A 140-dB Dynamic Range Digital PPG Front-end IC with An Integrated MoSe2 Photodiode for Wearable Non-invasive Pulse Oximetry

Photoplethysmography (PPG) has been widely applied as a useful non-invasive sensing technology for the assessment of blood pressure and blood oxygen levels in commercial medical devices. In contrast with conventional amplifier-based analog front-ends (AFEs), a light-to-digital converter (LDC), which digitizes optical signals right after a photodiode, has received increasing attention as a promising architecture of PPG front-end for its superior power efficiency and compactness. The design challenge, however, remains in how to achieve high dynamic range (DR) in direct light-to-digital conversion within the rather constrained power budget. To tackle this problem, we propose a highly-programmable digital front-end IC in this paper to enable low-dark-current sensing in a PPG sensor with a novel MoSe
2
photodiode. A programmable second-order continuous-time delta-sigma ADC utilizing 1.5-bit quantizer serves as the core of this front-end IC. Its programmable architecture allows a minimally detectable input current of 0.4 μA, leading to a state-of-the-art dynamic range of 140 dB. The proposed LDC is implemented in TSMC 0.18 μm process and achieves an average power consumption of 14.87 μW. The simulated SNDR with the transient noise is 80.1 dB . Furthermore, an LED Driver is integrated to the proposed front-end IC to provide an output current ranging from 1 mA to 127 mA (7 bit). Compared to prior art, this work features outstanding performance with a smaller footprint.