Printed circuit board (PCB) wiring plays a key role in high-speed circuits, but it is often one of the last steps in the circuit design process. High-speed PCB wiring has many aspects of problems, about this topic has been written a lot of literature. This paper mainly discusses the wiring problem of high-speed circuit from the Angle of practice. The main purpose is to help new users to pay attention to the many different issues that need to be considered when designing high-speed circuit PCB wiring. Another goal is to provide a review material for customers who have not been exposed to PCB wiring for some time. Due to limited space, it is not possible to discuss all the issues in detail in this article, but we will discuss the key parts that are most effective in 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 for wiring most other high-speed analog circuits. When operational amplifiers operate at very high RF frequencies, the performance of the circuit is largely dependent on PCB wiring. What looks like a good high-performance circuit design on the “drawing” may end up with mediocre performance if it is affected by careless wiring. Forethought and attention to important details throughout the wiring process can help ensure expected circuit performance.
Schematic diagram
Although good schematics do not guarantee good wiring, good wiring begins with good schematics. Careful consideration must be given when drawing schematic diagrams, and the signal direction of the entire circuit must be considered. If there is a normal steady signal flow from left to right in the schematic, 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 absent, the customer will ask us to help solve the circuit problem, the designers, technicians and engineers who are engaged in this work will be very grateful, including us.
In addition to the usual reference identifiers, power consumption, and error tolerance, what information should be given in the schematic? Here are some Suggestions for turning a normal schematic into a first-class one. Add waveform, mechanical information about the housing, print line length, blank area; Indicate which components to place on the PCB; Give adjustment information, value range of components, heat dissipation information, printed line of control impedance, notes, brief description of circuit action…… (and others).
Don’t trust anyone
If you do not design your own cabling, be sure to leave plenty of time to carefully examine the cabling design. A small prevention at this point is worth a hundred times the remedy. Don’t expect the cabling people to understand your thinking. Your advice and guidance is most important at the beginning of the wiring design process. The more information you can provide, and the more you can get involved in the wiring process, the better the result will be. Set a tentative completion point for the wiring design engineer — a quick check of your desired wiring progress report. This “closed loop” approach minimizes the possibility of rework by preventing wiring from going astray.
Instructions to the wiring engineer include a brief description of the circuit function, a sketch of the PCB showing the input and output positions, and information about the layers of the PCB (for example, how thick and how many layers the board is, details of each signal layer and ground plane — power consumption, ground, analog, digital, and RF signals); The layers need those signals; The location of important components is required; The exact location of the bypass element; Which 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 kept away from each other (or close to each other); Which lines need to stay away from (or close to) each other; Which components need to be kept away from each other (or close to each other); Which components should be placed on the top and which on the bottom of the PCB. Never complain about giving someone too much information — too little? Is; Too much? Not at all.
A learning lesson: about 10 years ago, I designed a multilayer surface board with components on both sides. Use a lot of screws to hold the plate in a gold-plated aluminum case (because of the strict shockproof specifications). Pins providing offset feed-through pass through the board. The pin is connected to the PCB by welding wire. This is a very complicated device. Some of the components on the board are used to test the SAT. But I’ve specified where the components are. Can you guess where these components are installed? Oh, it’s under the board. Product engineers and technicians were unhappy when they had to take the whole thing apart, set it up and put it back together again. I haven’t made such a mistake since.
location
As in a PCB, position is everything. Where a circuit is placed on a PCB, where its specific circuit components are installed, and what other circuits are adjacent to it are all important.
Normally, the positions of the 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 pay off handsomely. Starting from the position of key components, according to the specific circuit and the whole PCB to consider. Specifying the location of key components and the path of signals from the outset helps ensure that the design achieves the desired work objectives. Getting the design right the first time reduces cost and stress — and thus leads to shorter development cycles.
Bypass the power supply
Bypassing power at the power side of the amplifier to reduce noise is an important aspect of the PCB design process – whether for high-speed operational amplifiers or other high-speed circuits. There are two commonly used configuration methods for bypass high speed operational amplifiers.
