Transimpedance amplifier for integrated SpO2 optic sensor

Abstract

The oxygen level in blood, usually referred as SpO2 (Saturation of hemoglobin with oxygen as measured by pulse oximetry) is an essential medical information. Measuring the oxygen level of the human blood using non- intrusive techniques is a vital achievement in modern medicine. This can be performed by processing the infrared and red light transmitted through the patient’s finger and received by a photoreceptor. Before being applied to an analog-to-digital converter (ADC), the incoming light has to be converted to a voltage and the range should be dynamically adjusted in order to use the full input range of the ADC. Since the photoreceptor generates an output current, a transimpedance amplifier (TIA) with gain control is required. The two-stage TIA proposed in this paper, uses a regulated common-gate (RCG), in the first stage, employing noise cancellation and balun operation using an additional common-source (CS) stage, while the adjustable gain is implemented in the second-stage, which is based on an intrinsically noiseless MOS parametric amplifier(MPA). This MPA operates in the discrete-time domain, thus, eliminating the need of an input sample-and-hold (S&H) block in the ADC. The proposed circuit has been designed in a 130 nm digital 1.2 V CMOS technology with a power consumption lower than 350µW.