Low-cost, high-precision measurement system for capacitive sensors

Abstract

This paper describes a low-cost realization of a precision capacitance measurement instrument. The method is based on synchronous modulation and demodulation operating at a frequency of 100 kHz. A two-channel multiplying digital-to-analogue converter is used to generate a sinusoidal drive voltage and to perform multiplication with the demodulation signal. The modulation and demodulation signals are generated using a direct digital synthesizer, which is implemented using a field programmable gate array. Experimental results show a reproducibility of better than ±0.005% in the range from 50 pF to 600 pF.

Microcontroller Based Capacitive Mass Measuring System

“…Accuracy, small size, low power consumption and freedom from imperfections made the capacitive sensors successful in the measurement of many physical quantities [9].The capacitive sensors usually have low capacitances and their measurement is affected by the stray capacitances and earth admittances. Fig.1(a) and (b) show a three terminal capacitor and its equivalent circuit respectively.…”

“…If a current is passed through an unknown capacitance, it will show a characteristic magnitude and phase shifts [9]. The phasor diagram between applied voltage and currents is depicted in Fig.3 The phase shift between the excitation voltage (V S ) and the resulting current (I X ) will depend upon the value of the capacitance and the conductance of the capacitive sensor.…”

Impedance measurements with second-order harmonic oscillator for testing food sterility

“…2) between point , at which and the measured impedance are connected, and ground will cause a part of the loop current to be directed to ground. This will result in a change of the oscillating frequency (4) and will result in a relative error for of (5) If the value of this parasitic capacitance is too high, one can use an electric-field shield that is connected to the inverting input of A1 (see Fig. 2); in this way, the initial parasitic capacitance is split in two partsand .…”

“…For sensor-interfacing purposes, this solution appears to be quite expensive and complicated. Attempts have been made to optimize and simplify the above-mentioned technique [4] and alternative methods have been sought to interface impedance sensors [5]- [8], but all of them are applicable in a frequency range below 100 kHz. This paper introduces a novel application-specific impedance-measurement technique for the frequency range up to 10 MHz.…”

Interface circuit for impedance measurement to test sterility of food products

“…For sensorinterfacing purposes it appears to be quite an expensive and complicated solution. There are attempts to optimize and simplify the above-mentioned technique [4] or to use alternative methods to interface impedance sensors [5] [6] [7] [8], but they are applicable in a limited frequency range, up to 100 kHz.…”