Grounding of the power supply: this method is most effective in most cases, using multiple shunt capacitors to directly ground the power supply pins of the operational amplifier. Two parallel capacitors are generally sufficient – but adding parallel capacitors may benefit some circuits.
Parallel capacitors with different capacitance values help ensure that the power pin can only see very low AC impedance over a very wide frequency band. This is especially important at the PSR attenuation frequency. The capacitor helps compensate for the amplifier’s reduced PSR. Grounding paths that maintain low impedance over many ten-octave ranges will help ensure that harmful noise does not enter the op amp. Figure 1 shows the advantages of using multiple shunt capacitors. At low frequencies, large capacitors provide a low impedance grounding path. But once the frequencies reach their own resonant frequencies, the capacitance of the capacitors diminishes and becomes increasingly sensible. This is why it is important to use multiple capacitors: when the frequency response of one capacitor begins to decline, the frequency response of the other capacitor begins to function, thus maintaining a very low AC impedance over many ten-octave ranges.
Practice guide for high-speed printed circuit board (PCB) wiring
Figure 1. Impedance of the capacitor in relation to frequency.
Directly start from the power pin of operational amplifier; Capacitors with minimum capacitance and minimum physical size should be placed on the same side of the PCB as the op amp — and as close to the amp as possible. The grounding end of the capacitor should be connected directly to the ground plane with the shortest pin or printed wire. The above ground connection should be as close as possible to the load end of the amplifier to minimize interference between the power and ground ends. Figure 2 shows this join method.
Practice guide for high-speed printed circuit board (PCB) wiring
Figure 2. Shunt capacitors at bypass power supply end and ground.
This process should be repeated for capacitors with sub-large capacitance values. It is better to place an electrolytic capacitor with a minimum capacitance value of 0.01 F close to that of a 2.2-f (or a little larger) capacitor with a low equivalent series resistance (ESR). The 0508 shell size 0.01 F capacitor has very low series inductance and excellent high frequency performance.
Power end to power end: another configuration method USES one or more by-pass capacitors to bridge between the positive and negative power ends of the operational amplifier. This is usually done when it is difficult to configure four capacitors in a circuit. The disadvantage is that the housing size of the capacitor may increase, since the voltage at both ends of the capacitor is twice the voltage value of the single-source bypass method. Increasing the voltage requires increasing the rated breakdown voltage of the device, i.e. increasing the housing size. However, this approach can improve PSR and distortion performance.
Since each circuit and wiring is different, the configuration, number, and capacitance of the capacitors will depend on the actual requirements of the circuit.
Parasitic effects
Parasitic effects are those tiny (literally) bugs that slip into your PCB and cause havoc on the circuit, headaches, and unexplained problems. They are parasitic capacitors and inductors that infiltrate into high-speed circuits. This includes a parasitic inductance formed by a package pin and a printed wire that is too long; Parasitic capacitance formed between pad to ground, pad to power plane and pad to printed wire; The interplay between through-holes and many other possible parasitic effects. Figure 3 (a) shows a typical operating amplifier schematic. However, if parasitic effects are considered, the same circuit may look like figure 3 (b).
Practice guide for high-speed printed circuit board (PCB) wiring
Figure 3. Typical operational amplifier circuit, (a) original design drawing, and (b) diagram with parasitic effects taken into account.
In high-speed circuits, very small values can affect the performance of the circuit. Sometimes a few dozen pF capacitors are enough. Example: if there is only 1 pF additional parasitic capacitance at the inverting input, it can cause a spike pulse of approximately 2 dB in the frequency domain (see figure 4). If the parasitic capacitance is large enough, it may cause instability and oscillation of the circuit.
Practice guide for high-speed printed circuit board (PCB) wiring
Figure 4. Additional spike pulse caused by parasitic capacitance.
Several basic formulas for calculating the dimensions of these parasitic capacitors may be used when looking for problematic hosts. Formula (1) is the formula for calculating parallel plate capacitors (see figure 5).
Practice guide for high-speed printed circuit board (PCB) wiring
(1)
C represents the capacitance, A represents the plate area in cm2, k represents the relative dielectric constant of PCB material, and d represents the distance between plates in cm.
Practice guide for high-speed printed circuit board (PCB) wiring
Figure 5. Capacitance between two plates.
