Wireless current detection circuit design overview and advantages

The process of measuring current through a shunt resistor might seem straightforward at first glance—amplify the voltage and read it with an ADC to determine the current. However, things become more complex when the voltage across the shunt resistor is far from the system ground. In such cases, traditional methods face significant challenges. A common solution involves eliminating the voltage difference either in the analog or digital domain. But here’s a different approach: wireless sensing. This method offers a unique way to overcome many of the limitations of conventional techniques. Analog current-sense ICs are compact but have limited voltage tolerance due to semiconductor constraints. Most devices can't handle more than 100V, making them unsuitable for high-voltage applications. These circuits also struggle with rapid changes or large fluctuations in the common-mode voltage of the shunt resistor. Digital isolation solutions, like magnetic or optical isolators, offer better performance and higher voltage withstand capabilities. They provide precise measurements but tend to be bulky and require isolated power supplies. In some cases, they can be integrated into the isolator itself, which helps reduce complexity. Wireless current sensing eliminates many of these issues. The entire circuit can float along with the common-mode voltage of the shunt resistor, and data can be transmitted wirelessly without any physical connections. This allows the shunt resistor to be placed anywhere, without the need for long cables. With ultra-low power consumption, even a small battery can power the system for years. Figure 1 shows a block diagram of the design. It includes a low-power chopper-stabilized op-amp (LTC2063) to amplify the voltage drop across the shunt resistor. A micropower SAR ADC (AD7988) digitizes the signal and communicates via SPI. A SmartMesh IP radio module (LTP5901-IPM) handles wireless communication and networking. A low-power DC-DC converter (LTC3335) powers the circuit and includes a coulomb counter to track battery usage. The LTC2063 is ideal for battery-powered systems, offering ultra-low power consumption and high accuracy. Its offset voltage is less than 10μV, ensuring reliable measurements even for very small signals. The AD7988 provides excellent DC accuracy and low power consumption, making it suitable for low-rate sampling applications. Power management is handled by the LTC3335, which efficiently converts battery voltage to a stable output while monitoring charge consumption. The LTP5901-IPM integrates a microprocessor, radio transceiver, and SmartMesh IP firmware, enabling seamless wireless communication and network formation. In this setup, the wireless node not only measures current but also acts as a relay, contributing to a robust and energy-efficient mesh network. The microprocessor inside the LTP5901-IPM manages the ADC, coulomb counter, and op-amp, optimizing power use during idle periods. The total power consumption of the system is extremely low, with the measurement circuit using less than 5μA and the radio consuming about 40μA. This makes it possible to run on small batteries for extended periods. Figure 3 shows a complete wireless current-sensing circuit implemented on a small PCB. The only physical connection is a banana jack for the current input. The radio module is visible on the right, and the circuit is powered by two AAA batteries. This design demonstrates how combining advanced signal chain components, efficient power management, and wireless networking enables true wireless current sensing. It's a powerful solution for applications where traditional methods fall short. Author: Kris Lokere is a strategic application manager for signal chain products, joining ADI after the merger with Linear Technology. He specializes in integrating multiple product lines for system-level designs. Over the past 20 years, he has been involved in amplifier design, team building, and product strategy. Kris holds several patents and has a master’s degree in electrical engineering from the University of Leuven and an MBA from Babson College.

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