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.