PCB Blog - High-speed printed circuit board wiring practice guide

Printed circuit boards wiring plays a key role in high-speed circuits, but it is often one of several steps in the circuit design process. 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 the same good signal flow on the PCB board. 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 board; give adjustment information, component value ranges, heat dissipation information, control impedance printed lines, comments, and summary Circuit description. 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 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 PCB board you will get. Set a tentative completion point for the wiring design engineer-a quick check according to the wiring progress you want. This "closed loop" method can prevent the wiring from going astray, thereby reducing 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 board indicating the input and output positions, PCB board stacking information (for example, how thick the board is, how many layers there are, and the detailed information of each signal layer and ground plane -Power consumption, ground wire, analog signal, digital signal and RF signal); which signals are required for each layer; require the placement of important components; the exact location of bypass components; which printed lines are important; which lines need to control impedance printing Lines; which lines need to match the length; the size of the components; which printed lines need to be far away from each other; which lines need to be far away (or close) to each other; which components need to be far away (or close) to each other; which components need to be placed on the PCB board Above and which ones are placed below. Never complain that there is too much information for others-too little? Is it too much? No. Just like in a PCB board, position is everything. Where to place 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 board. 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 can reduce costs and pressure-and shorten the development cycle. Bypassing the power supply at the power end 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 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 a very low AC impedance can be seen on the power supply pin over a wide frequency band. This is especially important for the op amp power supply rejection ratio attenuation frequency. 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 begins to drop, the frequency response of the other capacitor begins to work, so it can maintain a very low AC impedance in many ten-octave ranges. Start directly from the power supply pin of the operational amplifier; the capacitor with the capacitance value and physical size should be placed on the same side of the PCB as the operational amplifier-and as close as possible to the amplifier. The ground terminal of the capacitor should be directly connected to the ground plane with a short pin or printed wire. The above ground connection should be as close as possible to the load terminal of the amplifier in order to reduce the interference between the power terminal and the ground terminal. This process should be repeated for capacitors with the next largest capacitance value. With 0.01 of 0508 case size, the capacitor has very low series inductance and excellent high frequency performance. Power supply to power supply: Another configuration method uses one or more bypass capacitors connected across the positive and negative power supply terminals of the operational amplifier. This method is usually used when it is difficult to configure four capacitors in the circuit. Its disadvantage is that the size of the capacitor's case may increase because the voltage across the capacitor is twice the value of the voltage in the single-supply bypass method. Increasing the voltage requires increasing the rated breakdown voltage of the device, that is, increasing the size of the case. However, this method can improve PSR and distortion performance. Because each circuit and wiring is different, the configuration, number and capacitance value of capacitors should be determined according to the requirements of the actual circuit. The so-called parasitic effects are those small faults that sneak into your PCB and cause great damage in the circuit, headaches, and unexplained causes. They are the parasitic capacitance and parasitic inductance that penetrate into the high-speed circuit. Including the parasitic inductance formed by the package pins and the long traces; the parasitic capacitances formed from the pad to the ground, the pad to the power plane, and the pad to the trace; the mutual influence between the vias, and Many other possible parasitic effects. In high-speed circuits, a small value will affect the performance of the circuit. Sometimes dozens of picofarads are enough. Related example: If there is only 1 pF of additional parasitic capacitance at the inverting input, it can cause almost 2 dB spikes in the frequency domain. If the parasitic capacitance is large enough, it will cause instability and oscillation of the circuit. Strip inductance is another parasitic effect that needs to be considered. It is caused by too long printed lines or lack of ground planes.

