4.4 A sub-nW 80mlx-to-1.26Mlx self-referencing light-to-digital converter with AlGaAs photodiode

Wearable sensors are increasingly common and continue to grow more diverse in their sensing modalities, ranging from glucose to heart rate monitoring. One compelling sensing modality for wearable sensors is cumulative light exposure. For instance, reduced light exposure exacerbates depression, while high levels of sunlight exposure are well known as the primary cause of skin cancer. In order for wearable sensors to determine how much light radiation the skin is receiving, a light-to-digital converter (LDC) is needed to both measure instantaneous light intensity and record the total accumulated light energy. LDCs targeting wearable devices will have tight constraints on area (cost) and power. At the same time, the wide dynamic range of light (~10 ⁶ ) from 0.1lx to 100klx places difficult performance requirements on the LDC. Previous LDCs integrate photodiode current and use a transimpedance amplifier followed by ADC conversion [1-2], resulting in continuous power consumption up to 100s of μW, which is not sustainable for wearable devices with small batteries. Recently, a low-power LDC based on logarithmic digital-to-resistance conversion was proposed for heart rate monitoring [3], but its input dynamic range is too small for light energy monitoring. An alternate approach converts light intensity to frequency [4], which is then measured with a frequency-to-digital converter. However, this approach consumes significant power at strong light levels as oscillation frequency increases linearly with light intensity. Also, this approach requires an accurate reference timer circuit that may not be available during sleep/standby modes common in wearable devices.