Abstract
Photoplethysmography (PPG) enables wearable vitals monitoring. Nevertheless, it is still limited by the few mA of the LEDs driving current. We present a PPG sensor integrating an array of dedicated pinned-photodiodes (PPD) with a full readout chain integrated in a 0.18 μm CMOS Image Sensor (CIS) process. The sensor features a total input referred noise of 0.68 e-rms per PPD, independently of the input light, and achieves a 4.6 μW total power consumption, including the 2 μW LED power, at 1.38 bpm heart rate average error.
Highlights
N OWADAYS continuous and reliable health monitoring is becoming more and more important
PPDs have been first developed for chargedcoupled device (CCD) technology for their enhanced performances, such as low dark current and good effective quantum efficiency (EQE), as illustrated in Fig. 2(c) [14]
The overall analog-to-digital conversion (ADC) is operated at 1.8 V power supply, apart from the four input switches CKs and CKf which are driven at a 3.3 V gate voltage to cope with the amplifier’s output that can take values larger than 1.8 V
Summary
N OWADAYS continuous and reliable health monitoring is becoming more and more important. This work achieves the same signal-to-noise ratio (SNR) at a significantly lower LED power. This PPG sensor integrates an array of pinned-photodiodes (PPD), commonly used in CMOS imagers for achieving subelectron noise [14]. The full CMOS integration allows to dramatically reduce the parasitic capacitance at the sensing node leading to a larger conversion gain and a lower noise This approach provides higher miniaturization and lower cost compared to traditional solutions with off-chip PDs. The use of an array enables spatial averaging leading to further noise reduction.
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