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Wireless Current Sensing made possible with a Micropower, Zero-Drift Op Amp

Editor’s note: I really like it when an neat alternative solution is provided for a challenging analog problem. Chris Lokere do that for us in this following tech blog. Now that Linear Technology is a part of Analog Devices, we have here a virtually complete solution that has been tested and verified by the combined teams of engineers with components from both of these companies including power management with an integrated coulomb counter, wireless , precision reference, data conversion and precision analog.

Many current sense circuits follow the same simple recipe: develop a voltage drop across a sense resistor: amplify the voltage, read it with an ADC, and now you know the current. But if the sense resistor is at a voltage that is very different from system ground, things can quickly get complicated. Typical solutions bridge the voltage difference in either the analog or digital domain. But here is a different approach – wireless.

High-side current sense amplifiers operate in the analog domain. The ICs are compact, but the voltage difference that they can withstand is limited by semiconductor processes. Devices rated for more than 100V are rare. And these circuits often lose accuracy if the sense resistor common mode voltage changes quickly or swings both above and below system ground.

Magnetic or optical isolators usually break the isolation barrier in the digital domain. The hardware can be a bit more bulky, but works without loss of accuracy and can typically withstand thousands of volts. These circuits need an isolated power supply but that can sometimes be integrated in the isolator component. If the sense resistor is physically separated from the main system, you may also need to run long wires or cables.

Recent low-power signal conditioning and wireless technologies offer a new approach. By allowing the entire circuit to float with the common mode of the sense resistor, and transmitting the measured data wirelessly over the air, there are no voltage limitations. The sense resistor can be located anywhere, without the need to run cables. If the circuit is very low power, then you don’t even need an isolated power supply and can instead run for many years from a small battery.

Wireless Current Sense

The current sense circuit in Figure 1 employs on the LTC2063 chopper-stabilized op amp to amplify the voltage drop across a sense resistor. The micropower SAR ADC AD7988 digitizes the value and reports the result via an SPI interface. The LTP5901-IPM is the wireless module that automatically forms an IP-based mesh network with other nearby nodes. It also has a built-in microprocessor which reads the AD7988 ADC SPI port. The LTC3335 is a nanopower buck-boost regulator which converts the battery voltage to a constant output voltage. The nanopower buck-boost regulator also includes a Coulomb counter which reports cumulative charge pulled from the battery.

Figure 1

A low-power wireless current sense circuit is formed by a low power chopper op amp to amplify the sense voltage, is digitized using a low power ADC and reference, and is connected to a SmartMesh IP™ wireless radio module. A low-power DC/DC converter conditions the battery and keeps track of the charge drawn from the battery.

A low-power wireless current sense circuit is formed by a low power chopper op amp to amplify the sense voltage, is digitized using a low power ADC and reference, and is connected to a SmartMesh IP™ wireless radio module. A low-power DC/DC converter conditions the battery and keeps track of the charge drawn from the battery.

Micropower Zero-Drift Op Amp

To minimize heat dissipation in sense resistors, the voltage drop is typically limited to 10mV-100mV. To measure this requires an input circuit with low offset errors, such as a zero-drift op amp. The op amp is an ultralow power, chopper-stabilized op amp with a maximum supply current of 2µA. Because the offset voltage is less than 10µV, it can measure even very small voltage drops without loss of accuracy. Figure 2 shows the chopper-stabilized op amp configured to amplify and level shift the voltage across a 10mΩ sense resistor.

Figure 2

The current sense circuitry floats with the sense resistor voltage. The chopper-stabilized op amp amplifies the sense voltage and biases it mid-rail for the ADC. The LT6656-3 provides the precision 3V reference.

The current sense circuitry floats with the sense resistor voltage. The chopper-stabilized op amp amplifies the sense voltage and biases it mid-rail for the ADC. The LT6656-3 provides the precision 3V reference.

