The pulse oximeter is a non-immersive instrument used to monitor patient oxygen saturation and is benefiting from an expensive discrete component solution to a more integrated design. However, integration means making a difficult decision about which processing architecture to use.
Pulse oximetry relies on pulse strength or pulsating flow to make measurements, so there must be good blood flow in the area being monitored, and any blockage can cause measurement errors. The pulse oximeter can also calculate the pulse rate, so most pulse oximeters have this function. Typical pulse oximeters measure between 70% and 100% saturation, and readings below 70% are unreliable.
The working principle of the pulse oximeter is based on the change in the amount of light absorbed during arterial pulsation. Two light sources, respectively visible in the visible red spectrum (660 nm) and the infrared spectrum (940 nm), alternately illuminate the area under test (typically fingertips or earlobe). The amount of light absorbed during these pulsations is related to the oxygen content in the blood. The microprocessor calculates the ratio of the two spectra absorbed and compares the results to a table of saturation values ​​stored in the memory to obtain hemorrhagic oxygen saturation.
Typical components of a pulse oximeter include: a microprocessor, memory (EPROM and RAM), two digital-to-analog converters that control the LEDs, devices that filter and amplify signals received by the photodiodes, digitize the received signals to provide An analog to digital converter for the microprocessor. The LED and photodiode are placed in a small probe that is in contact with the patient's fingertip or earlobe. Pulse oximeters also include displays and plotters, which are typically discrete components.
Several analog microcontrollers with A/D and D/A converters, microcontroller cores, memory and other peripheral functions are available from several manufacturers. Only external filtering is required for filtering, signal conditioning, and amplification. If the selected MCU is powerful enough, signal conditioning can be done in the digital domain to further simplify the system.
However, the choice of integrated solutions is challenging because of the wide variety of analog microcontrollers, such as 8-bit, 16-bit and 32-bit CISC and RISC architectures, as well as a variety of A/D, D/A and Other peripheral options.
For MCUs, designers should first consider 8-bit cores, such as the traditional 8051. These cores are well known and easy to understand and use. In addition, there is a large amount of code and proven tools available.
Although 16-bit and 32-bit MCU cores are not popular in data acquisition systems, they are gaining acceptance. Using these wide bus MCUs makes it easier to handle wide byte data than with 8-bit MCUs, so they are easier and faster to calculate for 12-bit, 16-bit, and 24-bit A/D data. With their faster operation speeds, these wide bus MCUs are more efficient at performing complex algorithms than 8-bit MCUs. Tool providers have begun to provide toolkits and support for these 16/32-bit integrated solutions.
For a simple pulse oximeter, the 8-bit MCU core is fully functional, but 16/32-bit devices may be a better choice if more complex filtering, math, or data processing is required. The ideal solution would be: a 16/32-bit MCU with internal program flash and RAM (for storing operating programs, saturation lookup tables and captured data), two D/A converters for driving the light source, A multi-channel A/D converter (which can also be used to monitor other parameters such as battery life) with at least 12-bit resolution to quantify the data received by the photodiode. It would be better if there were some configurable glue logic on the board.
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