Selecting Power over Coax Inductors for ADAS and Infotainment

Designers of automotive systems are increasingly turning to Power over Coax (PoC) architectures to deliver both power and high-speed data over a single cable. This approach reduces wiring bulk and simplifies system design, particularly in data-intensive subsystems such as advanced driver assistance systems (ADAS), infotainment, and sensor-driven powertrains. However, implementing a reliable PoC bias-T circuit requires careful selection of the inductor to manage competing demands for power and data delivery.

This article provides an overview of PoC bias-T circuit design. It then introduces PoC inductors for automotive applications from Murata and shows how they support designers in optimizing bias-T circuits for powertrain, vehicle safety, and infotainment systems. Two useful inductor selection tools will also be described.

The importance of bias-T circuit design for automotive PoC

Bias-T circuits play a crucial role in PoC systems by separating DC power from high-speed data signals. A typical automotive PoC application uses coaxial cables to connect sensors and control units, with bias-T circuits at each end managing the power and signal paths (Figure 1). Inductors in these circuits block high-frequency signals to keep the power supply clean, while capacitors prevent DC voltage from reaching sensitive serializer/deserializer (SerDes) modules.

Figure 1: A typical PoC application uses bias-T circuits at each end of a coaxial cable to separate DC power from high-speed data signals. (Image source: Murata)

Designing the inductive portion of a bias-T circuit requires careful attention to detail. To prevent signal leakage onto the power line, the inductor must maintain high impedance across a broad frequency range. If this impedance is inadequate, residual signal energy can introduce power fluctuations that degrade system performance.

In practice, a single inductor may not provide sufficient signal blocking across all frequencies of interest. As a result, designers often employ multiple inductors, along with their attendant resistors and capacitors, to suppress anti-resonances. Anti-resonances are narrow frequency bands where impedance dips and filtering becomes ineffective. These effects arise from the interaction of multiple components with different frequency responses.

PoC bias-T circuit design challenges

Although multi-inductor configurations can extend the blocking range of a bias-T circuit, they also introduce complexity, with each added inductor increasing the risk of anti-resonance. Managing these interactions requires additional resistors and capacitors, which further increase the component count and design effort.

To reduce this complexity, designers often aim to minimize the number of inductors required. A single, wide-bandwidth inductor reduces the likelihood of anti-resonance effects, improves filtering consistency, and helps conserve valuable board space. The latter is critical in compact automotive subsystems.

The difficulty lies in selecting an inductor with the right mix of impedance, frequency response, and other electrical characteristics. This is challenging in part because PoC applications have diverse requirements. Powertrain, ADAS, and infotainment systems have different mixes of power delivery and data bandwidth demands, and there is considerable variance among application requirements within each of these categories.

A single-inductor approach to PoC bias-T circuit design

To address these needs, Murata has developed a family of inductors engineered explicitly for automotive PoC bias-T circuits. The family encompasses a wide range of performance characteristics, providing designers with the flexibility to meet the needs of various applications.

Each solution in this family delivers high impedance across a broad frequency range, a performance characteristic that would previously require multiple inductors to achieve. This feature enables effective signal blocking while minimizing the size and complexity of the bias-T circuit. Additional features that support automotive design goals include:

• Current ratings up to 1 ampere (A): Suitable for powering high-current loads such as sensors and actuators in powertrain and ADAS systems
• High current saturation: Maintains inductance under load, preventing performance degradation in the presence of large magnetic fields
• Low DC resistance: Reduces losses when separating power and signal, improving overall power efficiency
• Shielded construction: Minimizes electromagnetic interference (EMI), supporting the reliable operation of surrounding circuitry
• A compact form factor: Saves board space and enables use in dense automotive layouts
• A -40°C to +125°C operating temperature range: Supports performance in harsh automotive environments
• AEC-Q200 qualification and RoHS compliance: Meets automotive and environmental standards

Together, these features make the Murata PoC inductor family well suited for a range of applications, including automotive safety systems, infotainment, and powertrain control.

