Summary of LOW COST HIGH ACCURACY STM32 FFT LCR METER
This article details a DIY LCR meter project designed for university use, featuring mixed-signal architecture with an analog front end and DSP processor. It supports test frequencies of 1, 10, and 100 KHz, measuring L, C, R, Z, and auto-classifying components within a 0.1 Ohm to 10 MOhm range. The device uses a 128×64 LCD display and costs approximately £55.
Parts used in the DIY LCR Meter:
- Analog front end
- DSP processor
- Digital synthesizer
- Resistive current shunt
- Difference amplifier U1
- Gain stages
- Source V1
- 128x64 Jelly Bean LCD
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.
The approach I took was a mixed signal one where a capable analog front end would be paired up with a beefy DSP processor to compute the Impedance. Most importantly, in this scheme, the DSP is responsible for discriminating the phase between the sampled voltage and current waveforms; this approach is preferred because it leads to good accuracy and calibration stability.
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.
The LCD is a jelly bean 128×64 type and has been divided into a primary display consisting of the measured quantity and a secondary display showing the current measuring range and the impedance representation currently being displayed. The overall cost came to about £55.
Measurement Theory:
Passive Shunt:
The first method is the traditional and broadband technique commonly referred to as the “I-V method”. In this a resistive current shunt is placed in series with the DUT and the voltage across the shunt and the DUT are read off, allowing a calculation of Z.
However, this method has severe practical limitations described briefly below.
Limited I-V gain which is coupled to burden voltage: To keep the burden voltage of the shunt small, the shunt itself is typically, a few mOhms. As a result, I-V gain i.e. the differential voltage across the shunt is very small and on top of a much larger common mode signal. This places very strict performance requirements on the difference amplifier U1 and any subsequent gain stages. The only way to practically increase this signal is to increase the value of RSHUNT, which in turn increases the burden voltage. The source V1 then needs to be increased to keep VZDUT, often a strictly specified test parameter, constant
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- How does the DSP improve accuracy?
The DSP discriminates the phase between sampled voltage and current waveforms, which leads to good accuracy and calibration stability. - What test frequencies are supported?
The device supports digitally synthesized test frequencies of 1, 10, and 100 KHz. - Can the meter classify components automatically?
Yes, the software includes an auto mode that classifies the DUT based on its impedance phase. - What is the measurement range of this LCR meter?
With simple calibration, the impedance measurement range is tested from 0.1 Ohm to 10 MOhm. - What method is used for passive shunt measurements?
The project uses the traditional I-V method where a resistive current shunt is placed in series with the DUT. - Why is increasing the shunt value problematic?
Increasing the shunt value increases the burden voltage, requiring the source to be increased to keep the DUT voltage constant. - How many voltage and current ranges does the signal path allow?
The highly reconfigurable analog signal path allows about 100 voltage and current ranges. - What is the approximate cost of building this project?
The overall cost of the project came to about £55.
