Electromagnetic Compatibility Consulting and Training
Grounding of Mixed Signal PCBs
A question that I hear often is: How do I prevent digital logic ground currents from contaminating my low level analog circuitry? This is a good question without a simple answer. Most A/D converter manufacturer's data books and application notes provide little if any useful information on the subject. If they do provide information, it is usually only applicable to a simple system containing only one A/D converter.
The Split Ground Plane Approach
Some people suggest splitting the ground plane in order to isolate the digital ground currents from the analog ground currents. Although the split plane approach can be made to work, it has many potential problems especially in large complicated systems. Some of these problems include:
- Inability to route traces over the split in the plane.
- Creation of big loops, resulting in radiation and high ground inductance.
If you must split the ground plane and run traces across the split, it should be done in a way that provides a return path for the current directly underneath each of the traces. This can be achieved by connecting the planes together at one point (a bridge) and routing all traces so that they cross at this bridge point. By doing this, you will minimize the loop area and ensure that high-frequency digital ground return currents remain in the digital section of the board and do not interfere with the analog signals.
A Single Ground Plane with Partitioning
Instead of using a split ground plane, another approach is to use a single ground plane and partition the PCB into digital and analog routing sections. Analog signals are then routed only in the analog section, and digital signals are routed only in the digital section. This approach, when combined with proper routing discipline, can solve layout problems without the additional issues caused by a split ground plane.
Additional Considerations
While some A/D converter manufacturers recommend connecting both the AGND and DGND pins to the analog ground plane, this approach may create additional problems. It is important to carefully consider the ground connections and the return paths for the current flow in order to optimize the board layout and reduce the risk of noise interference in the circuit.
To test the effectiveness of a single ground plane, you can conduct an experiment by layouting a board with a split ground plane but connecting the planes together with jumpers or zero ohm resistors every 1/2 inch. Properly route the board with digital traces only in the digital section and analog traces only in the analog section. Then, compare the functionality and EMC performance of the board with the single ground plane connected and disconnected. In most cases, you will find that the single ground plane provides better results.
Summary
Partition your PCB with separate analog and digital sections. Do not split the ground plane. Use one solid ground plane under both analog and digital sections of the board. Route digital signals only in the digital section of the board. This applies to all layers. Route analog signals only in the analog section of the board. This applies to all layers. The key to a successful PCB layout is the use of routing discipline.
By following these guidelines, you can optimize the layout of your mixed signal PCB and minimize the risk of interference between analog and digital signals.
When it comes to optimizing the performance of your electronic devices, it's essential to consider the layout and partitioning of your PCB. Electromagnetic compatibility is crucial in ensuring that your analog and digital signals do not interfere with each other. By utilizing a single ground plane and employing proper routing discipline, you can successfully separate and protect your analog and digital circuitry. This approach minimizes the risk of noise interference and enhances the overall functionality and EMC performance of your board.