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Issue #214 May 2008
Where Analog And Digital Collide
An Easy-To-Use LCR Meter
Third Place Microchip 2007 Design Contest
by Miguel Rusch
Start | LCR Meter | Back to Basics | The Big Picture | Creating a Wave | Analog Stages | Signal Conditioning | User Interface | Firmware | Take a Measurement | System Performance | Further Development | What's Next? | Sources & PDF
SYSTEM PERFORMANCE
The prototype system’s performance is very pleasing. The user interface is easy to use, push button operation is handy, and the graphic LCD is clear and easy to read. Measurements appear to meet the required accuracy (less than 1%), and repeatability is excellent. Because the system can automatically choose the most suitable frequency and circuit type, making basic measurements is very easy.
Currently, the program implements a five-cycle averaging function on the result to improve stability and accuracy. While this is a useful function, it has the disadvantage of increasing the overall test cycle time. A test on “automatic” will take up to 5 s to finish. A fast test feature is needed in the next prototype to enable quick sorting of parts at a lower accuracy.
Typically, LCR meters implement short-circuit and open-circuit calibration. Tests on this device have shown that use of the four-wire method has resulted in a series resistance error that is significantly less than the lower-limit impedance that can be measured. Open-circuit tests, however, have shown that there is approximately 3.5 pF of parallel capacitance in the test leads. At frequencies below 10 kHz, this capacitance represents an impedance value significantly above the measurement ceiling. Future revisions of this design will add a calibration cycle to measure and compensate for the parallel capacitance.
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