A Fully Integrated CMOS 0.3 V 335 nW PWM-Based Light-to-Digital Converter for Optoelectronic Sensing Systems in Biomedical Applications

Abstract

This letter reports on the design of a novel, fully integrated stand-alone light-to-digital converter (LDC) for optoelectronic sensors in wearable/implantable biomedical applications. The architecture designed in TSMC 180 nm standard Si CMOS technology integrates a Si photodiode (PD), a ring oscillator, two digital counters, and a voltage-to-pulsewidth modulation (PWM) stage as the basic block of the light-to-digital converter in a Si area of 0.018 mm². The modulation stage, composed of ten transistors and a capacitor, provides a square waveform whose pulse width varies as a function of its input voltage provided by the Si photodiode operating in a photovoltaic mode that linearly depends on the light intensity impinging on its sensitive area. The value of the pulse width is digitalized by two digital counters driven by the ring oscillator. The complete system, powered at 0.3 V, has been fully characterized by postlayout simulations demonstrating an overall sensitivity of 0.062 LSB/lx, a power consumption of 335 nW, and a sample rate of 3 kS/s. A comparison with similar solutions in the literature shows that the proposed system achieves the best performance in power consumption, Si area, and supply voltage with a good sample rate value.