Wireless current sensing with a micropower zero-drift Op Amp: Page 2 of 4

June 13, 2018 // By Kris Lokere
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.

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 LTC2063 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 LTC2063 configured to amplify and level shift the voltage across a 10mΩ sense resistor. 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 AD7988 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 LT6656 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.

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.

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