The pulse oximeter measures the oxygen content of the blood in a non-invasive manner, measured as a percentage of the full saturation level, expressed as a single value, the so-called percentage of blood oxygen saturation, often referred to as SpO2.
The Principle and typical architecture of pulse oximeter system
The pulse oximeter measures the oxygen content of the blood in a non-invasive manner, measured as a percentage of the full saturation level, expressed as a single value, the so-called percentage of blood oxygen saturation, often referred to as SpO2.
This measurement is based on the light absorption properties of hemoglobin in the blood. In the visible and near-infrared spectra, oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) have different absorption curves. The red light frequency absorbed by Hb has more light, and the infrared light (IR) frequency has less light. HbO2, on the other hand, absorbs less red light at a higher frequency and more infrared light (IR).
The red and infrared LEDs are as close to each other as possible, transmitting light through a single tissue location in the human body. Red and infrared LEDs use time multiplexing to transmit light and therefore do not interfere with each other. After the ambient light is estimated, it is subtracted from the red and infrared signals.
A photodiode capable of responding to red and infrared light receives light, and then a trans conductance amplifier produces a voltage proportional to the intensity of the received light. The ratio of red and infrared light received by the photo diode is used to calculate the percentage of oxygen in the blood. Based on the pulse characteristics of blood flow, the pulse rate and intensity are also determined and displayed during the measurement cycle.
The pulse oximeter includes a transmit path, a receive path, a display and backlight, a data interface, and an audio alarm. The transmission path includes a red LED, an infrared LED, and a DAC for driving the LED. The receive path includes a photodiode sensor, signal conditioning, an analog to digital converter, and a processor.
The Pulse oximetry system design considerations and major challenges
When designing a pulse oximeter system, multiple problems need to be solved, such as low blood perfusion, exercise and skin moisture, stray light interference, carboxyhemoglobin and methemoglobin interference.
• Low blood perfusion (small signal level). Photodiode measurements require signal conditioning with wide dynamic range and low noise gain to capture pulse events. The transmit and receive paths require high quality, low noise LED driver circuits with high resolution DACs and high precision analog front end circuits with high resolution ADCs.
• Exercise and skin moisture. Movement can cause artifacts, which can be solved by software algorithms, or by an accelerometer such as the ADXL345.
• Stray light interference. Photodiodes are used to respond to red and infrared light, which is easily disturbed by ambient light. Therefore, the algorithm used to filter out the red and infrared target signals is very important, which means that the signal processing is more complicated. In this case, a DSP with higher signal processing capability is required.
• Carbooxyhemoglobin and methemoglobin. Carbon monoxide (CO) readily binds to hemoglobin, making the blood more red-like HbO2, resulting in a falsely high SpO2 value. The iron in the heme matrix is in an abnormal state and cannot carry oxygen (Fe+3 instead of Fe+2), resulting in a decrease in hemoglobin and a falsely low reading of SpO2. Using more wavelengths can improve accuracy, but this requires a higher performance digital processing DSP. Processing time is critical.
Analog Devices offers a full line of high performance linear, mixed-signal, MEMS and digital signal processing technologies for pulse oximeter design. Our data converters, amplifiers, microcontrollers, digital signal processors, RF transceivers and power management products are backed by leading design tools, application support and system experience.
Pulse oximeter functional block diagram at belows
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