A balun transimpedance amplifier with adjustable gain for integrated SPO2 optic sensors

The oxygen level in blood, usually referred as SPO2 (Saturation of hemoglobin with oxygen as measured by pulseoximetry) 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 by 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 always 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 in first stage employing noise cancellation and balun operation using an additional 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. The electrical simulations show that the overall power consumption is lower than 250 μ W and input referred noise power density is extremely low.