Simple Interface for Resistive Sensors Based on Pulse Width Modulation

A resistance to pulse width converter with improved stability and temperature compensation is described. The converter is therefore suited as a signal conditioner for resistive sensors. The sensor interface is based on a multivibrator in monostable or astable configuration and combines the single point calibration used in direct sensor-to-microcontroller interface. Analyses show that a resolution better than 14.2 b (for 20 MHz time base) with maximal relative error of 0.11% in the range 1000-3000 Ω is achievable. In this paper, a comparative analysis in terms of measuring uncertainty, speed, and resolution between the multivibrator in monostable and astable operation is performed. A method for defining optimal time constant that results in best speed/resolution trade-off is proposed.