The gain is selected so that +/-10mV full-scale at the sense resistor (corresponding to +/-1A of current) maps to a near full-scale range at the output, centered around mid-supply. This amplified signal is fed into a 16-bit SAR ADC. The ADC was selected for its very low standby current and good DC accuracy. At low sample rates, the ADC automatically shuts down in between conversions, resulting in average current consumption as low as 10µA at 1ksps. The precision voltage reference consumes less than 1µA, and biases the amplifier, the level-shift resistors and the ADC’s reference input.

Industrial-Strength Wireless Mesh

SmartMesh wireless modules such as LTP5901-IPM include the radio transceiver, embedded microprocessor, and networking software. When multiple SmartMesh motes are powered up in the vicinity of a network manager, the motes automatically recognize each other and form a wireless mesh network. All motes in a network are automatically time-synchronized, which means that each radio is only powered on during very short, specific time intervals. As a result, each node can function as a source of sensor information, as well as a routing node to relay data from other nodes toward the manager. This creates a highly reliable, low power mesh network, where multiple paths are available from each node to the manager, even though all nodes, including the routing nodes, operate on very low power.

This SmartMesh IP Node Wireless Mote Module includes an ARM Cortex-M3 microprocessor core which runs the networking software. In addition, users may write application firmware to perform tasks specific to the user application. In this example, the microprocessor inside the wireless module reads the SPI port of the current measurement ADC (AD7988) and reads the I2C port of the Coulomb counter (LTC3335). The microprocessor can also put the chopper op amp in shutdown mode, further reducing its current consumption from 2µA to 200nA. This provides additional power savings in use models with extremely long intervals between measurements.

Nanopower Coulomb Counter

Typical power consumption for a mote reporting once per second is less than 5µA for the measurement circuit and can be 40µA for the wireless radio. In practice, power consumption depends on various factors, such as how often the signal chain takes a reading, and how the nodes are configured in the network.

The example circuit is powered from two alkaline primary battery cells. The battery input voltage is regulated by the nanopower buck-boost converter with integrated Coulomb counter module. It can provide a regulated 3.3V output from an input supply between 1.8V and 5.5V. Load current in duty-cycled wireless applications can vary from 1µA to 20mA, depending on whether the radio is in active or sleep mode. This IC has a quiescent current of just 680nA at no load, which keeps the entire circuit very low power when the radio and signal chain are in sleep mode. Still, this device can output as much as 50mA, which provides enough power during radio transmit/receive and for a variety of signal chain circuits.

In high-reliability wireless sensor deployments it is not acceptable for batteries to ever run out. At the same time, replacing batteries too often incurs unwanted cost and downtime. The upshot: Accurate battery drain circuitry is needed. The LTC3335 has a built-in Coulomb counter as a bonus. Whenever the regulator switches, it keeps track of the total charge that it draws from the battery. This information can be read out using an I2C interface, and can then be used as a predictor of the timing for battery replacement.

Summary

Combining Linear Technology and Analog Devices signal chain, power management, and wireless networking products enables the design of a truly wireless current sense circuit. Figure 3 shows an example implementation. The new ultralow power chopper op amp can accurately read small voltage drops across a sense resistor. The entire circuit, including micropower ADC and voltage reference, floats with the common mode of the sense resistor.

Figure 3

A complete wireless current sense circuit is implemented on a small circuit board. The only physical connections are the banana jacks for the current to be measured. The wireless radio module is shown on the right. The circuit is powered from two AAA batteries connected on the back of the board.

A complete wireless current sense circuit is implemented on a small circuit board. The only physical connections are the banana jacks for the current to be measured. The wireless radio module is shown on the right. The circuit is powered from two AAA batteries connected on the back of the board.

The nanopower switcher can power the circuit for years from a small battery, while reporting cumulative battery usage with its built-in Coulomb counter. The wireless module manages the entire application and automatically connects to a highly reliable SmartMesh IP network.

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