Relaxation Oscillator Based Improved Resistance to Frequency Converter

Abstract

The relaxation oscillator-based circuits find utility in various resistive and capacitive sensor interfaces, thanks to their quasi-digital output. The integration of relaxation oscillator with Wheatstone bridge brings the unique advantage of a high-resolution resistive sensing system without any additional analog-to-digital converters. However, the conventional topologies of a Wheatstone bridge circuit with relaxation oscillator encounter challenges, such as dc offset voltage and output nonlinearity, particularly for single-element resistive sensors. This letter introduces an improved relaxation oscillator-based configuration to mitigate these issues. It employs a dc-servo loop between the Wheatstone bridge and the relaxation oscillator for offset voltage compensation. In addition, the quarter bridge circuit is modified to linearly convert the resistance change into output frequency change across a broad dynamic range of “$\Delta R$ .” The design and experimental results are discussed. The results show that quarter and full bridge configurations are immune to offset voltage variations as high as 100 and 60 mV, respectively, and provide linear frequency response for the “$\Delta R/\text{R'}$’ variations up to 190% and 90% in quarter and full bridge configuration, respectively, with nonlinearity error of less than 0.01%.