design once can reduce costs and pressure, and therefore shorten the development cycle. Bypass power supply Setting a bypass power supply at the power end of the amplifier to reduce noise is a very important direction in the PCB design process, including for high-speed operational amplifiers and other high-speed circuits. There are two common configuration methods for bypassing high-speed operational amplifiers. * This method of grounding the power supply terminal 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 the power supply pin has a very low alternating current (AC) impedance 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. (Picture 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 compatibility of the capacitor will be weakened 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.