A Novel Ring Shaped Photodiode for Reflectance Pulse Oximetry in Wireless Applications

We present a pulse oximeter for use in home-care applications in a sticking patch with integrated electronics. The core in the pulse oximeter is a large ring shaped backside silicon pn photodiode placed around a Ledtronics dual LED with wavelengths of 660 nm and 940 nm. The concentric photodiode gives optimal gathering of light and thereby enabling lower LED drive currents and lower power consumption. To further optimize the photodiode a two layer SiO₂/SiN interference filter is employed yielding 98% transmission at the wavelengths of the LED and damping of other wavelengths. The presented photodiode has an inner-outer radius of 3.29 -4.07 mm and an area of 18 mm² , however, photodiodes with ring center radii ranging from 2.8 -4.9 mm have been fabricated. Using the pulse oxymetry sensor photoplethysmograms clearly showing the cardiovascular cycle are recorded. An on-chip integrated Au thermistor for surface body temperature measurements is found to have a repeatability of plusmn0.26degC.