Understanding the Differences Between One and Two Copper Ground Planes in PCB Design

Printed circuit board (PCB) design is crucial for ensuring reliable and efficient electronic circuits. One of the key aspects of PCB design is the layout and configuration of ground planes. This article explores the differences between having a single copper ground plane and a dual ground plane setup, and the benefits and implications of each approach.

The Role of Ground Planes in PCBs

The ground plane in a PCB serves as a common reference point for all circuit components, helping to reduce noise, improve signal integrity, and ensure stable power delivery. When several ground planes are used, it typically results in a decreased impedance between each node and the ground, which in turn reduces cross-talk noise caused by ground currents.

Advantages of Dual Ground Planes

Hybrid/Mixed Circuits: In hybrid/mixed circuits where both digital and analog components coexist, the use of two ground planes offers significant advantages. Each plane can be designated for either digital or analog signals, thereby reducing potential interference between the two signal types. For instance, in biomedical instrumentation, low-level analog signals are particularly sensitive to ground noise. By separating digital and analog grounds, high ground current spikes from digital devices can be isolated from low-level analog signals, maintaining signal integrity.

Star Ground Connection

Although having two separate ground planes enhances performance, it is essential to ensure proper connection between them. A Star Ground connection is a common method to achieve this, involving a central point that connects all separated ground paths. This central point, often referred to as a "star ground connection," helps ensure that no floating ground planes exist. For a visual representation, refer to the diagram below.

Applications in High-Frequency Circuits

In high-frequency circuits such as RF or microwave circuits, the configuration of the ground planes is critical for defining the characteristic impedance of transmission lines. These lines can be either striplines or microstrips, each requiring a specific number of ground planes.

Stripline vs. Microstrip

Stripline requires two ground planes (each in green) to shield the outside world from the electromagnetic (EM) field generated by the signal inside the dielectric. This setup provides better isolation against external interference.

Microstrip uses only one ground plane, which means it 'contaminates' half of the world with the EM field. The characteristics of both stripline and microstrip, including impedance and propagation delay, depend on the dielectric material (such as FR4) and the dimensions/separation between the structures.

For example, the characteristic impedance in ohms and the propagation delay in picoseconds per inch for microstrip can be calculated using the following formula:

characteristic impedance Z0 87 / √ (er) Ω propagation delay td 1 / (60 × er0.5) ps/inch

These formulas provide a general guideline for designing high-frequency circuits where precise control over signal behavior is crucial.

General Advice for PCB Designers

Based on these considerations, it is generally advisable to implement multiple ground and power planes if sufficient metal layers are available. This approach ensures better signal integrity, reduced noise, and enhanced overall circuit performance.

Conclusion

Choosing between a single copper ground plane and a dual ground plane setup in PCB design depends on the specific requirements and applications of the circuit. Dual ground planes offer improved performance in hybrid circuits, and proper implementation through techniques like star grounding ensures optimal signal integrity. Understanding the implications of these design choices is essential for achieving reliable and efficient electronic circuit designs.