Circuit Selection | From signal chain, power management to wireless networking, the real wireless current detection circuit is like this
Measuring the current through a sense resistor seems simple enough—
Amplify the voltage and read it with ADC, and you will know the current.
However, if the voltage across the sense resistor itself is far from the system ground voltage, sensing becomes more difficult. The typical solution is to eliminate the voltage difference in the analog or digital domain. But here is a different approach - wireless.
Analog current-sensing ICs are compact solutions, but the voltage differences they can tolerate are limited by semiconductor processes. It is difficult to find devices rated for voltages greater than 100 V. These circuits cannot accurately measure if the sense resistor common-mode voltage changes rapidly or swings above or below the system ground voltage.
Digital isolation techniques (magnetic or optical) are somewhat bulky but operate with high accuracy and can typically withstand thousands of volts. These circuits require an isolated power supply, but this can sometimes be integrated into the isolator. If the sense resistor is physically separated from the main system, long wires or cables may also be required.
Wireless current sensing circuits overcome many of these limitations. By letting the entire circuit float along with the common-mode voltage of the sense resistor and transmitting the measurement data wirelessly over the air, voltage limitations are eliminated. The sense resistor can be located anywhere, and no cables need to be run. If the circuit power consumption is very low, then an isolated power supply is not even required and a small battery can keep it running for years.
Figure 1 shows the block diagram of the design. The current sensing circuit is based on the chopper-stabilized op amp LTC2063, which is used to amplify the voltage drop across the sense resistor. The micropower SAR ADC AD7988 digitizes the value and reports the result through the SPI interface. The LTP5901-IPM is the radio module that contains not only the radio but also the networking firmware required to automatically form an IP mesh network.
In addition, the LTP5901-IPM has a built-in microprocessor to read the AD7988 ADC SPI port. The LTC3335 is a low power DC/DC power supply that converts the battery voltage into a constant output voltage. The LTC3335 also contains a coulomb counter to report the accumulated charge drawn from the battery.
Figure 1: The low-power wireless current sensing circuit consists of a low-power chopper op amp to amplify the sense voltage, digitized using a low-power ADC and reference voltage source, connected to the SmartMesh IP radio module. A low-power DC/DC converter conditions the battery while recording the charge drawn from the battery.
The LTC2063 is an ultralow power, chopper-stabilized op amp. With a maximum supply current of 2µA, it is particularly suitable for battery-powered applications. The offset voltage is less than 10µV, so it can measure very small voltage drops without losing accuracy. Figure 2 shows the LTC2063 configured to amplify and level-shift the voltage across a 10mΩ sense resistor. The gain is chosen so that the ±10mV full-scale range of the sense resistor (corresponding to ±1A current) is mapped to a close 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 because of its very low standby current and good DC precision. At lower sampling rates, the ADC automatically shuts down between conversions, consuming an average of as little as 10µA at 1ksps. The LT6656 is used to bias the amplifier, level-shift resistors, and ADC reference input. The LT6656 reference consumes less than 1µA, can drive up to 5mA loads, and has a low dropout voltage, so it can easily output a precision 3V even from a 3.3V system supply.
There are three roughly equal sources of offset error in this signal chain, which together contribute approximately 0.5% error relative to a ±10mV full-scale input. This includes the offset voltages of the LTC2063 and AD7988, as well as the mismatch of the level-shifting resistors (0.1% resistors are recommended). A single-point calibration can largely remove this offset. Gain error is generally dominated by the inaccuracy of the available sense resistors, which are often worse than the 0.05%, 10ppm/°C specification of the LT6656 voltage reference.
Figure 2: Current sensing circuit with sense resistor voltage floating. The LTC2063 chopper op amp amplifies the sense voltage and biases it to the midrail of the AD7988 ADC. The LT6656-3 provides a precision 3V reference.
The LTC3335 is a nanopower buck-boost converter with an integrated coulomb counter. It is configured to generate a regulated 3.3V output from an input supply ranging from 1.8V to 5.5V. This allows the circuit to be powered from a source such as two alkaline primary batteries. For duty-cycled wireless applications, the load current can easily vary from 1µA to 20mA, depending on whether the radio is in active mode or sleep mode. The LTC3335 consumes only 680nA of quiescent power at no load, so the entire circuit consumes very little power when the radio and signal chain are in sleep mode. In addition, the LTC3335 can output up to 50mA of current, which can easily provide enough power during radio transmit/receive, making it suitable for a variety of signal chain circuits.
The LTC3335 also has a convenient internal coulomb counter. When switching, it records the total charge drawn from the battery. This information can be read out via the I2C interface and used to predict when the battery needs to be replaced.
The LTP5901-IPM is a complete radio module that includes a radio transceiver, an embedded microprocessor, and SmartMesh IP networking software. The LTP5901-IPM performs two functions in this application: wireless networking and management microprocessor. When multiple SmartMesh IP motes are powered on near the network manager, they 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 for very short, specific time intervals. Therefore, each node is not only a source of sensor information, but also acts as a routing node to pass data from other nodes to the manager. This forms a high-reliability, low-power mesh network with multiple paths from each node to the manager, but all nodes (including routing nodes) operate at very low power consumption.
The LTP5901-IPM includes an ARM Cortex-M3 microprocessor core that runs networking software. In addition, the user can write application firmware to accomplish tasks specific to the user's application. In this example, the microprocessor inside the LTP5901-IPM reads the SPI port of the current measurement ADC (AD7988) and the I2C port of the coulomb counter (LTC3335). The microprocessor can also put the chopper op amp (LTC2063) into shutdown mode, further reducing its power consumption from 2µA to 200nA. This can save even more power in applications where the measurement interval is extremely long.
The total power consumption of the complete application circuit depends on many factors, including how often the signal chain takes readings, how the nodes are configured in the network, etc. For a mote reporting once per second, the measurement circuitry will typically consume less than 5µA, and the radio might typically consume 40µA, allowing it to operate for years on a very small battery.
Figure 3: A complete wireless current sensing circuit implemented on a small PCB. The only physical connection is the banana jack for the current to be measured. The radio module is shown on the right. The circuit is powered by two AAA batteries connected to the back of the board.
The combination of Linear Technology and Analog Devices signal chain, power management, and wireless networking products allows the design of a truly wireless current sensing circuit. Figure 3 shows an implementation example. The new ultra-low power chopper op amp, the LTC2063, can accurately read the small voltage drop across the sense resistor. The entire circuit, including the micropower ADC and reference, floats with the common-mode voltage of the sense resistor. With just a small battery, the nanopower LTC3335 switching regulator can power the circuit for years while reporting the cumulative battery usage using the built-in coulomb counter. The LTP5901-IPM wireless module manages the entire application, automatically connecting to a high-reliability SmartMesh IP network.
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