Dual-Mode Additive Noise Rejection in Wearable Photoplethysmography

This paper presents a mixed-signal photo detection architecture that provides DC offset rejection of up to x5 beyond the dynamic range of the front-end amplifier while retaining the DC signal content of the physiological signal being detected. Closed-loop control of the mean input current is used to prevent saturation of the detector's front-end amplifier while frequency modulation of the illumination source enables homodyne detection of the absorption properties of the blood vessels being investigated. As modulation creates a copy of the desired signal at high frequency, the bandwidth of the current feedback loop is allowed to overlap with low frequency physiological signals (e.g. respiration rate) without rejecting them from the homodyne output. Use of lattice wave digital filters enables a photo plethysmography system to be implemented with up to 1,000 samples per second in real-time by a low-power microcontroller. Experimental validation of the dual-mode noise rejection technique shows that it is robust against high static ambient light levels as well as rapid transitions in light levels.