The present paper describes the development of a low cost, highly accurate low capacitance measurement system that can be used over a range of 0 – 400 pF with a resolution of 1 pF. The range of capacitance may be easily altered by a simple resistance or capacitance variation of the measurement circuit. This capacitance measurement system uses quad two-input NAND Schmitt trigger circuit CD4093B with hysteresis for the measurement and this system is integrated with PIC 18F2550 microcontroller for data acquisition purpose. The microcontroller interacts with software developed in the PC end through USB architecture and an attractive graphical user interface (GUI) based system is developed in the PC end to provide the user with real time, online display of capacitance under measurement. The system uses a differential mode of capacitance measurement, with reference to a trimmer capacitance, that effectively compensates lead capacitances, a notorious error encountered in usual low capacitance measurements. The hysteresis provided in the Schmitt-trigger circuits enable reliable operation of the system by greatly minimizing the possibility of false triggering because of stray interferences, usually regarded as another source of significant error. The real life testing of the proposed system showed that our measurements could produce highly accurate capacitance measurements, when compared to cutting edge, high end digital capacitance meters.
A. Chatterjee, S. Munshi, M. Dutta, and A. Rakshit, "An artificial neural linearizer for capacitive humidity sensor," Proc. 17th IEEE Instrumentation and Measurement Technology Conference (IMTC 2000), Baltimore, USA, May, 2000.
A. L. Hugill, "Displacement transducers based on reactive sensors in transformer ratio bridge circuits," Journal of Physics E: Sci. Instrm. vol. 15, pp. 597-606, 1982.
D. M. G. Preethichandra and K. Shida, "A simple interface circuit to measure very small capacitance changes in capacitance sensors," IEEE Tran. Instrumentation and Measurement, vol. 50, no. 6, pp. 1583-1586, December 2001.
D. Mariolo, E. Sardani, and A. Taroni, "Measurement of small capacitance variation," IEEE Tran. Instrumentation and Measurement, vol. 40, no. 2, pp. 426-428, April 1991.
F. N. Toth and G. C. M. Meijer, "A low-cost, smart capacitive position sensor," IEEE Tran. Instrumentation and Measurement, vol. 41, no. 6, pp. 1041-1044, December 1992.
J. C. Lotters, W. Olthuis, and P. Bergveld, "A sensitive differential capacitance to voltage converter for sensor applications," IEEE Tran. Instrumentation and Measurement, vol. 48, no. 1, pp. 89-96, February 1999.
M. Yamada, T. Takebayashi, S. Notoyama, and K. Watanabe, "A switched-capacitor interface for capacitive pressure transducer," IEEE Tran. Instrumentation and Measurement, vol. 41, no. 1, pp. 81-86, February 1992.
S. M. Huang, A. L. Stott, R. G. Green and M. S. Beck, "Electronic transducers for industrial measurement of low value capacitances," Journal of Physics E: Sci. & Instrum., vol. 21, pp. 242-250, 1988.
 Y. Masuda, M. Nishikawa, and B. Lchijo, "New methods of measuring capacitance and resistance of very high loss materials at high frequencies," IEEE Tran. Instrumentation and Measurement, vol. IM-29, pp. 28-36, 1980.
 K. Kobayashi, S. Okamoto, and M. Sukigara, "A capacitance relaxation method for studying surface states at the semiconductor-liquid junction," J. Electroanal. Chem., vol. 225, pp. 79-92, 1987.
 A. C. Corney, "A universal four-pair impedance bridge," IEEE Tran. Instrumentation and Measurement, vol. IM-28, pp. 211-215, 1979.
 J. K-Rasmussen and W. Q. Wang, "A compact electrical capacitance tomography system," Proc. IEEE International Workshop on Imaging Systems and Techniques ( IST 2008), Chania, Greece, September 10–12, 2008.
 CD4093BM CD4093BC Quad 2-Input NAND Schmitt Trigger. Texas Instruments, Literature Number: SNOS369A.
 Online. ww1.microchip.com/downloads/en/devicedoc/39632c.pdf.
 Online. http://en.wikipedia.org/wiki/Schmitt_trigger.
Y. C. Chung, N. N. Amarnath, and C. M. Furse, "Capacitance and inductance sensor circuits for detecting the lengths of open- and short-circuited wires," IEEE Tran. Instrumentation and Measurement, vol. 58, no. 8, pp. 2495-2502, August 2009.