Pulse oximeter design

Pulse oximeters measure arterial oxygen saturation by sensing the absorption properties of deoxygenated and oxyhemoglobin using different wavelengths of light. Basic meters consist of sensors placed on the patient's earlobe, toe, finger or other body part and a data acquisition system that calculates and displays oxygen saturation, heart rate and blood flow.

Typically, pulse oximeters require ultra-low power and low noise power rails; to support extended battery life and precision measurements. TI's buck-boost converters provide support for Li-Ion battery technology and 96% efficiency. High PSRR LDOs can also be used for other low noise power rails. TI's linear Li-Ion low single-cell charger family meets the requirements of wall plug and USB port charging. The innovative next-generation fuel monitoring solution introduced uses "Impedance Track" to automatically understand/detect battery characteristics, thereby extending battery life and system operation time.

Low-end portable pulse oximeter

To accommodate low-end designs, TI's family of highly integrated MSP430 ultra-low-power microcontrollers (MCUs) reduces the number of external components required in a design because the signal chain, power management and display driver elements are integrated into the MCU.

Signal Acquisition Challenges: Inverting feedback resistor configurations are often used with gain amplifiers in signal chains. However, large feedback resistor values ​​can drive very high output swings with slight changes in light intensity due to circuit sensitivity levels. Some designs can benefit from pushing the output swing to or below ground. Dual-supply auto-zero transimpedance amplifiers allow the output to swing to ground and single-supply devices to swing very close to ground. When the output voltage is very close to 0V, a pull-down resistor to –5V allows the output to swing slightly below ground to minimize errors. TI offers a range of transimpedance amplifiers that offer very high precision, excellent long-term stability, and very low 1/f noise.

Mid-range and high-end portable pulse oximeters

Mid- to high-end implementations may require higher performance processors with low supply current and higher precision analog components. TI's low-power DSP technology can remove signal distortion caused by other light sources or movement when reading the information, extracting only the important signal. Through complex algorithms, DSP technology can accurately read very low-level signals. This additional processing capability is very useful in pulse oximeters, which can measure absorption at other wavelengths to detect the saturation of other types of oxygenated hemoglobin.

Signal acquisition challenges: TI's precision switched integrator transimpedance amplifiers do not have the thermal noise of feedback resistors nor the stability issues often found in transimpedance amplifiers that use large feedback resistors. Using a photodiode with two integrator transimpedance amplifiers eliminates dark current and ambient light errors because the errors common to both are subtracted. In addition, these amplifiers allow simultaneous sampling at integer multiples of the AC line frequency, providing extremely high noise rejection. The transimpedance gain can be easily changed through an on-chip setting. In addition, TI's high-precision ADCs provide a small package single-chip solution with excellent AC/DC performance for measuring photodiodes.