PoC inductor for powertrain applications

Powertrain subsystems often involve sensor-driven communications with relatively low data rates. For these applications, a high-inductance solution such as the LQW43FT180M0HL (Figure 2) can help ensure stable power delivery. This device’s 18 µH rating provides high impedance at low frequencies, effectively blocking unwanted signal content. This performance holds up through its self-resonant frequency of 40 MHz, which aligns well with the frequency range of typical powertrain data streams.

Figure 2: The LQW43FT180M0HL is a compact, wide-bandwidth inductor that serves the role of several inductors within automotive PoC bias-T circuits. (Image source: Murata)

The inductor is rated at 600 milliamperes (mA), making it suitable for many peripheral subsystems within the powertrain. Its low DC resistance of 160 mΩ helps minimize power loss, while the 1812 package, measuring 4.5 mm × 3.2 mm × 3.7 mm, offers a compact alternative to multi-inductor designs.

PoC inductor for ADAS applications

ADAS applications must handle significantly higher data rates from sensors, such as high-resolution cameras. The LQW32FT2R2M0HL (Figure 3) is a medium-inductance 2.2 µH solution designed for this purpose. Its self-resonant frequency of 200 MHz enables effective signal blocking for high-bandwidth communications.

Figure 3: The LQW32FT2R2M0HL is designed for PoC applications, offering both high power and high data rates. (Image source: Murata)

In addition to high data rates, ADAS subsystems often have significant power requirements. This inductor’s 1 A rating is designed to serve these higher-power needs, including LiDAR sensors or the actuators used in automated lane-keeping systems. It is housed in a 1210 package measuring 3.2 mm × 2.5 mm × 2.5 mm.

PoC inductor for infotainment applications

Infotainment systems typically operate at high data rates but draw relatively little power. The LQW21FT2R0M0HL (Figure 4) offers a compact solution tailored to these needs. With a 2 µH inductance and a self-resonant frequency of 230 MHz, it provides effective signal blocking in the frequency range commonly used by high-speed audio, video, and navigation data streams.

Figure 4: The LQW21FT2R0M0HL supports high-data-rate automotive PoC. (Image source: Murata)

The inductor is rated for 400 mA, making it suitable for lower-power endpoints such as infotainment displays and multimedia control modules. It is housed in an 0805 package measuring just 2.0 mm × 1.2 mm × 1.8 mm, making it well suited for applications where space on the board is limited.

Useful tools for selecting bias-T inductors

Designing and characterizing a bias-T circuit can be complex, particularly when balancing electrical performance against size and system constraints. To streamline the process, Murata offers two free online tools that help engineers evaluate and select suitable inductors for PoC applications.

The first is the Bias-T Inductor Design Support Tool (Figure 5). This tool enables designers to input parameters such as DC current, ambient temperature, and cable characteristics, allowing them to generate a complete bias-T configuration. The tool automatically recommends inductors along with appropriate resistors and capacitors, and also provides simulated performance plots, including S-parameters and impedance curves, for deeper insight into circuit behavior.

Figure 5: The Bias-T Inductor Design Support Tool streamlines automotive PoC inductor selection and bias-T circuit characterization. (Image source: Murata)

In addition to simulating the bias-T circuit, drop-down menus allow users to select specific reference criteria for calculations, including different communication protocols and cable parameters. An option to account for stray capacitances is also included for enhanced understanding of real-world performance.

For more general exploration, Murata’s inductor selection tool (Figure 6) features a part number search function and the ability to filter by application, electrical characteristics, and size. Once a selection has been made, the user can simulate the frequency characteristics of the inductor as well as S-parameters for series and shunt configurations. This eliminates time spent combing through datasheets and product pages to find suitable components.

Figure 6: The inductor selection tool enables fast evaluation of inductor performance for a wide range of applications. (Image source: Murata)

Conclusion

The challenge of designing a complex bias-T circuit for PoC can be significantly simplified by using Murata’s wide-bandwidth inductors, each of which can replace a circuit that previously required multiple components, enabling space savings and greater system stability. By providing a family of application-specific inductors and powerful online design tools, Murata removes significant barriers, making it easier for engineers to develop and adopt PoC technology.