A 0.5V <4µW CMOS photoplethysmographic heart-rate sensor IC based on a non-uniform quantizer

Photoplethysmographic biosensing has been of recent interest, since it provides electrode-free operation for continuous health-monitoring applications. This paper presents a <;4μW heart-rate (HR) monitor IC based on measurement of fluctuations in the light intensity passing through tissue such as a finger. As shown in Fig. 22.3.1, key components of the system include a logarithmic, digital-to-resistance converter (DRC) that forms the load of an external photodiode, and a non-uniform quantizer that provides error information to a digital accumulator controlling the DRC input. As the photodiode current fluctuates due to HR-induced pulsing of bloodflow through the tissue, the feedback loop adjusts the DRC load resistance in order to maintain constant voltage across the photodiode. As such, the accumulator output, OUTADC[k], provides a digital representation of HR-induced fluctuations of the photodiode current, and the logarithmic implementation of the DRC accommodates 3 orders of magnitude in light intensity. In order to obtain instantaneous HR frequency, HR[k], off-chip digital signal processing is performed on OUTADC[k].