A triangular/square-wave generator with current-controllable frequency and amplitude is presented. The generator utilizes operational transconductance amplifiers as switching elements. A prototype circuit built with commercially available components exhibits less than 2% nonlinearity in its current-to-frequency transfer characteristic from 1 to 10 kHz and -150 ppm//spl deg/C temperature coefficient of frequency over 15/spl deg/C to 35/spl deg/C. The circuit also displays wide sweep capability and good linearity of current to amplitude. The application of the circuit to a current-controllable saw-tooth waveform generator is also presented.

Triangular/square-wave generator with independently controllable frequency and amplitude

This paper introduces a new simple Schmitt trigger circuit using a plus-type differential voltage-current conveyor (DVCC+) and only two grounded resistors. The proposed circuit is very simple and enjoys adjustable lower and higher threshold voltages as well as the output saturation levels. The application of the proposed Schmitt trigger circuit to the square/triangular wave generator is also given. Moreover, a current feedback operational amplifier (CFOA)-based square/triangular wave generator is derived from the proposed DVCC+-based circuit. Simulation and experimental results are presented to exhibit the performance of the proposed circuits.

A Simple Schmitt Trigger Circuit with Grounded Passive Elements and Its Application to Square/Triangular Wave Generator

Similar to phase-locked loops, frequency-locked loops (FLLs) are useful in many applications involving waveform synchronization or synthesis. Simple logic circuit-based relaxation oscillators convert capacitance to frequency, which is a characteristic inverse relationship between output frequency and input capacitance. The oscillator's logic level square-wave output can be fed into an all-digital FLL that will frequency lock to the input signal and produce a digital output word N, where N is inversely proportional to the input frequency. The result is that N is linearly proportional to the unknown capacitance in the oscillator. This novel approach allows a simple FFL implementation for capacitance measurement and is demonstrated in hardware using a capacitive sensor that measures the mass of small quantities of water with an output capacitance range of 75-185 pF.

A Digital Frequency-Locked Loop System for Capacitance Measurement

A new electronic interface circuit is presented. The circuit is built around a single plus-type second-generation current conveyor (CCII+) and can be used with resistive, capacitive and resistive-capacitive sensors. Experimental results are provided.

A current conveyor-based relaxation oscillator as a versatile electronic interface for capacitive and resistive sensors

Microcontrollers with embedded timers can measure resistances or capacitances by determining the charging or discharging time of an RC circuit. The microcontroller-based interfaces proposed for capacitive sensors have not been analyzed in detail, and basic information such as capacitance range, stray capacitance compensation, and accuracy is not available. This paper analyzes the performance of these interfaces when measuring capacitances in the picofarad range. The effects of stray capacitances are evaluated and reduced by applying the three-signal calibration technique. For the PIC16F873 microcontroller, the absolute error achieved is below 4% FSR for 1 pF < C/sub x/ < 10 pF, and below 1.5% for 10 pF < C/sub x/ < 100 pF.

A low-cost microcontroller interface for low-value capacitive sensors

Oscillation periods of a relaxation oscillator consisting of an op-amp-based integrator and a comparator are measured over a wide range of RC integration constants, and the results are compared with theoretical values derived in terms of the finite gain-bandwidth (GB) product and the slew rate of an op-amp and the response delay of a comparator The comparison validates the theoretical derivation which gives the design criteria of a relaxation oscillator for capacitance measurements and sensor signal processing

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Yili Liu

Papers

Limitations of a relaxation oscillator in capacitance measurements