A Novel Light-to-Frequency Converter Based Analog Front-End for Optical Sensing Applications

This paper reports on a novel light-to-frequency converter circuit based on the variation of the internal junction capacitance of a Si-photodiode under light illumination of its sensitive area. In particular, the proposed analog front-end circuit is capable of measuring light intensity variations by providing the frequency modulation of a square wave output voltage signal. Thus, this solution allows for a quasi-digital output that can be measured using simple digital frequency meters, avoiding the use of transimpedance amplifiers and analog-to-digital converters. The characteristics and performances of the circuit, developed in the current-mode approach employing second-generation current conveyors, have been studied both theoretically and numerically. Moreover, the proposed architecture has been implemented by using commercial off-the-shelf discrete components. A series of experimental findings are presented validating the solution and demonstrating its performances experimentally in terms of sensitivity and resolution as well as its real-time response to light intensity variations by using a laser operating in steady-state and pulsed regimes. By varying the laser power up to 8 µW, the resulting circuit sensitivity and resolution are equal to 5.32 kHz/µW and 8 nW, respectively. These results, combined with the small range of the employed laser power, achievable also with LED sources, demonstrate that the proposed solution is suitable for low-voltage low-power portable/wearable and implantable devices and systems.