A Nonuniform Sampling Lifetime Estimation Technique for Luminescent Oxygen Measurements

Abstract

This paper presents a nonuniform sampling technique for measuring the lifetime of luminescent materials for oxygen sensing. The system features a switched-capacitor circuit to implement fixed-voltage steps for quantization, enabling long integration times without saturating the front-end amplifier. A control circuit automatically tunes the light emitting diode (LED) excitation pulses to avoid overpowering or starving the front end as photodiode current varies with changes in the partial pressure of oxygen. Time gating of the front-end integrator removes the need for optical filtering. The analog front end (AFE) has a gain bandwidth product of 10 MHz and an input-referred noise of 124 $\mu V_{rms}$ (measured 200 Hz - 100 kHz). The circuit was realized in 180 nm CMOS technology. The AFE and LED driver consume a maximum of 16 $\mu J$ per calculation. We have demonstrated the entire system's functionality by measuring oxygen concentrations from 0 to 240 mmHg in a controlled gas vessel. The results indicate satisfactory linearity on a Stern-Volmer plot covering the human-relevant range of 50 to 150 mmHg.