A 9.84-μW 148.9-dB Total DR Light-to-Digital Converter With Current-Integration SAR Quantizer for Multi-Wavelength PPG Applications

This work presents a low-power, large cross-scale dynamic range (DR) successive approximation register (SAR) based light-to-digital converter (LDC) designed for wearable multi-wavelength photoplethysmogram (MWPPG) readout. The proposed SAR-LDC employs a current-integration SAR (CI-SAR) quantizer that directly integrates the photocurrent onto a binary capacitive digital-to-analog converter (DAC) and utilizes low-power SAR logic for direct digitization. This approach releases the various trade-offs present in the recent LDC designs and further maximizes the LDC’s power efficiency. At the circuit level, an AC/DC correlated DR enhancement loop, controlled by a digital reconfigurable unit (DRU), enhances both the DC and AC DR of the proposed system. The proposed architecture is fabricated in standard 180-nm CMOS technology. By utilizing a 10-bit SAR-LDC, it achieves a maximum effective number of bits (ENOB) of 9.93 and 12.56 in the Nyquist bandwidth and PPG bandwidth, respectively. The SAR-LDC reads MWPPG on a single channel through time multiplexing, consuming 9.84 μW only while achieving a cross-scale AC DR of 103.4 dB and a total DR of 148.9 dB without using any hybrid circuit technique. The proposed SAR-LDC is further validated using with a commercial PPG sensor for finger MWPPG measurement.