Temperature-less-sensitive trans-impedance amplifier with background light cancellation loop

A temperature-less-sensitive photoplethysmography (PPG) sensor integrated circuit with a background light loop for monitoring vital signs is proposed. The proposed PPG sensor contains a low-power trans-impedance amplifier (TIA) with a background light (BGL) loop and a temperature sensing circuit (TSC). The background light loop assists in rejecting the DC photocurrent up to 15 μA, improving the total harmonic distortion (THD) by 15.7X. Meanwhile, the TSC improves the TIA's temperature robustness by more than 14X over a wide range of temperature variations from −40°C to 85°C. The proposed circuit is designed and analyzed using the signal-flow graph (SFG) strategy to prove the functionality's performance. In TSMC 0.18 μm CMOS technology, the circuit is simulated and occupies a silicon area of 320 μm x 345 μm, providing seamless integration with the wearable devices. The pre- and post-layout simulation results are matched and consistent with a relative error less than 1 %. Statistical analysis based on Monte-Carlo (MC) and process–voltage–temperature variations (PVTs) have been executed to guarantee the well-functionality performance throughout future manufacture. The post-layout simulation results indicate the midband gain reaches 115 dB and the power consumption is 27 μW with a single power supply of 1.8 V. Furthermore, the high-pass corner frequency, the low-pass corner frequency, and the total input referred noise reach 60 mHz, 112 Hz, and 2 pArms, respectively. Out of the figure of merits (FOMs), the functionality performance of the proposed circuit has been verified with a perfusion index (PI) of 3.3 % and performs superior to the prior arts.