A 44μW 140dB-DR Hybrid Light-to-Digital Converter with Current-Tracking Dynamic Zoom and Power-Scaling OTA

Highly sensitive optical sensors have been widely used in chemical analysis, backlight control, gas detection, and medical devices. For instance, photoplethysmography (PPG) sensors can assess one's heart rate, oxygen saturation (SpO
2
), and pulse wave velocity. These applications often require light-to-digital converters (LDCs) with large dynamic range (DR) in excess of 120dB to sense currents from a few pA to hundreds of μA. State-of-the-art LDCs that operate in the time or frequency domain, such as slope ADCs [1]–[2] and Hourglass ADCs [3], achieve excellent DR by employing auxiliary DC servo loop [1]–[2] or predictive DAC [3] to cancel the baseline input current before the
$\Delta\Sigma$
conversion. However, these auxiliary feedback loops have limited bandwidth (BW), and therefore their slow response cannot track fast-changing current disturbances. Continuous-time
$\Delta\Sigma$
modulators (CT-
$\mathrm{I}-\Delta \Sigma \mathrm{M})$
based on an incremental transimpedance amplifier (TIA) [4] or a truncation baseline-servo loop [5] achieve over 136dB DR in kHz BW. However, the former dissipates > 1mW due to the power-hungry loop filter and segmented DAC, while the latter is not compatible with time-multiplexed input for two-wavelength SpO
2
or linear pixel array.