εr represents the relative permeability of the PCB board material. T represents the thickness of the PCB board. D1 represents the diameter of the land surrounding the through hole. D2 represents the diameter of the isolation hole in the ground plane. All dimensions are in cm. A through hole on a 0.157 cm thick PCB board can increase the parasitic inductance of 1.2 nH and the parasitic capacitance of 0.5 pF; this is why it is necessary to keep alert when wiring the PCB board, and the influence of parasitic effects Down. The ground plane acts as a common reference voltage, provides shielding, can dissipate heat and reduce parasitic inductance (but it also increases parasitic capacitance). Although there are many benefits to using a ground plane, care must be taken when implementing it, because it has some restrictions on what can and cannot be done. Ideally, one layer of the PCB should be dedicated as a ground plane. This will produce results when the entire plane is not destroyed. Never misappropriate the area of the ground plane in this dedicated layer to connect to other signals. Since the ground plane can eliminate the magnetic field between the conductor and the ground plane, the printed line inductance can be reduced. If a certain area of the ground plane is destroyed, unexpected parasitic inductance will be introduced to the printed lines above or below the ground plane. Because the ground plane usually has a large surface area and cross-sectional area, the resistance of the ground plane is maintained at a value. In the low frequency band, the current will choose the path of resistance, but in the high frequency band, the current will choose the path of impedance. However, there are exceptions, and sometimes a small ground plane is better. If the ground plane is moved away from the input or output pads, the high-speed operational amplifier will work better. Because of the parasitic capacitance introduced at the ground plane of the input end, the input capacitance of the operational amplifier is increased, and the phase margin is reduced, thereby causing instability. As seen in the discussion of the parasitic effects section, a 1 pF capacitance at the input of an op amp can cause very obvious spikes. Capacitive loads at the output-including parasitic capacitive loads-cause poles in the feedback loop. This reduces the phase margin and causes the circuit to become unstable. If possible, analog and digital circuits-including their respective ground and ground planes-should be separated. A fast rising edge can cause current glitches to flow into the ground plane. The noise caused by these fast current spikes can destroy the analog performance. The analog ground and digital ground should be connected to a common ground point to reduce circulating digital and analog ground currents and noise. In the high frequency range, a phenomenon called "skin effect" must be considered. The skin effect causes current to flow on the outer surface of the wire-as a result, the cross-section of the wire becomes narrower, thus increasing the DC resistance. Although the skin effect is beyond the scope of this article, here is a good approximation formula for the skin depth in the copper wire (in cm, low-sensitivity electroplated metal helps reduce the skin effect. Wiring and Shielding, there are various analog and digital signals on the PCB, including high to low voltage or current, from DC to GHz frequency range. It is very difficult to ensure that these signals do not interfere with each other.

Reducing the length of the long parallel wires in the same PCB board and the proximity between the signal printed wires can reduce inductive coupling. Reducing the length of long traces in adjacent layers can prevent capacitive coupling. Signal traces that require high isolation should go on different layers and-if they cannot be completely isolated-should go on orthogonal traces, and place the ground plane between them. Orthogonal wiring can reduce capacitive coupling, and the ground wire will form an electrical shield. This method can be used when forming a controlled impedance printed line. High-frequency signals usually flow on the control impedance printed line. That is, the printed line maintains a characteristic impedance, such as 50Ω. Two common controlled impedance printed lines, the microstrip line 4 and the strip line 5 can achieve similar effects, but the implementation methods are different. H represents the distance from the ground plane to the signal trace, W represents the width of the trace, and T represents the thickness of the trace; all dimensions are in mils. εr represents the dielectric constant of the PCB board material. The strip-shaped control impedance printed line uses two layers of ground planes, and the signal printed line is clamped in it. This method uses more printed lines, requires more PCB layers, is sensitive to changes in dielectric thickness, and is more costly-so it is usually only used in demanding applications. High-level PCB layout is very important for successful operational amplifier circuit design, especially for high-speed circuits. A good schematic diagram is the basis of good wiring; close cooperation between circuit design engineers and wiring design engineers is fundamental, especially with regard to the location of components and wiring. Issues that need to be considered include bypassing the power supply, reducing parasitic effects, using ground planes, the impact of opamp packaging, and printed circuit boards wiring and shielding methods.