Practical and simple circuitry for the measurement of small capacitance

Practical and cost-effective circuitry with high sensitivity has been developed to measure a small capacitance using current compensation method. The circuitry uses an electronic switch to periodically connect or separate the capacitor under test $(Cx)$ from a reference capacitor $(Cr)$⁠. When $Cx$ is connected in parallel with $Cr$ the total capacitance becomes $Cx+Cr$⁠. On the other hand, as $Cx$ is separated from $Cr$⁠, the total capacitance is only $Cr$⁠. This periodic change of the capacitance generates a periodic square-wave output with an amplitude in proportion to the capacitance of $Cx$⁠. A high sensitivity of $ΔV∕ΔC=202.2mV∕pF$ has been achieved, making the circuitry a powerful tool in measuring small capacitances. Three applications have been performed to present its capability: (a) displacement, (b) height of liquid, and (c) angle of tilt. The experimental results demonstrate the performance of the circuitry.