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

Abstract

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 $\mu$A, 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 $\mu$m CMOS technology, the circuit is simulated and occupies a silicon area of 320 $\mu$m x 345 $\mu$m, 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 $\mu$W 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.