A Measuring Circuit for Small Values of Capacitance Based on Capacitance-to-phase Conversion

Existing capacitance-measuring instruments can only reach the range level of a micro-farad; for smaller capacitors, they often fall short of the range requirement. This study provides a method for detecting pill capsule products whose contents are out-of-weight-specifications. A difference in the weight of the capsule stuffing leads to a difference in the capacitance of the capsule. However, the capacitance of a capsule tends to be small, ranging from a few to tens of picofarads, which cannot be measured by general measuring instruments. Based on this fact, this study designs a capacitance-to-phase conversion circuit with a capacitance sensor, consisting of a cylindrical hollow pipe and two half-cylindrical copper electrodes. When the measured capsule passes through the pipe, the capacitance sensor captures its capacitance which is converted into a series of output pulses by the measuring circuit. The pulse width of these pulses represents the difference between the tested capsule and standard reference capacitance. The feasibility of the circuit was verified using LTSpice simulation. The test results show that this design can convert the capacitance value into pulses with a resolution of 7.6us/1pF. The relative error between the pulse width read by the microcontroller and that measured by the oscilloscope is less than 5%.