Micro-capacitance measuring instrument with accuracy higher than 1pF

This example shows an inexpensive but very accurate capacitance measuring instrument. This is a micro-capacitance measuring instrument with an accuracy higher than 1pF, which is specially used for capacitive oil level gauges. The user can arbitrarily adjust the capacitance range of the measuring instrument by modifying the component values. Because the meter was originally designed for oil level measurement, its power supply components are designed to withstand voltage transients that may occur during the power supply of a motorcycle or car.

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 455kHz. The charging current flows through R8, and the differential amplifier U4 filters and amplifies the charging current. The average current of the discharge current 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 pressure drops of D1 depends on the temperature, which can exactly offset the temperature change of D3.

If the output voltage is 5V, assuming the amplifier gain is AV, the formula for calculating the maximum capacitance (C) is as follows:

C = 5V / (5.74V & TImes; AV & TImes; R8 & TImes; 455,000Hz)

In this example, AV = 10, so C = 128pF. To decrease the value of C, just increase the value of R8.

The capacitor under test must be fully charged and discharged, but when measuring large-capacity capacitors, the value of R8 cannot be reduced too much. The best way is to reduce the frequency of charge and discharge, such as using 4060Hz, and then select the appropriate output circuit. Then, fine-tune by adjusting the gain of the R8 amplifier.