A 1.5 mW front-end readout circuit for a small-sized melanin sensor

This study presents a novel readout circuit that serves to evaluate quantity of skin melanin in a noninvasive way via utilizing a photoplethysmography (PPG) optical sensor. PPG sensors are widely used nowadays for noninvasive diagnosis due to their salient features such as low cost and easy-to-useness. PPG signal is known highly correlated with human skin condition and cardiovascular system health. A new readout circuit featuring high-bandwidth, low-noise and low-power is designed in this study to evaluate quantitatively the melanin content of human skin based on PPG signals with high accuracy. This new readout circuit is intended to convert and amplify the electric current of the photo diode in the PPG sensor to voltage signals, and then being further properly amplified to the full dynamic range of the chip circuit fabricated by the TSMC 0.18 μm process for maximum S/N ratios. The designed circuit has a front-end transimpedance amplifier (TIA), 60 dB in gain, with a degenerate resistor to perform a low noise less than 3 µV/√Hz while operating by a 3.3 V supply. In related circuit designs, the mismatch in current sources caused by CMOS process variation is mitigated by a current steering digital-analog-converter (DAC) with least significant bit (LSB) corresponding to 1.25 µA. Both TIA and DAC are designed with effort to render high accuracy in the digital outputs for the levels of sensed melanin. The power consumption of chip design is as low as 1.5 mW, while the error rate between measurement and simulation result is less than 3 %.