研究目的
To develop a monolithic light-to-frequency converter that balances low power consumption and fast tracking speed for blood oxygen saturation (SpO2) sensing in wearable devices, addressing the contradiction between these two characteristics through adaptive power scaling based on light intensity.
研究成果
The proposed light-to-frequency converter successfully achieves low power consumption (scalable from 1.9 mA to 0.7 mA) and fast tracking (within two output cycles) for blood SpO2 sensing. The adaptive power scaling technique in the analog domain reduces power without adding noise, making it suitable for wearable devices. Measurement results confirm competitive performance with industry standards, though improvements in signal processing algorithms could enhance accuracy under low perfusion conditions.
研究不足
The sensor may show deviations in SpO2 measurement under extremely low perfusion index conditions (e.g., up to 14% deviation at low pulse rates). Calibration is not required for normal use but might be needed for low perfusion applications. The design relies on specific CMOS technology (0.35 μm), which may limit scalability to more advanced nodes.
1：Experimental Design and Method Selection:
The design uses a switched-capacitor integrator scheme with an adaptive bias current scaling technique. A light-intensity-positively-correlated (LIPC) control voltage is generated to dynamically tune the amplifier bias current, reducing power consumption at low light intensities without introducing switching noise. The chip is fabricated using 0.35 μm CMOS technology.
2：35 μm CMOS technology.
Sample Selection and Data Sources:
2. Sample Selection and Data Sources: The photodetector consists of a 7x7 Nwell-Psub photodiode array. Measurements are conducted using a Keysight oscilloscope MSO9254A and an LED light integration sphere for noise and performance evaluation. Blood oximeter system tests use a Fluke Index2 oximeter simulator as a gold standard.
3：List of Experimental Equipment and Materials:
Equipment includes a Keysight MSO9254A oscilloscope, LED light integration sphere, Fluke Index2 oximeter simulator, STM32F103 microcontroller, and various light sources (660 nm red and 960 nm infrared LEDs). Materials include the fabricated CMOS chip, photodiode array, and peripheral circuits like bandgap reference and LDO.
4：Experimental Procedures and Operational Workflow:
The chip is tested for output frequency noise, transient pulse response, current consumption, and dynamic range under different light intensities. In the blood oximeter system, output frequencies for red and infrared light are measured to calculate SpO2, with comparisons to commercial products.
5：Data Analysis Methods:
Data is analyzed using oscilloscope measurements for frequency and noise, with statistical methods for standard deviation and signal-to-noise ratio. SpO2 accuracy is evaluated against the Fluke simulator using deviation calculations.
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