A 50μW Fully Differential Interface Amplifier With a Current Steering Class AB Output Stage for PPG and NIRS Recordings

Abstract

This presents a low-power fully differential optical sensor interface amplifier for photoplethysmography (PPG) and near infrared spectroscopy (NIRS) recordings. The proposed amplifier employs a stacked current reuse input stage that provides a DC biasing voltage for the photodiode, reducing the parasitic capacitance. The output stage exploits a fully differential current steering class AB topology with a self-regulated common mode voltage, providing a $70\mu \text{A}$ input current range with minimized quiescent current. Moreover, the interface amplifier can be reconfigured as either single-ended mode suitable for photo detectors biased at a high voltage or fully differential mode for those biased at a low voltage. This amplifier is implemented in a standard $0.18\mu \text{m}$ CMOS process and characterized experimentally. Measurement results show that, with a 1.5V supply, the amplifier can provide up to 1V biasing voltage for the photodiode and consumes a static power of around $50\mu \text{W}$ . It achieves a dynamic range of 97dB in fully differential mode with an input referred noise of 67pArms in 32Hz band and 102dB in single -ended mode with an input referred noise of 582pArms.