Op-amp capacitance meter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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The principle of operation of a capacitor capacitance meter from several picofarads to 5 μF is based on measuring the alternating current flowing through the capacitor under test.
An alternating current of a rectangular shape is generated by an oscillator on the operational amplifier A1. The repetition frequency of the pulses issued by the generator depends on the capacitance of one of the capacitors C1-C6 and the position of the variable resistor R5 slider. Depending on the measurement subrange, it varies from 100 Hz to 200 kHz. Trimmer resistor R1* symmetricalize the shape of rectangular oscillations at the output of the generator. In order for the measurement error in the very first subrange (50 pF) not to exceed 10%, the internal resistance of the device must be at least 3 kOhm.
The required measurement range is set by switch S1. The capacitance meter is powered by a stabilized bipolar source with a voltage of 8 ... 15 V. The values of the frequency-setting capacitors C1-C6 may differ from those indicated in the diagram by 20%, but must have high temperature and time stability. The instrument is calibrated using reference capacitors. Since the alternating voltage at the output of the generator is bipolar, it is impossible to measure the capacitance of electrolytic capacitors with this device.
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