Lock-In Based Phase Fluorometric Dissolved Oxygen Sensor Interface With 4 kHz – 150 kHz Tunable Excitation Frequency and Frequency Error Calibration

Abstract

Optical dissolved oxygen (DO) concentration measurement based on phase fluorometric theory is one of the most promising DO concentration measurement methods with advantages of insensitivity to the solution, short response time, and satisfying maintainability. However, most phase fluorometric DO concentration measurement systems have limitations such as strong dependence with the chemiluminescent films, low level integration and large noise interference. In this paper, a fully integrated lock-in based phase fluorometric DO sensor interface is proposed with a wide range tunable excitation frequency to be compatible with various chemiluminescent films. The lock-in technique is adopted to improve the sensitivity with large background noise. The proposed interface chip consists of driver for excitation light, transimpedance amplifier (TIA) to read out the weak current of the optical detector, and lock-in amplifier (LIA) for phase-shift detection. A tunable sinusoidal current generator (SCG) based on digital recursive oscillator (RDO) is adopted to generate excitation signal, meeting the requirements of different chemiluminescent films, while a counter-based orthogonal signals generator (OSG) is applied to generate orthogonal demodulating signals. The frequency error between the demodulating signals and excitation signal, which would cause phase-shift detection error, is cancelled by digital calibration method, which relaxes the challenge to match the demodulation signals with the tunable excitation signal. The TIA with DC cancellation is utilized to suppress the DC current of the photodiode (PD) and thus to improve the accuracy of phase-shift detection. The customized interface chip is implemented using a 0.18 μm CMOS process with a core area of 660 μm*464 μm. The test results show that the developed system has an accuracy of 0.06 degrees and a precision of 0.2 degrees in terms of the excitation frequency range from 4 kHz to 150 kHz.

A Microfluidic, Flow-Through, Liquid Reagent Fluorescence Sensor Applied to Oxygen Concentration Measurement

“…This yields the possibility for a design of a high electrical bandwidth optical measurements system that can allow for reliable measurements of short fluorescence decay times. The latter opens a variety of different sensing opportunities, as short decay time dyes/reagents cover a broad range of different analytes [ 33 , 34 , 35 , 36 , 37 , 38 , 39 ]. The proposed microfluidic setup was demonstrated as an on-line sensing system for the measurement of dissolved oxygen in water, using two different fluorescent dyes with decay times between 400 ns and 1 µs.…”