Measuring the current that flows through a sense resistor seems easy. Amplify the voltage, read it with an ADC, and now you know what the current is. But it gets more difficult if the sense resistor itself sits at a voltage that is very different from system ground. Typical solutions bridge that voltage difference in either the analog or digital domain. But here is a different approach – wireless.
Analog current sense ICs are compact solutions, but the voltage difference that they can withstand is limited by semiconductor processes. It is difficult to find devices that are rated for more than 100V. And these circuits often lose accuracy if the sense resistor common mode voltage changes quickly or swings both above and below system ground.
Digital isolation techniques (magnetic or optical) are a bit more bulky, but work 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 then you also may need to run long wires or cables.
A wireless current sense circuit overcomes many of these limitations. 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 at all. 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.
Figure 1 shows the block diagram of the design. The current sense circuit is based 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 SPI interface. The LTP5901-IPM is the radio module which contains not only the radio, but also the networking firmware needed to automatically form an IP-based mesh network. In addition, the LTP5901-IPM has a built-in microprocessor which reads the AD7988 ADC SPI port. The LTC3335 is a low-power DC/DC power supply which converts the battery voltage to a constant output voltage. The LTC3335 also includes a Coulomb counter which reports cumulative charge pulled from the battery.
The LTC2063 is an ultra-low power, chopper-stabilized op amp. With a maximum supply current of 2µA, it is uniquely suited for use in battery powered applications. Because the offset voltage is less than 10µV, it can measure even very small voltage drops without loss of accuracy. Figure 2 shows the LTC2063 configured to gain-up and level shift the voltage across a 10mΩ sense resistor. The gain is chosen 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 1.5V. This amplified signal is fed into a 16-bit SAR ADC. The AD7988 was chosen for its very low standby current and good DC accuracy. At low sample rates, the ADC automatically shuts-down in between conversions, resulting in an average current consumption as little as 10µA at 1ksps. The LT6656 biases the amplifier, the level-shift resistors and the ADC’s reference input. The LT6656 voltage reference consumes less than 1µA, and can drive up to 5mA loads with low drop-out, making it easy to output a precise 3V, even when powered from the 3.3V system supply.
There are three roughly equal sources to offset error in this signal chain, together contributing about 0.5% relative to a ±10mV full-scale input. They are the offset voltage of the LTC2063 and AD7988, as well as mismatch in the level-shift resistors (0.1% resistors are recommended). A one-point calibration step could largely eliminate that offset. Gain error is generally dominated by inaccuracies in available sense resistors, which tend to be worse than the 0.05%, 10ppm/°C specifications of the LT6656 voltage reference.
The LTC3335 is a nano-power Buck-Boost converter with integrated Coulomb counter. It is configured to provide a regulated 3.3V output from an input supply between 1.8V and 5.5V. This allows the circuit to be powered by, for example, two Alkaline primary battery cells. For duty-cycled wireless applications, the load current can easily vary from 1µA to 20mA, depending on whether the radio is in active or sleep mode. The LTC3335 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, the LTC3335 can output as much as 50mA, which easily provides enough power during radio transmit/receive and for a variety of signal chain circuits.
The LTC3335 also has a handy built-in Coulomb counter feature. When switching, 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 for when it may become time to replace the batteries.
The LTP5901-IPM is a complete wireless radio module, which includes the radio transceiver, embedded microprocessor, and SmartMesh IP networking software. The LTP5901-IPM performs two functions in this application: wireless networking and housekeeping microprocessor. When multiple SmartMesh IP motes are powered up in the vicinity of a network manager, the motes automatically recognize each other and form a wireless mesh network. The entire network is 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 not only as a source of sensor information, but also as a routing node to relay data from other nodes towards 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.
The LTP5901-IPM 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 LTP5901-IPM 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 (LTC2063) 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.
Overall Power Consumption
The total power consumption of the complete application circuit depends on various factors, including how often the signal chain takes a reading, and how the nodes are configured in the network. 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, allowing years of operation on small batteries.
Combining Linear Technology and Analog Devices signal chain, power management, and wireless networking products enable the design of a truly wireless current sense circuit. Figure 3 shows an example implementation. The new ultra-low power LTC2063 chopper op amp can accurately read small voltage drops across a sense resistor. The entire circuit, including micro-power ADC and voltage reference, floats with the common mode of the sense resistor. The nano-power LTC3335 switcher can power the circuit for years from a small battery, while reporting cumulative battery usage with its built-in Coulomb counter. The LTP5901-IPM wireless module manages the entire application and automatically connects to a highly reliable SmartMesh IP network.