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Signal conditioning


Wireless current sense circuit floats with sense resistor


Kris Lokere, strategic applications manager for signal chain products at Analog Devices, discusses a wireless current sense circuit that can measure current when a sense resistor sits at a voltage that is very different from system ground


M


easuring the current that flows through a sense resistor seems easy. Amplify the voltage, read it with an


ADC, and now you know what the current is. However, it gets more difficult if the sense resistor sits at a voltage that is very different from system ground. The typical solutions that bridge that voltage differ in either the analogue or digital domain. But here is a different approach - wireless. Analog current sense ICs are compact solutions.


However, the voltage difference that they can withstand is limited by semiconductor processes. It is difficult to find devices that are rated for more than 100 V, and these circuits often lose accuracy if the sense resistor’s common-mode voltage changes quickly or swings both above and below system ground. Digital isolation techniques (magnetic or optical)


are a bit more bulky, but work 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, then you also may need to run long wires or cables. A wireless current sense circuit overcomes many of these limitations. By allowing the entire


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


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 at all. The sense resistor can be located anywhere without the need to run cables. If the circuit is very low power, then you do not even need an isolated power supply and can instead run for many years on a small battery.


Design Overview Figure 1 shows the block diagram of the design. The current sense circuit is based on the LTC2063 chopper-stabilised op amp to amplify the voltage drop across a sense resistor. The micropower SAR ADC AD7988 digitises the value and reports the result via an SPI interface. The LTP5901-IPM is the radio module that contains not only the radio, but


January 2019 Instrumentation Monthly


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