A Highly Duty-Cycled PPG Sensor With Ultralow-Power Consumption and Wide Input Range

This article presents an innovative ultralow-power photoplethysmography (PPG) readout circuit designed for driving red (R) and infrared (IR) light-emitting diodes (LEDs) and extracting a PPG signal with high energy efficiency. Implemented in TSMC 130-nm CMOS technology, this new PPG sensor improves upon previous solutions in several ways including: 1) employing a current buffer between the photodiode (PD) and the transimpedance amplifier (TIA) to isolate the PD’s substantial parasitic capacitance, which among others, allows using shorter duty cycles than other solutions, thereby reducing the power consumption significantly; 2) utilizing a class AB output stage in the TIA to enhance the dynamic range and cope with large dc input components; and 3) using a novel LED driver circuit, which relaxes the power supply requirements and, consequently, reduces the overall power consumption. Employing the mentioned techniques, the TIA achieves a wide input current range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$180~\mu $ </tex-math></inline-formula>A, preventing circuit saturation. Also, with a 1% duty cycle (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$100~\mu $ </tex-math></inline-formula>s) and a PWM frequency of 100 Hz, the measured power consumption is 0.3 mW, with typical driving currents of 9 mA for IR, and 15 mA for red LEDs. The fabricated chip was incorporated into a finger-clip PPG sensor integrating LEDs and a PD for facilitating testing and validation in realistic conditions with five participants.