By Christopher Nunes
In the world of industrial communication, mastering voltage domain isolation is the key to building safer, smarter systems. Last month, we explored various transceiver architectures used to isolate field-bus channels. Understanding the trade-offs between these architectures is essential for selecting the most suitable option for your application. Today, we’ll dive into the trade-offs between two common voltage domain structures—but first, let’s examine why further separation of ground reference planes is critical in electrical systems.
Why Isolating Ground Reference Planes Matters
Safety is the primary reason for isolating multiple field-bus channels. Transients such as electrostatic discharge (ESD), electrical fast transients (EFTs), and surge events frequently occur on field-bus lines. When channels share a common ground reference, these transients can easily couple from one I/O port to another, potentially propagating across the system and damaging sensitive electronics.
By implementing multiple isolated voltage domains, you effectively break up the supply and ground reference planes. This segmentation acts as a barrier, preventing transients from permeating throughout the system and enhancing overall system robustness.
Preventing Ground Loops in Distributed Systems
A second key reason for isolating voltage domains is to prevent ground loops — especially in systems with field-bus channels extending to remote locations. As discussed in our first blog post on isolation, floating ground reference planes help eliminate harmful ground loop currents.
Consider a factory automation setup: a centralized control node communicates with machines distributed across the facility. If all devices share a common ground, differences in potential can create ground loops, leading to erratic behavior or equipment damage. Isolating the grounds at the control node ensures that imbalances don’t result in harmful ground loops.
Figure 1: Shared Voltage Domain vs. Multiple Voltage Domain Configuration
Choosing the Right Voltage Domain Structure for Field-Bus Isolation
When designing isolated field-bus systems, selecting the appropriate voltage domain structure is a critical decision that impacts system performance, safety, and complexity. Let’s explore two common approaches—Shared Voltage Domains and Multiple Voltage Domains—and the trade-offs associated with each.
Shared Voltage Domain
The shared voltage domain is the most straightforward to visualize. It consists of two reference planes: the logic side and the bus side. This setup supports any transceiver architecture, making it highly flexible. The key design consideration here is how to power the isolated side.
Figure 2: Shared Voltage Domain—Power Options
Powering Options
- Integrated isoPower transceivers offer maximum ease of use, simplifying design and reducing component count. However, they may not be the most power-efficient solution.
- External transformers provide higher power efficiency, especially when a higher voltage supply is available on the logic side. Designers can choose between:
- A transceiver with an integrated transformer driver
- Standard isolated transceivers paired with an external transformer driver
Trade-Offs
- Simple and easy to implement
- Vulnerable to overstress events—transients on one field-bus channel can couple onto others, potentially damaging downstream electronics
Multiple Voltage Domains
Multiple isolated voltage domains are ideal for applications where safety and noise immunity are top priorities. Contrary to popular belief, this doesn’t mean each field-bus channel must have its own domain—channels that communicate with the same physical area can share a domain.
Figure 3: Multiple Voltage Domain—Power Options
Powering Options
- Integrated isoPower transceivers remain the easiest option for powering isolated sides.
- External transformers are more efficient but introduce complexity:
- Multiple transformer structures consume significant PCB space
- Custom multi-winding transformers are costly and difficult to source
Trade-Offs
- Highest robustness against transients and ground loops
- Increased PCB space requirements, design complexity, and cost
Making the Right Choice
Choosing the correct voltage domain structure depends on your system’s isolation needs:
- If you only need isolation between I/O ports and the controller, a shared voltage domain is likely sufficient.
- If you're concerned about transients coupling between ports or require maximum safety, multiple isolated domains are the better choice.
This table should help you quickly assess which voltage domain structure best suits your application.
| Feature/Trade-Off | Shared Voltage Domain | Multiple Voltage Domains |
| Isolation Scope | Between controller and I/O ports | Between all I/O ports and controller; ports isolated from each other |
| Ease of Design | Very simple and straightforward | More complex due to multiple domains |
| Powering Options |
- Integrated isoPower (easy, less efficient) - External transformer (efficient, flexible) |
- Integrated isoPower (easy) - External transformer (efficient but complex and space-consuming) |
| PCB Space Requirements | Minimal | Higher due to multiple power domains and transformers |
| Safety & Robustness | Basic isolation; vulnerable to transient coupling | Maximum robustness; excellent protection from transients and ground loops |
Table 1: Voltage Domain Structure Quick Assessment
Conclusion
In the complex world of field-bus systems, robust voltage domain isolation isn't just a technical detail — it's the bedrock of system reliability and safety. By understanding the distinct advantages and trade-offs of both shared and multiple isolated voltage domains, you're now equipped to make informed decisions that prevent ground loops, mitigate transients, and ensure seamless operation. Choose wisely, and empower your designs with the precision and resilience they deserve.
See the blogs in the TranscendingConventionalFieldBus series.