Advances in Mixed-Signal PCB Design
Earlier, electronic equipment had multiple circuit boards handling different functions of the design. For instance, it was common to find a power supply board, a processor board, separate audio and video cards, and probably, a fan control board. According to Rush PCB Inc, electronic equipment are becoming smaller and are including additional functionality. Moreover, there is the added pressure to reduce manufacturing expenses. All this has culminated into replacing the multiple-board systems with one mixed-signal design.
A mixed signal layout looks essentially the same as any general purpose printed circuit board does. The designer has to place all component parts, route the nets, and connect the power and ground planes. On closer inspection, however, it is possible to discern unique requirements that can be a challenge for the designer. They must follow certain PCB design guidelines for a mixed signal design.
Component Placement for Mixed Signal Designs
For laying out a mixed signal PCB design, the designer must develop a floor-plan before starting the placement. Advanced CAD software tools are readily available for creating suitable floor-plans. While creating the floor-plan, the designer must partition circuit areas to separate different signals like low-frequency, high-frequency, low-power, and high-power, etc. This is an important step for minimizing various problems such as EMI/EMC, crosstalk, and for improving the overall signal integrity of the design.
After suitably partitioning the circuitry, the designer can begin to place the components. Mixed signal layouts are no different from other PCB designs, and the designer must follow industry-standard rules and clearances for the placement. Some best practices that experienced designers follow are:
Partitions: For the best signal integrity, it is essential to keep sensitive analog and digital components separated, and route them away from each other. Although it is tempting for the designer to pack the component density to reduce the PCB area, it is essential that different circuitry remain isolated from each other.
Large Processors and Memory: Most of these components take up a lot of space, and generate a lot of heat. With their higher pin-count, they also require a lot of routing. For better thermal management, it is best to place these hot components in the middle of the board. It is also necessary to place their related circuitry closer for the best signal integrity performance.
Bypass Capacitors: Mixed signal designs essentially require power integrity. However, ground bounces and power spikes can easily affect power integrity. To offset these, it is customary to place a bypass capacitor close to the supply and ground pins of each IC. The routing from the IC pins to the bypass capacitors needs to be short, broad, and direct.
Routing: Sensitive circuitry, for instance in op-amps, processors, and memory devices, usually have a signal path. Routing this signal path must observe proper clearance from noisy circuitry.
Power Supply: It is necessary to isolate the power supply circuitry from sensitive analog and digital circuitry. However, they must also be close to the devices they are powering. Different power supply modules also require adequate separation for thermal considerations. For proper power integrity, power parts need close placement among themselves.
Although mixed signal designs require proper placement of components for good signal and power integrity, the design must also offer maximum production yields. This requires following the DFM or design for manufacturability rules while the designer is placing parts. For instance, a robust ground system is essential for mixed signal designs.
Layer Stack Up: The stackup often decides the success of a board, with power and ground planes being the most important. Before starting to route, most designers will set up the layers to make them easily accessible. They typically set up a reference plane on an adjacent layer to allow a shorter signal return path for a component—this improves signal integrity and minimizes noise.
A good stackup layer also contributes to limiting the EMI. Although it is possible to save on manufacturing expenses by cutting down on the layer count, it may damage the overall signal integrity. It is necessary that the designer isolate high-speed and sensitive signals, and noisy power circuitry, from each other. Creating additional layers for the stackup and including adequate number of ground planes is essential for shielding sensitive signals from EMI.
The partitioning of the circuitry for laying out the PCB must focus on creating a robust ground system. With different types of circuit in the board, it is essential for the designer to decide:
- Should analog and digital grounds combine or remain separate
- Should analog and digital grounds remain completely isolated from each other
Typically, separating the analog and digital grounds in a mixed signal circuit is likely to introduce more EMI problems. Therefore, designers prefer to keep the grounds on the same plane. If the board is carrying high voltage circuits, creating separate grounds may improve the safety of the board.
Once the designer has carefully partitioned the circuitry, located components in their rightful places, and placed a robust ground plane system, the actual routing between components can begin. Some guidelines that designers follow while routing are:
- Keeping signal paths short and direct
- Following the signal path as in the schematic
- Being careful of not creating antennas while routing traces and vias
- Reducing inductance in power supply routing by using short, direct, and wide traces
- Isolating the routing of digital circuitry from analog circuitry
- All traces that must go between digital and analog circuit areas, must have a clear signal return path on the reference plane.
The designer must follow the design rules and constraints while routing traces in a mixed signal design.
Even after taking care of ground bouncing, crosstalk, power supply noise and other disturbances, circuits may suffer from electromagnetic interference or EMI. This may result in various issues like:
- Interference in wireless communications
- Disruptions in communications
- Sensor data corruption
- Component malfunction
- Software errors and failures
One of the effective means of dealing with EMI is to use adequate amounts of metal shielding. Preferably, the shielding should create a Faraday cage, covering the circuit from all six sides and a ground layer.
Although the use of shielding can block most of the incoming EMI, it must also address thermal cooling, and allow for signal inputs and outputs.
PCB layout tools offer other features that help in the design of mixed signal PCBs. These include simulation tools, inter-checking capabilities, IR drop viewer, and 3D viewing capabilities.
According to Rush PCB Inc, laying out a PCB for mixed signal designs can be challenging. However, the starting point must be the component floor plan. Managing the stack up and allowing adequate ground layers are also key points for the designer to consider for achieving optimum performance in mixed signal design layouts. Selecting an optimum and beneficial grounding scheme helps to improve not only the system performance, but also prevents issues like noise from return currents.