Abstract
To overcome light interference, including a large DC offset and ambient light variation, a robust photoplethysmogram (PPG) readout chip is fabricated using a 0.13-μm complementary metal–oxide–semiconductor (CMOS) process. Against the large DC offset, a saturation detection and current feedback circuit is proposed to compensate for an offset current of up to 30 μA. For robustness against optical path variation, an automatic emitted light compensation method is adopted. To prevent ambient light interference, an alternating sampling and charge redistribution technique is also proposed. In the proposed technique, no additional power is consumed, and only three differential switches and one capacitor are required. The PPG readout channel consumes 26.4 μW and has an input referred current noise of 260 pArms.
Highlights
The photoplethysmogram (PPG) is widely used for monitoring cardiovascular parameters such as heartbeat rate, blood vessel stiffness, and pulse transit time blood pressure [1,2,3,4]
The sensor system consists of photodiodes (PDs), red, infrared, or green light-emitting diodes (LEDs), and a transimpedance amplifier (TIA)
According to the measurement principle, in addition to the signal component of interest, two main interference components are induced by the ambient light intensity and optical path variation
Summary
The photoplethysmogram (PPG) is widely used for monitoring cardiovascular parameters such as heartbeat rate, blood vessel stiffness, and pulse transit time blood pressure [1,2,3,4]. According to the measurement principle, in addition to the signal component of interest, two main interference components are induced by the ambient light intensity and optical path variation. These interferences can be either a direct current (DC) or an alternating current (AC) component of a detected signal. Owing to variation in the ambient light intensity or optical path condition between the light source and the detector, the unwanted AC component is present in the detected signal. An ambient light, such as the sun or a fluorescent lamp, can be a noise source because it is unknown and unpredictable. (2) ambient light cancellation (ALC) through alternating sampling and charge redistribution, and (3) automatic emitted light compensation (AEC) against optical path variation
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