Wireless current sensing with a micropower zero-drift Op Amp

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.

Typical solutions bridge the voltage difference in either the analog or digital domain. But here is a different approach – wireless.

High-side current sense amplifiers operate in the analog domain. The ICs are compact, but the voltage difference that they can withstand is limited by semiconductor processes. Devices rated for more than 100V are rare. And these circuits often lose accuracy if the sense resistor common mode voltage changes quickly or swings both above and below system ground.

Magnetic or optical isolators usually break the isolation barrier in the digital domain. The hardware can be a bit more bulky, but works 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, you may also need to run long wires or cables.

Recent low-power signal conditioning and wireless technologies offer a new approach. 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. 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.

 

Wireless current sense

The current sense circuit in figure 1 employs 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 an SPI interface. The LTP5901-IPM is the wireless module that automatically forms an IP-based mesh network with other nearby nodes. It also has a built-in microprocessor which reads the AD7988 ADC SPI port. The LTC3335 is a nanopower buck-boost regulator 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.


Fig. 1:  A low-power wireless current sense circuit is formed by a low power chopper op amp to amplify the sense voltage, is digitized using a low power ADC and reference, and is connected to a SmartMesh IP wireless radio module. A low-power DC/DC converter conditions the battery and keeps track of the charge drawn from the battery.

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