Basic Design Guidelines for Mixed Signal PCB

In this article, we will be discussing the things to consider while designing a layout of mixed-signal PCB. Printed Circuit Boards are the backbone of almost all electronic devices. It is very important to design these boards properly as they are in everything from your phone to your computer. The manufacturing expenses associated with these boards are substantial, underscoring the need for designers to focus on creating compact and economically efficient PCB designs. While designing a PCB, following major points should be kept in mind:

1. Component Placement
2. Board Layering
3. Grounding
4. Routing

Typical Printed Circuit Board

Component Placement

Before starting with PCB layout, accurate library parts and the Layer stack-up should be defined properly. Once the stack-up and design properties are properly configured, the next step is to plan the floor of your PCB.

Just like before building any structure, it is essential to lay out the plan of the structure. Similarly, we plan the layout of PCB before placing the components. While developing the floor plan, it is advisable to provide a separate partition for Analog & Digital blocks in order to minimize the common return path. Designers must try to structure the layout so as to isolate analog, digital and power supply blocks from each other. Also, designers should keep in mind to follow the signal path of the schematic. To provide room for trace routing should also be kept in mind while placing the component.

While planning the floor, following things should be kept in mind:

1. Power supply section should be placed compactly together and proper decoupling techniques should be used to achieve best power integrity.
2. Decoupling capacitors should be placed as close as possible to the respective devices.
3. Connectors should be placed on the edge of the board.
4. Designers should try to follow the schematic flow for high frequency components.
5. Large memory devices & processors such as clock generators & controllers should be placed at the center of the board.

Board Layering

Once the floor planning is done, we can move forward to determining the type of board layers. It is important to determine board layers before starting with routing as these copper layers are responsible for managing the signal and current throughout the board.

The information below details a typical 4 layer PCB layers and its types:

1. First layer: Digital/ Analog signal
2. Second layer: Ground layer
3. Third layer: Power layer
4. Fourth layer: Aux signal( bottom layer)

Generally, high performance multi-layered boards should consist of 4 or more layers. The first layer generally contains digital and analog components/signals while the bottommost layer consists of Auxiliary signals.

The second layer is generally a ground layer while the third layer is a power plane consisting of all power current traces. The ground layer acts as a reference ground for all the high frequency impedance controlled signals.

Power and ground planes must be adjacent to each other as they provide additional interplane capacitance, which helps with the high frequency decoupling of the power supply.

Grounding

It is very important for designers to provide proper grounding to ensure a low impedance return path for signals. It is better to provide the entire layer as a ground plane in a typical mixed-signal PCB. Designers should decide whether to go with a single ground plane or separated ground for Analog and Digital circuits depending upon the need.

1. Single Ground Plane- For PCBs consisting of components that require low current, this would be the best approach. The return current in this case has an equal but opposite flow to the signal current. The return current will flow in the ground plane back to the source and will follow the path of least resistance. It will travel in a straight line for low frequency signals to complete the loop.
2. Split Ground Plane- A split ground plane is an approach where the ground plane is split in analog and digital planes. This technique is used where devices involving high currents are to be dealt with. These systems have a star ground junction where both analog and digital ground are joined together. The main purpose of having a star ground is to ensure that both digital and analog sections have the same reference ground. Figure below shows a system having Star ground with separate digital and analog grounds.

System with a split-ground plane

PCB Routing

With all the components properly placed at best locations, with proper ground plane being established, most of the routes will naturally follow the right paths. However, following guidelines should be kept in mind while trace routing:

1. It is better to keep digital circuitry isolated from analog circuitry.
2. Keep high-speed signal paths as straight and short as possible.
3. Layers having high-speed signal paths should have a ground layer adjacent to it to ensure proper return signal.
4. Try to follow the signal path of the schematic.
5. Sensitive high-speed components should be isolated from the Power supply.
6. Power Supply routing should be short, compact and should have wide traces(use zones if possible).
7. All the design rules and constraints should be kept in mind to avoid clearance issues .

At the end of the day, the best mixed signal PCB can be designed using step by step procedure as mentioned above in my suggestion. It is better to plan most of these points roughly in your mind before starting with your layout.

Thank you for reading!!