Microcapacitance measuring instrument with accuracy higher than 1pF
This example shows a low-cost but very accurate capacitance meter. This is a microcapacitance measuring instrument with an accuracy higher than 1pF, which is dedicated to capacitive oil level gauges. The user can arbitrarily adjust the capacitance range of the meter by modifying the component values. Since the gauge was originally designed for oil level measurement, its power components are designed to withstand voltage transients that may occur during motorcycle or car powering.
U3A is an oscillator using a ceramic resonator (Y1). The output circuit of U3C to U3F charges and discharges the capacitor at a frequency of 455 kHz. The charging current flows through R8, and the differential amplifier U4 filters and amplifies the charging current. The average current of the discharge current flowing through D3.R8 is proportional to the capacitance.
The charging voltage of the capacitor is equal to the fixed voltage of U2 - 5.74V plus the number of voltage drops of D1. The number of voltage drops on D1 depends on the temperature and just offsets the temperature change of D3.
If the output voltage is 5V, assuming the amplifier gain is AV, the maximum capacitance (C) is calculated as follows:
C=5V/(5.74V&TImes;AV&TImes;R8&TImes;455,000Hz)
In this example, AV=10, so C=128pF. If you need to reduce the value of C, just increase the value of R8.
The capacitor to be tested must be fully charged and discharged, but the value of R8 cannot be reduced too much when measuring large capacitors. The best way is to reduce the charge and discharge frequency, such as using 4060Hz, and then choose the appropriate output circuit. Then, fine tune by adjusting the gain of the R8 amplifier.
Atx Connector,Female Inset Connector,Molding Solder Type Connector,Receptacle Solder Type Connector
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