Low-cost Portable System for Capacitance Measurement based on Active Bridge and Relaxation Oscillator

In this paper, a capacitance measurement circuit based on an active bridge and relaxation oscillator has been developed to accurately measure capacitance in the picofarad range. The change of the measured capacitance value causes the unbalance of the active bridge, and the circuit converts the change of the capacitance value into the change of the output frequency. A diode is used to adapt voltage levels to the microcontroller (Arduino UNO). In addition, every 1000 frequency cycles are measured and averaged to reduce the effects of direct current (DC) errors and the jitter. The selection of LT1028, as the oscillator part's amplifier, improved measurement accuracy where the simulation result error is less than 1%, and the final measurement accuracy is below 3%. The design of a smaller PCB, adding a wireless module, and realization of Android App to control and switch the reference capacitance and using the LCD make it portable and IoT compatible. The experimental results of the circuit show that under the five different reference capacitors, accurate capacitance measurement is achieved from 80 pF to 900 pF, and the frequency range is between 9 kHz and 13 kHz. Furthermore, this circuit has good linearity. Therefore, it has the potential as a portable capacitance measurement device, which can be used in a dedicated capacitance measurements for example nanomaterials based capacitive sensors.