PCB manufacturing and PCB assembly factories
| 1. Component placement |
| Similar to building a house, a floor plan of the system must be created before circuit components can be placed. This step establishes the overall integrity of the system design and should help avoid interference from noisy signals. When developing a floor plan, it is recommended to follow the signal paths of the schematic, especially for high-speed circuits. Component placement is also a critical aspect of the design. Designers should be able to identify important functional blocks, signals, and connections between blocks to determine the best placement of each component in the system. For example, connectors are best placed on the edge of the board, while auxiliary components such as decoupling capacitors and crystal oscillators must be placed as close as possible to the mixed-signal device. |
| 2. Separation of analo and digital modules |
| To minimize the common return path for analo and digital signals, consider separating the analo and digital blocks so that analo signals are not mixed with digital signals. |
| 3. Power module |
| Power supplies are an important part of a circuit and should be handled properly. As a rule of thumb, a power module must be isolated from the rest of the circuit while still being close to the components it powers. |
| Devices in complex systems may have multiple power pins, in which case dedicated power blocks can be used for the analo and digital parts to avoid noisy digital interference. On the other hand, power traces should be short and straight, and use wide traces to reduce inductance and avoid current limiting. |
| 4. Decoupling technology |
| The power supply rejection ratio (PSRR) is one of the important parameters that designers must consider when achieving the system target performance. PSRR measures a device's sensitivity to power supply variations and will ultimately determine device performance. |
| To maintain optimum PSRR, it is necessary to prevent high frequency energy from entering the device. To do this, a combination of electrolytic and ceramic capacitors can be used to properly decouple the device supply to a low-impedance ground plane. The purpose of proper decoupling is to create a low noise environment for circuit operation. The basic rule is to allow the current to return easily by providing the shortest path. |
| 5. the circuit board layer |
| Once the component placement and floorplan are complete, we can look at another aspect of the board design—often referred to as the board layers. It is strongly recommended to consider the board layers before proceeding with the PCB layout, as this will determine the allowable return paths for the system design. |
| Board layer refers to the vertical arrangement of copper layers in a circuit board. These layers should manage current and signals throughout the board. |
| Typically, a high performance data collection system should have four or more layers. The top layer is typically used for digital/analo signals, while the bottom layer is used for auxiliary signals. The second layer (ground plane) acts as a reference plane for impedance-controlled signals to reduce IR drop and shield digital signals in the top layer. Finally, the power plane resides on the third layer. |
| The power and ground planes must be adjacent to each other because they provide additional interplane capacitance that aids high-frequency decoupling of the power supply. For the ground plane, recommendations for mixed-signal designs have changed over the years. |
| Splitting the ground plane into analo and digital parts made sense for many years, but for modern mixed-signal devices a new approach is suggested. Proper floor planning and signal separation should prevent problems associated with noisy signals. |
| 6. Ground plane: separated or not separated? |
| Grounding is an important step in mixed-signal PCB layout design. A typical 4-layer PCB must have at least one layer dedicated to the ground plane to ensure return signals return via a low-impedance path. All IC ground pins should be routed and connected directly to a low-impedance ground plane, minimizing series inductance and resistance. |
| For mixed-signal systems, separating the analo and digital grounds has become a standard grounding method. However, mixed-signal devices with low digital currents are best managed with a single ground. Further, the designer must consider which grounding practice is most appropriate based on the mixed-signal current requirements. Designers must consider two grounding practices. |
| 7. Single ground plane |
| For mixed-signal systems with a single low digital current ADC or DAC, a single solid ground plane would be the best approach. To understand the importance of a single-ground plane, we need to review the return current. |
| Return current is the current that returns to ground and the traces between devices to complete a loop. To prevent mixed-signal interference, every return path must be traced throughout the PCB layout. |
| For low frequency signals, the return current will follow the path of least resistance, usually a straight line between the device ground references. But for higher frequency signals, a portion of the return current tries to return along the signal path. This is because the impedance along this path is low and the loop formed between the outgoing and returning current is minimal. |
| 8. Here is the title of the separation of the small analo land and the honey word land |
| For complex systems where a solid grounding scheme is difficult, a separate ground may be more appropriate. |
| Splitting the ground plane is another common approach, where the ground plane is split into two: an analo ground plane and a digital ground plane. This is suitable for more complex systems with multiple mixed-signal devices consuming high digital currents. Figure 5 shows an example of a system with a split ground plane. |
| For systems with split ground planes, the simplest solution to achieving integral grounding is to eliminate the interruption of the ground plane and allow the return current to take a more direct route, returning through the star ground junction. The star ground is the junction where the analo and digital ground planes are joined together in a mixed-signal layout design. |
| In common systems, a star ground can be associated with a simple narrow continuous junction between the analo and digital ground planes. For more complex designs, star grounding is usually accomplished with a jumper shunt to the ground connection. There is no current flow in the star ground, so there is no need for high current-carrying splices and jumper shunts. The main purpose of the star ground is to ensure that both grounds have the same reference level. |
| It is important for the designer to check the grounding recommendations provided in each device's data sheet to ensure grounding requirements are met and to avoid issues related to grounding. On the other hand, mixed-signal devices with AGND and DGND pins can be tied to their respective ground planes because the star ground also connects both grounds at one point. |
| This way, all noisy digital currents flow through the digital supply, all the way to the digital ground plane, and back to the digital supply while being isolated from sensitive analo circuitry. The isolation of the AGND and DGND planes must be implemented on all layers of the multilayer PCB. |
| 9. Other common grounding practices |
| The following steps or checklist can be used to ensure that a proper grounding scheme is implemented in a mixed-signal/digital system: The connection to the star ground point should be formed by a wide copper trace. |
| • Check the ground plane for narrow traces, these connections are undesirable. |
| · It is useful to provide pads and vias so that the analo and digital ground planes can be connected if necessary. |
| 10. Conclusion |
| PCB layout for mixed-signal applications can be challenging, and creating a component floor plan is only the starting point. Proper management of board layers and proper grounding schemes are also among the key points that system designers must consider when striving to achieve the best performance in a mixed-signal system layout. |
| Developing a component floor plan will help establish the overall integrity of the system design, and properly organizing the board layers will help manage current and signals throughout the board. Ultimately, choosing the most favorable grounding scheme will improve system performance and prevent problems associated with noisy signals and return currents. |