Wireless current sensing with a micropower zero-drift Op Amp: Page 3 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.

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 LTC3335 nanopower buck-boost converter with integrated Coulomb counter. 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. 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 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. 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.

Fig. 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.

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