Printed circuit board (PCB) layout is an important activity when developing mixed-signal applications. A lot can go wrong, and everything needs to be correct for a successful design. Impedance control is an obvious consideration, but it needs to include trace resistance as well as parasitic inductances and capacitances. Maintaining signal integrity can be challenging. This FAQ reviews five of the many considerations when designing mixed-signal PCBs, including:
- Partitioning and component placement
- Trace routing
- Ground planes and signal integrity
- Mixed-signal ICs and ground planes
- Copper is not perfect
Beginning with correct partitioning is important to maintaining signal integrity and minimizing noise problems such as crosstalk and EMI between the digital and analog sections (Figure 1). Partitioning leads to placement considerations. Large digital components need room for routing and thermal management and often need to be placed near the center of the PCB. Bypass capacitors should be placed as close to the supply pins as possible. Placing these capacitors on the bottom of the board is advantageous in some designs. The analog components, connectors, and other devices need to be placed around the digital components to optimize the signal integrity of the layout. Power supplies can be particularly challenging, they need to be isolated from sensitive analog and digital circuitry, but also must be close to the device they are powering to ensure power integrity. Placement of the various components and the power supplies also needs to create a flat thermal gradient across the PCB with a minimum of (hopefully zero) hot spots.
Partitioning the PCB can simplify trace routing and the placement of ground planes. Factors to consider related to race routing include:
- Signal paths should be as direct and short as possible
- The same applies to power supply traces; they should be short, direct, and wide to reduce inductance
- High-speed circuits are especially sensitive and need to follow the signal path laid out in the schematic
- Watch out for the creation of antennas from traces or vias
- Routing needs to maintain the isolation between digital and analog circuit elements
- Grounding is important, especially for traces that connect the digital and analog partition areas
Ground planes and signal integrity
Correct partitioning can contribute to a robust grounding system. Good grounds serve several purposes:
- Provide an electromagnetic interference (EMI) shield for sensitive devices
- Help to contain and control internally-generated EMI
- A large ground plane creates a low impedance ground system
- A large ground plane can enhance thermal dissipation and spread heat more evenly
When designing a mixed-signal PCB, there is a choice between using a single ground plane or separate ground planes for the analog and digital partitions. Maintaining a single ground plane can minimize EMI concerns. But separate ground planes can be useful if the analog and digital partitions are completely isolated or if there are partitions with high-voltage currents that can benefit from additional isolation. Partitioning the design to enable the use of a single ground plane is often the best choice to ensure signal integrity across all areas of the PCB. However, if the design includes a large and/or sensitive mixed-signal IC, the use of a single ground plane may not be the best choice.
Mixed-signal ICs and ground planes
What’s the best grounding scheme for mixed-signal ICs? That depends. Some mixed-signal ICs have high digital currents, others have much lower digital currents and the two types can benefit from different grounding approaches. In either case, the PCB ground plane is usually split into an analog plane and a digital plane, and the AGND and DGND pins of an ADC or DAC can be tied together, creating the system ‘star’ ground at the mixed-signal device (left-hand side of Figure 2). The ‘star’ ground occurs where the analog and digital ground planes are joined together at the mixed-signal device. If there is more than one ADC or DAC, or when high digital currents exist, a simple ‘star’ ground may not be appropriate. If the analog and digital circuitry in the mixed-signal IC are well isolated, the connection between the two ground planes can be moved away from the AGND and DGND connections (right-hand side of Figure 2).
In the second approach, the Schottky diodes prevent low-frequency voltage spikes or large dc voltages from developing across the two planes and damaging the IC. Alternatively, a ferrite bead can provide a dc connection between the two planes but isolates them at frequencies above a few MHz.
Copper is not Perfect
Copper traces make great interconnects and ground planes, but copper is not perfect. The previous sections on grounding were interested in controlling capacitive and inductive parasitics in the PCB layout. But the resistance of copper can also be important in mixed-signal PCB layouts.
Most PCBs use 1-ounce copper, but high-power sections may use 2- or 3-ounce copper. The resistivity of copper is 1.724X10-6 Ω/cm at 25°C. The thickness of common 1-ounce copper foil is 0.036 mm (0.0014″) and its resistance is 0.48 mΩ/square. That’s not an inconsequential value. For example, 0.25 mm (10 mil) wide traces commonly used on PCBs have a resistance/length of about 19 mΩ/cm (48 mΩ /inch).
PCB trace resistance can be a source of error with mixed-signal ICs. In the case of a 16-bit, ADC with a 5 kΩ input resistance, driven through 5 cm of 0.25 mm wide 1 oz copper, the track resistance is 0.1 Ω and forms a divider with the 5 kΩ load, creating an error of 0.1/5 k (about 0.0019%), higher than 1 LSB (0.0015%) for 16 bits (Figure 3).
The actual situation may be worse since this ignores the return path and the 0.4%/°C temperature coefficient of copper at 25 °C. When dealing with low impedance precision circuits, the resistance of copper can be critical to a successful design.
PCB layouts for mixed-signal applications can be challenging. Correct partitioning of the circuitry is just the starting point. Subsidies related to parasitic impedances, ground considerations, and even the material properties of the copper traces must be included in the analysis to arrive at a successful design. The five considerations included in this FAQ provide a starting point but are not sufficient to ensure a comprehensive solution.
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