Summary of LOW COST HIGH ACCURACY STM32 FFT LCR METER
This university project details a low-cost, high-accuracy LCR meter built for personal lab use. It employs a mixed-signal design with an analog front end and a DSP processor to calculate impedance via phase discrimination. The device supports test frequencies of 1, 10, and 100 KHz, measures L, C, R, Z, and includes an auto-classification mode. With a cost of approximately £55, it achieves accurate measurements from 0.1 Ohm to 10 MOhm using a reconfigurable analog path and a 128×64 LCD display.
Parts used in the STM32 FFT LCR Meter:
- Analog front end
- DSP processor
- Resistive current shunt
- Difference amplifier U1
- Gain stages
- 128×64 LCD display
- Digital synthesizer circuitry
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
Read more: LOW COST HIGH ACCURACY STM32 FFT LCR METER
- What is the primary method used for passive shunt measurement?
The traditional broadband technique known as the I-V method places a resistive current shunt in series with the DUT. - How does the DSP processor contribute to accuracy?
The DSP discriminates the phase between sampled voltage and current waveforms to ensure good accuracy and calibration stability. - Which test frequencies are supported by this device?
The device digitally synthesizes test frequencies of 1, 10, and 100 KHz. - What components are required to increase the signal in the I-V method?
Increasing the value of RSHUNT increases the signal but also increases burden voltage, requiring a higher source V1. - Does the software support automatic classification of components?
Yes, the software includes an auto mode that classifies the DUT based on its impedance phase. - What is the measured impedance range of the instrument?
The tested impedance measurement range with simple calibration is from 0.1 Ohm to 10 MOhm. - How much did the overall project cost?
The overall cost came to about £55. - What type of display is used in the project?
A jelly bean 128×64 LCD is used, divided into primary and secondary display sections.
