Foreword: This article details what should be considered when designing the layout of a mixed-signal PCB. The paper will cover considerations in terms of component placement, board layering, and ground planes. The guidelines discussed in this paper provide a practical approach to mixed-signal board layout design and should be useful to engineers of all backgrounds.
Synopsis:
Mixed-signal PCB design requires a basic understanding of analogue and digital circuits to minimise, if not prevent, signal interference. The components that make up modern systems have elements that operate in both the digital and analogue domains and must be carefully designed to ensure signal integrity throughout the system.
As an important part of the mixed-signal development process, PCB layout can be daunting, and component placement is only the beginning. There are other factors that must be considered, including the layers of the board and how to manage them appropriately to minimise interference caused by parasitic capacitance, which can be accidentally generated between the PCB’s interplanar layers.
Grounding is also an important step in PCB layout design for mixed-signal systems. Although grounding is a frequently debated topic in the industry, developing a standardised methodology is not always the easiest task for engineers. For example, a single issue with high-quality grounding can affect the entire layout of a high-performance mixed-signal PCB design. Therefore, this aspect should not be ignored.
Component Placement
Similar to building a house, a floor plan of the system must be created before placing circuit components. This step will establish the overall integrity of the system design and should help avoid high noise signal interference.
When developing the floor plan, it is recommended that the signal paths of the schematic be followed, especially for high-speed circuits. The location of components is also a critical aspect of the design. The designer should be able to identify important functional modules, signals, and connections between modules to determine the best placement of each component in the system. For example, connectors are best placed at the edge of the board, while ancillary components such as decoupling capacitors and crystals must be placed as close as possible to the mixed-signal device.
Separation of analogue and digital modules
To minimise the common return path of analogue and digital signals, consider separating the analogue and digital modules so that analogue signals are not mixed with digital signals.
Figure 1. Separation of analogue and digital circuits
Figure 1 shows a good example of analogue and digital circuit separation. The following should be kept in mind when splitting the analogue and digital sections:
It is recommended that sensitive analogue components (such as amplifiers and reference voltage sources) be placed within the analogue plane. Similarly, noisy digital components (such as logic control and timing modules) must be placed on the other side/digital plane.
If the system contains a mixed-signal analogue-to-digital converter (ADC) or digital-to-analogue converter (DAC) with low digital currents, this can be treated in a similar way to the analogue components contained in the analogue plane.
For designs with multiple high-current ADCs and DACs, it is recommended that the analogue and digital power supplies be separated. That is, the AVCC must be tied to the analogue section and the DVDD should be connected to the digital section.
Microprocessors and microcontrollers can take up space and generate heat. These devices must be placed in the centre of the board for better heat dissipation and should be close to the circuit modules they are associated with.
Power Supply Module
The power supply is an important part of the circuit and should be handled properly. As a rule of thumb, the power supply 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 more than one power supply pin, in which case separate dedicated power supply modules can be used for the analogue and digital parts to avoid high noise digital interference.
On the other hand, power supply wiring should be short and straight and use wide alignments to reduce inductance and avoid current limiting.
Decoupling techniques
Power Supply Rejection Ratio (PSRR) is one of the important parameters that designers must consider when achieving the targeted performance of a system.PSRR measures the sensitivity of a device to power supply variations and will ultimately determine the performance of the device.
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 power 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 ease the return of current by providing the shortest path.
It is important for designers to note the high frequency filtering recommendations for each device. More importantly, this list will be used as a guide to general decoupling techniques and their proper implementation:
Electrolytic capacitors act as charge reservoirs for transient currents to minimise low-frequency noise on the supply, while low-inductance ceramic capacitors are used to reduce high-frequency noise. Alternatively, ferrite beads are optional but add high frequency noise isolation and decoupling.
Decoupling capacitors must be placed as close as possible to the power pins of the device. These capacitors should be connected to a larger area of the low-impedance ground plane through an over-hole or short traces to minimise additional series inductance.
Smaller capacitors (typically 0.01μF to 0.1μF) should be placed as close as possible to the power supply pins of the device. This arrangement prevents unstable operation when the device has multiple outputs switching at the same time. Electrolytic capacitors (typically 10μF to 100μF) should be placed no more than 1 inch from the power pins of the device.
To make implementation easier, instead of creating an alignment, the decoupling capacitor can be connected to the ground plane via a T-connection using an over-hole near the GND pin of the device. See Figure 2 for an example.
