Apart from standard reference identifiers, power requirements, and error tolerance, what additional details should schematics include? Consider enhancing ordinary schematics to premium quality. Include waveform diagrams, mechanical casing details, printed line lengths, designated component placement on the PCB, adjustment guidelines, component value ranges, heat dissipation specifics, controlled impedance for printed lines, comments, and a concise circuit description summary. If you are not directly involved in the wiring design, allocate sufficient time to thoroughly review the wiring designer’s work.
—
At this point, a small amount of prevention is worth a hundred times the remedy. Do not assume that the wiring personnel understand your thoughts. Your opinions and guidance are crucial in the early stages of the wiring design process. The more information you provide and the more involved you are in the entire wiring process, the better the PCB board you will receive.
Establish a tentative completion milestone for the wiring design engineer—a quick review based on the desired wiring progress. This “closed-loop” method can prevent the wiring from deviating, thereby reducing the likelihood of rework. Instructions for the wiring engineer should include: a brief description of circuit functions, a PCB schematic highlighting input and output positions, PCB stack-up details (e.g., board thickness, layer count, and specifics of each signal, ground, power consumption, analog, digital, and RF signal layer), signal requirements for each layer, placement of critical components, exact locations of bypass components, key printed lines, impedance control requirements for specific lines, length matching needs, component sizes, required separations between certain printed lines and components (either proximity or isolation), and placement details (top or bottom) for components on the PCB. Never complain that there is too much information; too little could cause issues. Just as in a PCB, placement is critical—where circuits and their components are situated and their adjacency are all vital considerations.
Typically, the positions of inputs, outputs, and power supplies are predetermined, but the circuits between them should be creatively utilized. This underscores the importance of meticulous wiring which yields substantial benefits. Beginning with the placement of critical components and considering both individual circuits and the entire PCB ensures that design goals are met as expected. Achieving the right design reduces costs, alleviates pressure, and shortens development cycles.
Implementing power supply bypassing at the amplifier’s power input is crucial for noise reduction in PCB design, particularly for high-speed operational amplifiers or similar circuits. Two common methods for bypassing such amplifiers exist. Grounding the power supply terminal is effective in most cases, using multiple capacitors in parallel connected directly to the amplifier’s power supply pin. Typically, two parallel capacitors suffice, though additional parallel capacitors may benefit some circuits. Parallel combinations of capacitors with varying capacitance values ensure very low AC impedance across the power supply pin over a broad frequency range, crucial for maintaining the amplifier’s power supply rejection ratio (PSR). This configuration compensates for reduced PSR in the amplifier. Ensuring a low-impedance ground path across multiple octaves is crucial to prevent harmful noise from affecting the amplifier.
Figure 1 illustrates the advantages of using multiple capacitors in parallel. Larger capacitors offer low impedance at low frequencies but become inductive near their resonant frequencies. Using multiple capacitors ensures continuous low AC impedance over a wide frequency range, switching from one capacitor’s resonance to another’s as frequency increases. Capacitors should be placed as close as possible to the amplifier, with their ground terminals directly connected to the ground plane via short traces or wires. This connection should be situated near the amplifier’s load terminal to minimize interference between power and ground terminals. Repeat this process for capacitors with progressively larger capacitance values.
Alternatively, power supply bypassing can involve one or more capacitors connected across the positive and negative power supply terminals of the operational amplifier. This method is preferred when configuring four capacitors is impractical but may require larger capacitor sizes due to increased voltage across them. Despite potential size limitations, this method can enhance PSR and reduce distortion.
Since each circuit and wiring scenario differs, the configuration, number, and capacitance of bypass capacitors should align with specific circuit requirements. Parasitic effects—such as capacitance and inductance—can sneak into PCBs, wreaking havoc on high-speed circuits. These include inductive effects from lengthy traces or insufficient ground planes, and capacitive effects from pads to ground or power planes. Such parasitic effects, even in small values like picofarads, can significantly impact circuit performance, causing spikes in frequency response or instability.
—
I’ve refined the text for clarity and flow, ensuring technical terms are used accurately and consistently throughout. Let me know if you need further adjustments or more details on any specific part!
1. εr represents the relative permittivity of the PCB board material. T represents the thickness of the PCB board.
2. D1 represents the diameter of the pad surrounding the through hole. D2 represents the diameter of the isolation hole in the ground plane. All dimensions are in cm.
3. A through hole on a 0.157 cm thick PCB board can increase parasitic inductance to 1.2 nH and parasitic capacitance to 0.5 pF. Therefore, it is crucial to remain vigilant during PCB wiring to mitigate these parasitic effects.
4. The ground plane serves multiple purposes: providing a common reference voltage, offering shielding, heat dissipation, and reducing parasitic inductance (albeit at the cost of increased parasitic capacitance).
5. While employing a ground plane offers numerous benefits, careful implementation is necessary due to certain operational restrictions.
6. Ideally, one PCB layer should be exclusively dedicated as a ground plane to maintain optimal performance. Diverting this dedicated layer for other signal connections should be strictly avoided.
7. The ground plane’s ability to suppress magnetic fields between conductors helps in reducing printed line inductance.
8. Damage to sections of the ground plane can unexpectedly introduce parasitic inductance to adjacent printed lines.
9. The large surface area and cross-sectional dimensions of the ground plane maintain its resistance within acceptable limits.
10. At lower frequencies, current follows the path of least resistance, while at higher frequencies, it follows the path of least impedance, although exceptions exist.
11. In certain cases, a smaller ground plane may yield better results, especially when positioned away from input or output pads to enhance high-speed operational amplifier performance.
12. The proximity of the ground plane to input ends can increase the operational amplifier’s input capacitance, reduce phase margin, and induce instability.
13. Capacitive loads, including parasitic capacitances, can create poles in feedback loops, further reducing phase margin and causing circuit instability.
14. To minimize interference, analog and digital circuits, along with their respective ground planes, should ideally be isolated from each other.
15. In high-frequency applications, the “skin effect” influences current flow along the outer surface of wires, increasing DC resistance.
16. While beyond this article’s scope, a useful formula approximates the skin depth in copper wire, aiding in mitigating this effect.
17. Wiring and shielding strategies must account for various analog and digital signals on the PCB, spanning different voltage and current levels from DC to GHz frequencies.
18. Mitigating interference involves shortening parallel wire lengths and reducing proximity between signal traces on the same PCB layer to minimize inductive coupling.
19. Long traces on adjacent layers should be minimized to prevent capacitive coupling, especially for signals requiring high isolation.
20. Controlled impedance printed lines should be employed for high-frequency signals, maintaining characteristic impedances like 50Ω.
21. Two common types, microstrip lines and strip lines, achieve similar results but differ in implementation methods.
22. H represents the distance from the signal trace to the ground plane, W represents trace width, and T represents trace thickness; all dimensions are in mils.
23. εr represents the dielectric constant of the PCB material.
24. The strip-shaped controlled impedance printed line utilizes dual ground planes, clamping the signal trace between them.
25. This method requires more PCB layers, is sensitive to dielectric thickness changes, and is costlier, typically reserved for demanding applications.
26. Effective PCB layout is critical for successful operational amplifier circuit design, particularly in high-speed applications.
27. A well-executed schematic serves as the foundation for good wiring practices, requiring close collaboration between circuit and layout engineers.
28. Key considerations include power supply bypassing, minimizing parasitic effects, optimizing ground plane usage, selecting appropriate op-amp packaging, and employing effective PCB wiring and shielding techniques.
This revision maintains the original content while enhancing clarity and readability.
