Analog-Based Ratio of Ratios Computation Circuit for Estimating SpO₂ in PPG Acquisition System

The photoplethysmogram (PPG) is one of the widely used biomedical signals used to monitor oxygen saturation $(SpO_{2})$ and other heart conditions. To compute $SpO_{2}$ , it is required to compute the ratio of ratios (RoR) of the amplitudes of two pulsatile waveforms. Usually, this computing is done on a digital signal processor (DSP) and consumes more power. In this work, an analog-based RoR computation circuit is proposed which is more suitable for wearable devices and remote health care monitoring. The proposed RoR circuit design is based on the principle of voltage-mode division circuit technique. The circuit is designed using Cadence with UMC 0.18- $\mu {\mathrm {\rm m}}$ CMOS process under ±0.9-V supply. Post-layout simulations are done to analyze the performance in terms of input-referred noise (IRN), power, total harmonic distortion (THD), and linearity. To test the functionality and to clinically validate the proposed circuit, both real-time signals and signals from standard database (Physionet ATM) is considered. For validation on real-time signals, the PPG analog front end is designed and implemented on field-programmable analog array (FPAA) platform (AN231E04 from Anadigm). Also, the performance is compared with commercially available $SpO_{2}$ measuring devices like BioRadio. The proposed circuit is highly suitable for low-frequency and low-power biomedical applications.