Zonal inductance is another parasitic effect to be considered due to the overlength of the printed wire or the lack of a ground plane. Formula (2) shows the formula for calculating Inductance of the printed line. See figure 6.
Practice guide for high-speed printed circuit board (PCB) wiring
(2)
W represents the width of the printed line, L represents the length of the printed line, and H represents the thickness of the printed line. All dimensions are in mm.
Practice guide for high-speed printed circuit board (PCB) wiring
Figure 6. Printed line inductance.
The oscillations in figure 7 show the effect of a printed line with a length of 2.54 cm at the in-phase input end of the high speed operational amplifier. Its equivalent parasitic inductance is 29 nH (10-9h), which is sufficient to cause a continuous low-voltage oscillation that lasts for the entire transient response period. Figure 7 also shows how the grounding plane can be used to reduce the effect of parasitic inductance.
Practice guide for high-speed printed circuit board (PCB) wiring
Figure 7. Impulse response with and without a ground plane.
Through hole is another parasitic source; They can cause parasitic inductance and parasitic capacitance. Formula (3) is the formula for calculating parasitic inductance (see figure 8).
Practice guide for high-speed printed circuit board (PCB) wiring
(3)
T represents the thickness of the PCB and d represents the through-hole diameter in cm.
Practice guide for high-speed printed circuit board (PCB) wiring
Figure 8. Size of through-hole.
Formula (4) shows how to calculate the parasitic capacitance caused by the through-hole (see figure 8).
Practice guide for high-speed printed circuit board (PCB) wiring
(4)
Epsilon r indicates the relative permeability of the PCB material. T represents the thickness of the PCB. D1 represents the diameter of the pad around the through hole. D2 represents the diameter of the isolation hole in the ground plane. All dimensions are in cm. On a piece of PCB 0.157 cm thick, a through-hole can increase the parasitic inductance of 1.2 nH and the parasitic capacitance of 0.5 pF. This is why PCB wiring must always be on the alert to minimize the effects of parasitic effects.
Ground plane
There’s actually a lot more to be said for this article, but we’ll highlight some key features and encourage readers to explore the topic further. References are listed at the end of this paper.
The ground plane ACTS as a common reference voltage, providing shielding, cooling and reducing parasitic inductance (but it also increases parasitic capacitance). While there are many benefits to using a ground plane, care must also be taken in implementing it, as it has some limitations on what can and cannot be done.
Ideally, the PCB should have a layer dedicated to the ground plane. In this way, the best results will be produced when the whole plane is not destroyed. Never divert an area of the ground plane in this special layer to connect to other signals. Because the ground plane eliminates the magnetic field between the conductor and the ground plane, the printed line inductance can be reduced. If an area of the ground plane is destroyed, an unexpected parasitic inductance may be introduced into the printed wires above or below the ground plane.
The resistance of the ground plane is kept to a minimum because the ground plane usually has a large surface area and cross-sectional area. At low frequencies, the current chooses the path with the least resistance, but at high frequencies, the current chooses the path with the least impedance.
There are exceptions, however, and sometimes a small ground plane is better. High-speed operational amplifiers work better if the ground plane is removed from the input or output pads. The parasitic capacitance introduced into the grounding plane of the input terminal increases the input capacitance of the operational amplifier and reduces the phase margin, thus causing instability. As seen in the discussion of parasitic effects section, the capacitance of the operational amplifier input terminal 1 pF can cause a sharp pulse. Capacitive loads on the output — including parasitic capacitive loads — create 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 rapid rising edge causes current burrs to flow into the ground plane. The noise caused by these rapid current burrs can damage the simulation performance. Analog and digital ground (and power supply) should be connected to a common ground to reduce circulating digital and analog ground currents and noise.
At high frequencies, a phenomenon called “skin effect” must be considered. The skin effect causes current to flow to the outer surface of the wire – resulting in a narrowing of the wire’s cross section, thereby increasing the direct current (DC) resistance. Although Skin effect is beyond the scope of this paper, a good approximate formula (in cm) for Skin Depth in copper wire is given:
Practice guide for high-speed printed circuit board (PCB) wiring
(5)