A New Light-to-Frequency Analog Front-End Circuit for Optical Sensing in Biomedical Applications

This paper reports on a new analog front-end circuit capable of measuring light intensity variations by providing the frequency modulation of a square wave output voltage signal. The circuit design allows for a quasi-digital output that can be measured using a simple digital frequency meter, avoiding the use of transimpedance amplifiers and analog-to-digital converters. Thus, the proposed solution is suitable to be employed, for example, as analog front-end in low-voltage, low-power wearable medical devices that make use of optical electrodes (optrodes) for telemedical diagnostic and therapeutic screening. The analog front-end circuit has been designed at the transistor level in TSMC 180 nm standard CMOS technology, and its main characteristics and properties have been studied both theoretically and numerically. Moreover, a simplified version of the proposed analog frontend circuit, implemented by using commercial off-the-shelf discrete components, is also presented to validate the solution and to demonstrate its performances and capabilities experimentally, especially in terms of sensitivity and resolution, as well as its real-time response to light intensity variations by using a laser operating in both steady-state and pulsed conditions.