28.2 A 400-to-1000nm 24μ W Monolithic PPG Sensor with 0.3A/W Spectral Responsivity for Miniature Wearables

Incorporating different wavelength (400 to 1000nm) LEDs, photoplethysmography (PPG) sensors allow wearable devices to monitor various health parameters such as heart rate (HR), oxygen saturation (SpO 2 ), and blood pressure (BP). Nowadays, PPG sensing technology at the wrist is well established. To cope with the large degree of motion turbulence presented at the wrist, PPG sensors use Green (Gr) LEDs together with multiple photodiodes (PD), and they are driven by wide-dynamic-range (DR) current-sensing front-ends [1]. It is attractive to use a near-infra-red (nIR) PPG sensor in a True Wireless Stereo (TWS), as the ear provides the best site to measure heart rhythm (more blood flow, constant distance from the heart, and less motion than at the finger or wrist). However, TWS requires a PPG sensor that is more stringent on size and power consumption (shown in Fig. 28.2.1). A promising solution [2, 3] is integrating an array of PDs with an ADC to dramatically reduce power while also providing monolithic integration. However, the limited DR (<80 dB) and the poor spectral responsivity remain challenging. This work advances [1] by demonstrating a CMOS monolithic PPG sensor, and improves spectral responsivity more than $4 \times (0.3\mathrm{A} /\mathrm{W}$ across 400 to 1000nm) compared to [2]. The sensor is fabricated by back-side illumination (BSI) CMOS technology providing 90dB DR (18dB improvement from [3]) while consuming only $24 \mu \mathrm{W}$ power and 5.5mm ² silicon area.