Design of an Analog Front-End (AFE) for Photoplethysmography (PPG) Acquisition System

This project implies a low power photoplethysmography (PPG) sensor circuit. PPG sensors are widely used in modern healthcare products for monitoring cardiovascular information. The PPG Signal can reveal diverse conditions, such as respiratory and cardiac arrest, systemic inflammatory response syndrome, renal insufficiency, cardiac insufficiency, systolic arterial pressure, among others. The low-power Analog Front-End (AFE) of the PPG sensor is designed in 0.18 µm CMOS process. The AFE amplifies the weak photocurrent from the photodiode (PD) and converts it to a strong voltage at the output by the means of Transimpedance Amplifier (TIA). To decrease the power consumption, the circuits are designed in subthreshold region; so, the total biasing current of the AFE is a few microwatts. The TIA core, consisting of a self-biased two-stage Miller OTA, operates with a Bandgap Reference (BGR) to manage supply voltage fluctuations. A voltage-to-current converter with a BGR ensures a stable and accurate current output, minimizing dependence on temperature and supply variations. Pseudo-resistance provides the large resistance needed for high TIA gain while maintaining a small chip area. DC offset, which affects TIA operation and amplification, is addressed. An IDAC controlling circuit is used for DC cancellation, enhancing the amplifier’s dynamic range by keeping the AC signal clear at the output.