An ultra-low noise and low-power PPG sensor towards continuous operations

Motivation

Nowadays, continuous and reliable health monitoring is becoming more and more important. Modern lifestyle often comes at the cost of increased stress levels and uncontrolled diets. 13% of the world adult population are today obese, meaning 650 million individuals having higher probability of cardio-vascular diseases (CVDs) and diabetes. In addition, the global population is getting older and older. Without a significant change of the modern lifestyle the healthcare costs are expected to explode the countries’ budgets. In this perspective, any system capable of continuous and reliable health monitoring would represent a game changer.

Objectives

Photoplethysmography (PPG) is a key technology allowing non-invasive monitoring of crucial vital indicators such as the heart rate (HR), the oxygen saturation (SpO2) and the blood pressure. A PPG signal is obtained by shining light from an LED at a given wavelength, visible or infrared, into a human tissue, e.g. finger, forehead, ear lobs. A photodetector (PD) detects the light transmitted through or reflected from the tissue and transforms it into a photogenerated current. Despite the goodness of the PPG sensor solutions presented so far, both in academia or in commercial products, their power consumption is still strongly affected by the few tens of mAs of the LED drivers and we should not expect dramatic improvements unless there are fundamental changes in the PPG sensor technology. This is particularly true on the PD side, being its parasitic capacitance one of the limiting factor in terms of power/noise. In this regard, the objective of this work is to re-design a PPG sensor with the aim of dramatically reducing the LED power towards truly continuous operations. We have presented a fully integrated sensor addressing the power issue by combining a high sensitivity PD together with an ultra-low noise and low power readout chain. Compared to conventional solution, 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 sub-electron noise. The PPDs array is implemented on the same chip together with the AFE including the analog-to-digital conversion (ADC). 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.

Results

The use of an array additionally enables spatial averaging leading to further noise reduction. Consequently, the LED power can be reduced dramatically. Indeed, the LED power can be reduced if the noise floor is reduced accordingly. A prototype has been implemented in a 180 nm CMOS Image Sensor (CIS) process, achieving 4.6 µW total power consumption, including 1.97 µW LED power, at 1.38 HR average error, compared to a chest device. In terms of power, this is from 1 to 2 order of magnitude better than other works. Last but not the least, the integration of the PD together with the electronic circuitry into a CMOS process make this solution particularly appealing for high volume production, such as the consumer market.

Publications

  1. A. Caizzone, C. Enz, and A. Boukhayma, “Comprehensive Noise Analysis in PPG Readout Chains,” 2017 International Conference on Noise and Fluctuations (ICNF), June 2017, DOI: 10.1109/ICNF.2017.7985971.
  2. A. Caizzone, A. Boukhayma, and C. Enz, “A 2.6 µW Monolithic CMOS Photoplethysmographic Sensor Operating with 2 µW LED Power,” in 2019 IEEE International Solid State Circuits Conference – (ISSCC), February 2019, DOI: 10.1109/ISSCC.2019.8662404, pp. 290–291.
  3. A. Caizzone, A. Boukhayma, and C. Enz, “A 2.6 µW W Monolithic CMOS Photoplethysmographic (PPG) Sensor Operating with 2µW LED Power for Continuous Health Monitoring,” IEEE Transactions on Biomedical Circuits and Systems – (TBIOCAS), pp. 1243-1253, vol. 13, n. 6, December 2019, DOI: 10.1109/TBCAS.2019.2944393. invited journal to the ISSCC special issue.
  4. A. Caizzone, A. Boukhayma, and C. Enz, “AC/DC Ratio Enhancement in Photoplethysmography Using a Pinned Photodiode,” IEEE Electron Device Letters – (EDL), pp. 1828-2831, vol. 40, n. 11, November 2019, DOI: 10.1109/LED.2019.2940063.
  5. A. Boukhayma, A. Caizzone, and C. Enz, “An Ultra-Low Power PPG and mm-Resolution ToF PPD-Based CMOS Chip Towards All-in-One Photonic Sensors,” IEEE Sensors Journal , pp. 11858-11866, vol. 19, n. 24, December 2019, DOI: 10.1109/JSEN.2019.2939479.
  6. A. Boukhayma, A. Caizzone, R. Capoccia, and C. Enz, “Design and Optimization of Low Power and Low Light Sensor,” accepted at the 2020 IEEE Custom Integrated Circuits Conference – (CICC). invited paper.