PCB ( printed circuit board ) wiring plays a key role in high-speed circuits. This paper mainly discusses the wiring problem of high-speed circuits from a practical point of view. The main purpose is to help new users become aware of the many different issues that need to be considered when designing PCB wiring for high-speed circuits. Another purpose is to provide a refresher material for customers who have not been exposed to PCB wiring for some time. Due to limited space, it is not possible to cover all the issues in detail in this article, but we will discuss the key parts that have the greatest impact on improving circuit performance, reducing design time, and saving modification time.
How to design PCB from a practical point of view
Although the focus here is on circuits related to high speed operational amplifiers, the problems and methods discussed here are generally applicable to wiring for most other high speed analog circuits. When operational amplifiers operate in very high radio frequency (RF) bands, the performance of the circuit is largely dependent on PCB wiring. What looks like a good high-performance circuit design on the “drawing board” can end up with mediocre performance if it suffers from sloppy wiring. Pre-consideration and attention to important details throughout the wiring process will help ensure the desired circuit performance.
Although good schematics do not guarantee good wiring, good wiring starts with good schematics. The schematic diagram must be carefully drawn and the signal direction of the entire circuit must be considered. If you have normal, steady signal flow from left to right in the schematic, you should have just as good signal flow on the PCB. Give as much useful information as possible on the schematic. Because sometimes the circuit design engineer is not available, the customer will ask us to help solve the problem of the circuit. The designers, technicians and engineers who do this work will be very grateful, including us.
Beyond the usual reference identifiers, power consumption, and error tolerances, what other information should be given in a schematic? Here are some suggestions for turning an ordinary schematic into a first-class schematic. Add waveform, mechanical information about the shell, printed line length, blank area; Indicate which components need to be placed on the PCB; Give adjustment information, component value range, heat dissipation information, control impedance printed lines, notes, concise circuit action description… (among others).
Don’t trust anyone
If you do not design your own wiring, be sure to allow plenty of time to double check the cabler’s design. A little prevention is worth a hundred times a remedy here. Don’t expect the cabling person to understand what you’re thinking. Your input and guidance is most important at the beginning of the wiring design process. The more information you can provide and the more involved you are in the wiring process, the better the PCB will be as a result. Set a tentative completion point for the cabling design engineer – a quick check of the cabling progress report you want. This “closed loop” approach prevents wiring from going astray and thus minimizes the possibility of rework.
Instructions to wiring engineers include: a short description of circuit functions, PCB sketches indicating input and output positions, PCB cascading information (e.g., how thick the board is, how many layers there are, details of each signal layer and grounding plane — power consumption, ground, analog, digital and RF signals); The layers need those signals; Require the placement of important components; The exact location of the bypass element; Which printed lines are important; Which lines need to control impedance printed lines; Which lines need to match the length; Dimensions of components; Which printed lines need to be far (or near) from each other; Which lines need to be far (or near) from each other; Which components need to be located away from (or near) each other; Which components should be placed on top and which on the bottom of the PCB? Never complain about having to give someone too much information — too little? Is; Too much? Not at all.
One learning lesson: About 10 years ago, I designed a multi-layer surface mount circuit board — the board had components on both sides. The plates are bolted to a gold-plated aluminum shell (because of the strict shockproof specifications). Pins that provide bias feed-through pass through the board. The pin is connected to the PCB by a welding wire. It’s a very complicated device. Some of the components on the board are used for test setting (SAT). But I’ve defined exactly where these components are. Can you guess where these components are installed? Under the board, by the way. Product engineers and technicians are not happy when they have to take the whole thing apart and put it back together after they’ve finished setting it up. I haven’t made that mistake since then.
As in PCB, location is everything. Where a circuit is placed on the PCB, where its specific circuit components are installed, and what other circuits are adjacent to it are all very important.
Normally, the input, output and power supply positions are predetermined, but the circuitry between them needs to be “creative”. This is why paying attention to the details of wiring can pay huge dividends. Start with the location of key components, consider the circuit and the entire PCB. Specifying the location of key components and the path of signals from the beginning helps ensure that the design works as intended. Getting the design right the first time reduces cost and stress — and thus development cycles.
Bypass the power supply
Bypassing the power side of the amplifier to reduce noise is an important aspect of the PCB design process — both for high-speed operational amplifiers and other high-speed circuits. There are two common configurations of bypass high speed operational amplifiers.
Power grounding: This method is most efficient in most cases, using multiple shunt capacitors to directly ground the power pins of the op amp. Two shunt capacitors are generally sufficient – but adding shunt capacitors may be beneficial for some circuits.
Paralleling capacitors with different capacitance values helps ensure that the power supply pins see only low AC impedance over a wide band. This is especially important at the operational amplifier power rejection ratio (PSR) attenuation frequency. The capacitor helps compensate for the reduced PSR of the amplifier. Grounding paths that maintain low impedance over many tenx ranges will help ensure that harmful noise does not enter the operational amplifier. Figure 1 illustrates the advantages of using multiple concurrent electrical containers. At low frequencies, large capacitors provide low impedance ground access. But once the frequencies reach their resonant frequency, capacitors become less capacitive and take on more sensuality. This is why it is important to have multiple capacitors: as the frequency response of one capacitor begins to decline, the frequency response of the other capacitor comes into play, thus maintaining a very low AC impedance over many ten-octaves.
Start directly from the power pin of the operational amplifier; Capacitors with minimum capacitance and minimum physical size should be placed on the same side of the PCB as the operational amplifier — as close to the amplifier as possible. The grounding terminal of the capacitor shall be directly connected to the grounding plane with the shortest pin or printed wire. The grounding connection mentioned above shall be as close to the load end of the amplifier as possible to minimize interference between the power and grounding end. Figure 2 illustrates this connection method.
This process should be repeated for sublarge capacitors. It is best to start with a minimum capacitance of 0.01 μF and place an electrolytic capacitor with a low equivalent series resistance (ESR) of 2.2 μF (or more) close to it. The 0.01 μF capacitor with 0508 housing size has very low series inductance and excellent high frequency performance.
Power-to-power: Another configuration uses one or more bypass capacitors connected between the positive and negative power ends of the operational amplifier. This method is often used when it is difficult to configure four capacitors in a circuit. The disadvantage is that the capacitor housing size may increase because the voltage across the capacitor is twice the value of the single-power bypass method. Increasing the voltage requires increasing the rated breakdown voltage of the device, which means increasing the housing size. However, this approach can improve PSR and distortion performance.
Because each circuit and wiring is different, the configuration, number, and capacitance value of capacitors will depend on the requirements of the actual circuit.
Parasitic effects are literally glitches that sneak into your PCB and wreak havoc, headaches, and unexplained havoc on the circuit. They are the hidden parasitic capacitors and inductors that seep into high-speed circuits. Which includes the parasitic inductance formed by the package pin and printed wire too long; Parasitic capacitance formed between pad to ground, pad to power plane and pad to print line; Interactions between through-holes, and many other possible effects.