Международна научна конференция “УНИТЕХ’10” – Габрово I-194 ‘ 10 INTERNATIONAL SCIENTIFIC CONFERENCE 19 – 20 November 2010, GABROVO SELF CALIBRATION CAPACITANCE METER Aleksandar Č. Žorić Faculty of Technical Sciences-University of Priština, Serbia Slobodan Obradović urđe Perišić Faculty of Computer Sciences Faculty of Information Technologies Megatrend University, Serbia Slobomir P University, Republic of Srpska, BiH Abstract This paper describes a self calibration method for capacitance measurement on the base of MCU. The method uses a software calibration technique for first order errors elimination as: offset and gain error, error caused by supply voltage variations, uncertainty error of comparator’s threshold voltage and its temperature drift as well as uncertainty and temperature drift of resistance. The method do not removes absolute measurement error of capacitance of the reference capacitor. Remaining sources of errors can be leakage currents of I/O pins of MCU, nonlinearity of applied passive components and discrete quantization error of ±1 instruction cycle. The measurement resolution is 16 bits. Keywords: MCU, capacitance, instruction cycle, timer, comparator, interrupt, DMM. INTRODUCTION Recent digital methods for capacitance measure- ment are based mainly on the well known principles of A/D conversion with time or frequency quantization. Electronic circuits of this kind are in principle simple, mainly based on the re- laxation oscillators and digital counters [1, 6]. However, serious disadvantage of such solu- tions is degradation of the measurement quan- tity because of multiple conversions. Namely, the total conversion error is result of capaci- tance to time interval conversion error and quantization error of time interval. Influence of parasitic effects even at the moderate ope- rating oscillator frequencies, instability of comparator’s tresholds voltage as well as un- certainty of passive components parameters are main causers of conversion error of capaci- tance to time interval. Converter resolution has dominant influence to the quantization error. With the advent and development of micro- controllers, made possible the realization of capacitance meter as intelligent devices [3]. In addition to managing the work of devices and complex processing measurement results, using the microcontroller can be achieved and a qualitatively new feature, connect the meter to a computer. Suitable design of the measuring algorithm can than eliminate the influence of parasitic capacitance attaching lines as well as some of the above mentioned errors. This paper describes software measurement procedure that uses a modified principle of capacitance to time interval conversion and quantization of the time [2]. Thanks to the microcontroller, the proposed method automa- tically compensates first-order errors that are result of temperature’s and long-term instability of the component’s parameters like uncertainty in determining the thresholds of comparison, power supply voltage and values of passive components. Taking into account 16-bits measure- ment resolution, the dominant influence on the total measurement error has the uncertainty of determining the capacitance of reference capa- citor. By selecting a stable reference capacitor with high quality dielectric, whose exact value of capacitance is software stores, the total measure- ment error can be kept within the desirable limits. Improved accuracy of measuring capaci- tance using software calibration technique, a small number of required external compo- nents, the relatively low consumption and low