22.4 A 172µW compressive sampling photoplethysmographic readout with embedded direct heart-rate and variability extraction from compressively sampled data

Abstract

Heart rate (HR) and its variability (HRV) provide critical information about an individual's cardiovascular and mental health state. In either application, long-term observation is crucial to arrive at conclusive decisions and provide useful diagnostic feedback [1]. Photoplethysmographic (PPG) estimation of HR and HRV has emerged as an attractive alternative to ECG, as it provides electrode-free operation increasing patient comfort. However, PPG monitoring systems robust to low ambient light conditions and low perfusion conditions require a LED as a light source, which strongly dominates the power consumption of the complete system. Compressive sampling (CS) based PPG readouts promise to mitigate this LED power consumption [2], yet require large computational power to recover the signal, hindering real-time embedded processing on energy-scarce wearable devices. This paper presents a fully integrated, low-power PPG readout ASIC, completely integrating a single-channel readout front-end (AFE) and a 12b SAR ADC and a digital back-end (DBE) for embedded energy-efficient real-time information extraction, that advances the state-of-the-art on the following fronts: 1) By smartly duty-cycling all system components synchronously on a sparse non-uniform CS sampling pulse stream, the LED driver power is reduced up to 30x, without significant loss of information. 2) Moreover, the necessity of wireless off-loading, or for computationally intensive embedded signal reconstruction, is circumvented by enabling the direct extraction of HR and HRV information from the compressed data in real-time on the ASIC, while consuming only 172μW for the complete system.