Two Novel Relaxation Oscillator Interface Circuits for Resistive and Capacitive Sensors

Abstract

Presented in this thesis is the design and testing of two novel interface circuits for capacitive and resistive sensors. In both cases, the interface circuit is a modified CMOS inverter based relaxation oscillator, which correlates the sensor’s capacitance/resistance to the fundamental frequency of its output square wave. Additionally, in both cases, the relaxation oscillator modulates the output signal onto the current signal of the power supply, which is recovered by a current transformer circuit at the power supply, obviating the need for a separate physical output signal connection. The first interface circuit is designed for use with a single resistive sensor. It is demonstrated with a thermistor that is evaluated via testing inside a freezer over a temperature range of 13 °C to -30 °C. Over that temperature range, the maximum error in the measured frequency of the oscillator interface circuit is 160 Hz. The second version uses two interface circuits, incorporating two capacitive sensors which are evaluated with a deionized water drop test. Both output signals are modulated onto the current signal of the power supply. The fundamental frequencies of the two relaxation oscillators are sufficiently different so that the output information from each interface circuit is recoverable by a frequency domain analysis of the current signal at the power supply. Both interface circuits demonstrate functionality and offer opportunities for further development and research.