In many current-sense circuits, the basic approach involves creating a voltage drop across a sense resistor, amplifying that voltage, converting it with an ADC, and then determining the current value. However, complications arise when the voltage at the sense resistor is significantly different from the system’s ground. Traditional solutions often address this by eliminating voltage differences in either the analog or digital domain. But there's another innovative approach: using wireless technology.
High-side current-sense amplifiers operate in the analog domain. These compact ICs have limitations due to semiconductor processes, typically handling voltages below 100V. Their accuracy can also degrade if the common-mode voltage of the sense resistor fluctuates rapidly or swings above and below ground.
Magnetic or optical isolators are commonly used to maintain isolation in the digital domain. They are robust and can withstand high voltages, but they may be bulky and require an isolated power supply. If the sense resistor is physically separated from the main system, long cables might be necessary.
The recent development of low-power signal conditioning and wireless technology has introduced a new solution. By allowing the entire circuit to float with the common-mode voltage of the sense resistor and wirelessly transmitting data, voltage limitations become irrelevant. The sense resistor can be placed anywhere without needing a physical connection. If the circuit consumes very little power, an isolated power supply isn’t even required, enabling operation for years on a small battery.
**Wireless Current Detection**
The current sense circuit shown in Figure 1 uses the LTC2063 chopper-stabilized op amp to amplify the voltage drop across the sense resistor. The AD7988 micropower SAR ADC digitizes the signal and reports the results via SPI. The LTP5901-IPM is a wireless module that automatically creates an IP-based mesh network with nearby nodes. It includes a microprocessor to read the ADC’s SPI port. The LTC3335 nanopower buck-boost regulator converts battery voltage into a stable output and includes a coulomb counter to track battery usage.
Figure 1: A low-power wireless current-sense circuit featuring a chopper-type op amp, a low-power ADC, and a SmartMesh IPTM wireless module. A DC/DC converter regulates the battery and tracks its charge.
**Micropower Zero-Drift Operational Amplifier**
To reduce heat in the sense resistor, the voltage drop is usually limited to 10mV to 100mV. Measuring such small voltages requires low-offset input circuits, like zero-drift op amps. The LTC2063 is a low-power, chopper-stabilized op amp with a maximum supply current of 2μA. Its offset voltage is below 10μV, ensuring accurate measurement of small voltage drops. Figure 2 shows the LTC2063 configured to amplify and level shift the voltage across a 10mΩ sense resistor. The gain is selected to map ±10mV (±1A) to a full-scale range centered on the intermediate supply. This amplified signal goes to a 16-bit SAR ADC, such as the AD7988, which offers low standby current and good DC accuracy. At low sample rates, the ADC powers down between conversions, consuming as little as 10μA at 1ksps. The LT6656 provides a precise 3V reference.
Figure 2: The current sense circuit floats with the sense resistor voltage. The LTC2063 amplifies the sense voltage and biases the ADC with the LT6656 reference.
**Industrial-Strength Wireless Grid**
SmartMesh wireless modules, like the LTP5901-IPM, include radio transceivers, microprocessors, and networking software. When multiple nodes are powered near a network manager, they automatically form a mesh network. Time synchronization ensures each node only powers up during short intervals, reducing energy use. Nodes act as both sensors and routers, creating a reliable, low-power network with multiple paths to the manager.
The LTP5901-IPM contains an ARM Cortex-M3 microprocessor running network software and allows custom firmware for specific applications. In this example, the microprocessor reads the ADC (AD7988) over SPI and the coulomb counter (LTC3335) over I2C. It can also put the LTC2063 op amp into shutdown mode, reducing current consumption from 2μA to 200nA, further saving power between measurements.
**Nanopower Coulomb Counter**
In wireless sensor applications, a single node may consume less than 5μA per second, while the radio transceiver can draw up to 40μA. Power consumption depends on factors like sampling rate and network configuration.
The circuit in this article runs on two alkaline batteries. The LTC3335 nanopower buck-boost converter regulates the battery voltage and includes a built-in coulomb counter. It provides a stable 3.3V output from 1.8V to 5.5V inputs. The LTC3335 has a quiescent current of just 680nA, keeping the circuit extremely low-power when the radio is in sleep mode. It can also deliver up to 50mA, sufficient for radio transmission and other circuits.
Accurate battery monitoring is crucial in high-reliability systems. The LTC3335’s coulomb counter tracks total battery usage, which can be read via I2C to predict when the battery needs replacement.
**Summary**
Linear Technology’s combination of Analog Devices’ signal chain, power management, and wireless networking products enables true wireless current sensing. Figure 3 shows an example: the ultra-low power LTC2063 accurately measures the small voltage drop across the sense resistor. The entire circuit, including the micropower ADC and reference, floats with the common-mode voltage. The LTC3335 nanopower converter powers the circuit for years on a small battery while tracking battery usage. The LTP5901-IPM wireless module manages the application and connects to a reliable SmartMesh IP network.
Figure 3: A complete wireless current-sense circuit on a small board. The only physical connection is a banana jack. The wireless module is on the right, and the circuit is powered by two AAA batteries.
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