Microcapacitance measuring instrument with accuracy higher than 1pF
This example presents a low-cost yet highly accurate capacitance meter, specifically designed for measuring microcapacitance with an accuracy better than 1pF. It is tailored for use in capacitive oil level gauges and offers flexibility through component adjustments, allowing users to customize the measurement range as needed. The design includes robust power components that can handle voltage transients commonly encountered in automotive environments, such as those from motorcycles or cars.
The circuit uses U3A as an oscillator, which is driven by a ceramic resonator (Y1). The output from U3C to U3F charges and discharges the capacitor at a frequency of 455 kHz. The charging current flows through R8, while the differential amplifier U4 filters and amplifies this current. The average discharge current through D3 and R8 is directly proportional to the capacitance being measured.
The capacitor's charging voltage is determined by the fixed output of U2, which is 5.74V, plus the voltage drops across D1. These voltage drops are temperature-dependent and help compensate for the thermal variations in D3, ensuring more stable readings under different environmental conditions.
If the output voltage is set to 5V and the amplifier gain is AV, the maximum measurable capacitance (C) can be calculated using the formula:
C = 5V / (5.74V × AV × R8 × 455,000Hz)
In this specific example, with AV = 10, the maximum capacitance is 128pF. To measure smaller capacitances, simply increase the value of R8. However, when measuring larger capacitors, it’s important not to reduce R8 too much, as this could affect the accuracy of the readings.
To improve performance with large capacitors, consider reducing the charge and discharge frequency—such as using 4060Hz instead of 455kHz—and then adjust the output circuit accordingly. Fine-tuning can also be achieved by modifying the gain of the amplifier connected to R8, ensuring optimal accuracy across a wide range of capacitance values.
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