Figure 2. Decoupling Techniques for Power Pins
Circuit Board Layers
Once the component placement and floor plan drawings are complete, we can look at another aspect of circuit board design – often referred to as circuit board layers. It is highly recommended to consider the circuit board layers before proceeding with PCB routing, as this will determine the allowable return paths for the system design.
Circuit board layers refer to the vertical arrangement of copper layers in a circuit board. These layers should manage the current and signals throughout the board.
Figure 3 Example of a four-layer PCB
Figure 3 shows a visual representation of the circuit board layers. Table 1 details the setup of a typical 4-layer PCB:
Typically, a high-performance data collection system should have four or more layers. The top layer is typically used for digital/analogue signals, while the bottom layer is used for auxiliary signals. The second layer (ground plane) acts as a reference plane for the impedance control signals, which are used to reduce the IR voltage drop and shield the digital signals in the top layer. Finally, the power plane is located in the third layer.
The power and ground planes must be adjacent to each other because they provide additional interplanar capacitance, which aids in high-frequency decoupling of the power supply.
For the ground plane, recommendations for mixed-signal designs have changed over the years. For many years it made sense to divide the ground plane into analogue and digital sections, but for modern mixed-signal devices a new approach is recommended. Proper planarisation and signal separation should prevent problems associated with highly noisy signals.
Ground plane: to separate or not to separate?
Grounding is an important step in mixed-signal PCB layout design. A typical 4-layer PCB must have at least one layer dedicated to ground plane to ensure that return signals are returned via a low impedance path. All IC ground pins should be routed and connected directly to the low impedance ground plane to minimise series inductance and resistance.
For mixed-signal systems, separating analogue and digital grounding has become a standard grounding method. However, mixed-signal devices with low digital currents are best managed through a single ground. Further, designers must consider which grounding practice is most appropriate based on mixed-signal current requirements. Designers are required to consider two grounding practices.
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 return current. Return current is the current that is returned to ground and the alignment between devices to form a complete loop. To prevent mixed-signal interference, it is important to trace each return path throughout the PCB layout.
Figure 4. Return current for a system with a solid ground plane
The simple circuit in Figure 4 shows the advantages of a single solid ground plane over a separated ground plane. The signal current has a return current of equal magnitude but opposite direction. This return current flows back to the source in the ground plane and it will follow the path of least impedance.
For low frequency signals, the return current will follow the path of least resistance, which is usually a straight line between the device ground reference points. However, for higher frequency signals, a portion of the return current will attempt to return along the signal path. This is because the impedance along this path is lower and the loop formed between the outgoing and returning current is minimised.
Separation of analogue and digital ground
For complex systems where a solid grounding scheme is difficult to use, separate grounding may be more appropriate. Separating the ground plane is another common approach, where the ground plane is split in two: the analogue ground plane and the digital ground plane. This is suitable for more complex systems with multiple mixed-signal devices that consume high digital currents. Figure 5 shows an example of a system with a split ground plane.
Figure 5. Return Current for a System with Separate Ground Planes
For systems with separated ground planes, the simplest solution to achieving an integral ground is to eliminate ground plane interruptions and allow the return current to take a more direct route, flowing back through the star ground junction. The star ground is the junction where the analogue and digital ground planes are connected together in a mixed-signal layout design.
In common systems, star ground can be associated with a simple narrow continuous junction between analogue and digital ground planes. For more complex designs, star grounding is often implemented with a jumper shunt to the ground connector. There is no current flow in star grounding, so connectors and jumper shunts carrying high currents are not required. The main purpose of a 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 datasheet to ensure that grounding requirements are met and to avoid grounding-related problems. On the other hand, mixed-signal devices with AGND and DGND pins can be connected to their respective ground planes because the star ground also connects both grounds at one point. In this way, all noisy digital currents flow through the digital power supply all the way to the digital ground plane and back to the digital power supply, while being isolated from sensitive analogue circuits.The isolation of the AGND and DGND planes must be implemented on all layers of a multilayer PCB.
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:
Star ground plane connections should consist of wider copper alignments.
Check the ground plane for narrower alignments where these connections are not required.
It is useful to provide pads and vias so that analogue and digital ground planes can be connected if necessary.
Conclusion
PCB layout for mixed-signal applications can be challenging. Creating a component floor plan is only the starting point. Proper management of board layers and development of an appropriate grounding scheme are also among the key points that system designers must consider when striving to achieve optimal performance in a mixed-signal system layout. Developing a component floor plan will help establish the overall integrity of the system design. Properly organising the board layers will help manage the currents and signals throughout the board. Finally, selecting the most advantageous grounding scheme will improve system performance and prevent problems associated with high noise signals and return currents.