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Signal conditioning


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-


Figure 2. The current sense circuitry floats with the sense resistor voltage. The LTC2063 chopper op amp amplifies the sense voltage and biases it mid-rail for the AD7988 ADC. The LT6656-3 provides the precision 3V reference


also the networking firmware needed to automatically form an IP-based mesh network. In addition, the LTP5901-IPM has a built-in microprocessor that reads the AD7988 ADC SPI port. The LTC3335 is a low power, dc-to-dc power supply that converts the battery voltage to a constant output voltage. The LTC3335 also includes a coulomb counter that reports the cumulative charge pulled from the battery.


Signal Chain


The LTC2063 is an ultralow power, chopper- stabilised 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 10 µV, it can measure 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 the ±10mV full-scale input 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 of 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 dropout, 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 per cent 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 per cent 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 per cent, 10ppm/°C specifications of the LT6656 voltage reference.


Power ManageMent


The LTC3335 is a nanopower buck-boost converter with an integrated coulomb counter. It is configured to provide a regulated 3.3V output from an input supply between 1.8V and 5.5V. This


Instrumentation Monthly January 2019


Figure 3. A complete wireless current sense circuit is implemented on a small PCB. 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.


43


allows the circuit to be powered by 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 680 nA at no load, which keeps the entire circuit operating at 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. 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.


wireleSS networking


The LTP5901-IPM is a complete wireless radio module that includes the radio transceiver, embedded microprocessor, and SmartMesh IP networking software. The LTP5901-IPM performs two functions in this application: wireless networking and housekeeping functions (processes). 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-synchronised, 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 toward the manager.


M3 microprocessor core that 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.


ConCluSion


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 LTC2063 chopper op amp can accurately read small voltage drops across a sense resistor. The entire circuit, including the micropower ADC and voltage reference, floats with the common mode of the sense resistor. The nanopower 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.


Analog Devices www.analog.com


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