Create A Low-Cost, High-Accuracy LCR Meter With An STM32 MCU And A Two Op-Amp Circuit

I have always wanted to build a fairly capable LCR meter that could cope with real world use in my own personal lab. This would mean reasonably good accuracy across a wide range of L, C and R. Fortunately, I got the time to do just that this year in the 3rd year Instrumentation module at my University. Although this justified spending time on such a project, I was motivated to do a good job so the end result would be usable as an actual piece of test equipment.

Create A Low-Cost, High-Accuracy LCR Meter With An STM32 MCU

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The specifications and features were basically designed to mimic a commercial LCR meter. The test frequencies can be chosen from 1, 10 and 100 KHz and are all digitally synthesised. The software supports displaying L, C, R, Z and also an auto mode that classifies the DUT based on its impedance phase. The impedance measurement range with simple calibration has currently been tested from 0.1 Ohm to 10 MOhm with very good accuracy; this range is achieved by a highly reconfigurable analog signal path that allows about 100 voltage and current ranges, most of which are not used to allow easier calibration.

Having a good LCR meter was something which [Adil] had wanted for his personal lab, so as any good university student (and former Hackaday contributor) does, he ended up building his own. Using a Nucleo-F446RE board for the MCU side and a custom PCB for the side that does the actual measuring, he created a meter that reportedly comes pretty close to commercial meters, and for the low price of 55.

Thanks! Accuracy is pretty good as I have checked it with the main LCR meters our university gives us in labs and they match pretty well. The only problem I have now is that on some of the range settings such as milliOhm and tens of MegaOhms calibrating it is very hard.

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Current through the DUT is measured in both directions using U5B opamp in transimpedance configuration. Advantage of this circuit is that it keeps its inverting input at the same voltage as its non-inverting input (which is held at half of reference voltage) - this means that while current is within measurement range (ensured by R34) the voltage on test terminal 1 is exactly the same as half of ADC reference voltage as well. And due to this the measurement of voltage across the DUT could be very nicely measured in both polarities using only positive power supply rail. Two voltage sensing ranges are used (U4B for 10 Ohm range and U4A for 100 Ohm range of measured impedance). I recommend reading similar device build log here on HAD: -a-low-cost-high-accuracy-lcr-meter-with-an-stm32-mcu/.

Furthermore, the measurement unit, which often replaces a benchtop instrument, must be designed for small form factor devices and a low power profile, while maintaining performance close to gold standard instruments. Application-specific integrated circuit (ASIC) based devices fulfill these requirements, such as the ones developed to perform online electrochemical impedance spectroscopy (EIS) for characterization of lithium-ion battery packs [18,19,20]. This technology can be extended to other applications in areas where size and power consumption are crucial. For instance, in our research we have been developing technology for using admittance spectroscopy to determine the physical state of plants [21]. However, this study relied on bulky and expensive lock-in detectors, not appropriate for an IoT implementation. A cheaper solution developed by S. Grassini is to use an Arduino-based electrochemical impedance spectroscopy (EIS) system [22] for in situ corrosion monitoring of metallic works of art [23]. The Arduino-based EIS is already a huge improvement over conventionally used RCL-bridges (resistance, capacitance, inductance) and lock-in detectors. Similarly, the ASIC-based miniaturized system for Online-EIS proposed by Manfredini [19] shows how versatile the ASIC device is, being capable of measuring not only the impedance of commercial batteries, but also capacitive and resistive sensors. The device is based on the SENSIPLUS, which is a System on a Chip (SoC) solution that uses minimal external hardware, and shows performance on a par with gold standard instruments. The Arduino has been also used as a platform to measure capacitances, as explained, for example, by Campbell [24]. It has been used to deploy a digital LCR meter [25] to measure single parameters (not combinations), although this depends on the known nominal values of external components.

Set-up of a pure load capacitance meter (C-meter). (a) Equivalent circuit of two analog I/O ports bridged with a load capacitance (CLOAD). (b) Reduction of the equivalent circuit to an impedance divider. (c) Pseudo-code used to implement the fast acquisition mode. 2ff7e9595c