A Fully-Analogue Light-to-Frequency Converter Circuit for Optical Sensing Applications

This paper reports on a novel analog front-end circuit designed to measure light intensity variations for portable/wearable and implantable sensor applications in different fields spanning from industry to biomedicine. The circuit is capable to follow variations of light intensity detected by a photodiode through the frequency modulation of a generated square wave output voltage signal. Therefore, the circuit can be seen as a light-to-frequency converter allowing for a quasi-digital output that can be measured by simple digital frequency meters so avoiding the use of transimpedance amplifiers and analog-to-digital converters. The proposed architecture of the analog front-end circuit has been designed at the transistor level in TSMC 180 nm standard CMOS technology in CADENCE Design System environment. The resulting characteristics and performances have been studied both theoretically and numerically. Moreover, a simplified version of the proposed circuit has been also implemented by using commercial off-the-shelf discrete components to study experimentally its main features. In this sense, the paper presents a series of experimental findings that fully validate the solution and prove its performances in terms of sensitivity and resolution as well as of time response to light intensity variations by using a laser operating in both steady-state and pulsed regimes. By varying the laser power up to $3.75~\mu \text{W}$ , the resulting circuit sensitivity and resolution are equal to 69.4 kHz/ $\mu \text{W}$ and 432 pW, respectively. In general, these results combined with small power variations of lasers or LED devices, demonstrate that the proposed solution is suitable for low-voltage, low-power applications, as required in portable/wearable and implantable devices and systems.