Pulse Oximeter design solution from Maxim Integrated
Oxygen saturation, sometimes called “fifth vital sign” along with body temperature, blood pressure, pulse (heart rate), and breathing rate (respiratory rate), is a parameter that shows how much oxygen is carried in your blood in comparison to its full capacity. Low oxygen saturation can cause serious damage to the internal organs, such as heart and brain, and needs to be promptly addressed.
The pulse oximeter is an optoelectronic device that measures oxygen saturation level in the bloodstream by utilizing light absorption characteristics of the red blood cells, especially heamaglobin. Oxygenated heamoglobin absorbs more infrared light than red light, whereas deoxygenated haemoglobin absorbs more red light than infrared light. By transmitting red and infrared waves using LEDs from one side of the body part (usually finger or ear) and detecting on the other side with a photodiode, the percentage of oxygen in the blood can be calculated. Pulse oximeters can be implemented as a part of bedside patient monitoring equipment or as a stand-alone battery-powered handheld device.
In this article, we are featuring a pulse oximeter design solution from Maxim Integrated. The system diagram below highlights the components they provide to support the design of pulse oximeters.
Maxim Integrated Pulse Oximeter – Block diagram
The light transmitter and detection circuits for pulse oximeter typically consist of two LEDs, a LED driver, a photodiode, and the analog signal conditioning circuit. Designing a complete sensor signal chain on your own can be quite a challenging task, considering the level of precision and accuracy required for this type of medical application.
MAX30101 (190-4792) module combines the transmitter with a detector on a single IC for highly precise implementation of the pulse oximetry and heart rate monitoring. The module has an on-chip temperature sensor for compensating errors in oxygen saturation reading due to ambient temperature changes. The tiny footprint (5.6mm × 3.3mm × 1.55mm) of the MAX30101 could be a decisive factor for adopting this sensor solution in smartphones and wearable devices.
You might want to try MikroElektronika’s Heart Rate 4 click (139-3650) board with an in-built MAX30101 module during initial prototyping stages.
Replacing the sensor signal chain with a single sensor-chip with digital output simplifies the microcontroller selection process as well. The 32-bit MAX32660 microcontroller operating at 96MHz is an ideal low power and low-cost option for this application. It is based on the Arm® Cortex®-M4 processor with a floating-point unit (FPU) and has the industry’s smallest form factor: 1.6mm x 1.6mm. 256KB of flash memory and 96KB of RAM should be sufficient to store application and sensor code.
For continuous monitoring of the oxygen saturation level, the data from the sensors needs to be logged in the system with timestamps. Having a discrete real-time clock (RTC) for this purpose in your device is optional since the majority of microcontrollers will have the function built-in. However, battery-powered devices cannot afford to power their microcontrollers constantly to keep the internal RTC running. DS3231 (103-9916) is a low-cost, extremely accurate I2C RTC with an integrated temperature-compensated crystal oscillator (TCXO) and crystal, which has an incorporated battery-powered input allowing to run the clock while the microcontroller is powered down.
Power and Battery Management
Maxim’s MAX14676 is an ideal battery-charge management solution for this application which combines a linear battery-charger with a Smart Power Selector™, ModelGauge™ fuel gauge, and several power-optimized peripherals in a single IC.
When installing new or replacing old batteries, a secure authentication method of some sort needs to be put in place. This is especially true for Li-Io battery technology, where safety is a major concern. The DeepCoverTM Secure Authenticator DS28E15 (191-4791) is designed to provide strong cryptographic security based on the SHA-256 algorithm. It features 512 bits of user-programmable EEPROM for storing application data and additional secure memory for holding a key for SHA-256 operations. The DS28E15 communicates with the application processor through the 1-WireTM bus, reducing the number of pins required to implement battery authentication.
The pulse oximeter requires a simple LED or LCD display that is compact but also does not drain much power if the device is mainly operated from a battery. MAX1553 step-up converter can be utilized to drive from 2 to 10 LEDs in series to provide display backlight for LCD displays. Backlighting is not necessary for LED displays.
Push-button switches can be added to the device to implement reset functionality and/or perform any specific action. MAX16054 (751-2623) pushbutton on/off controller can be used to eliminate bounce during switch opening and closing. It converts a noisy input from a mechanical switch to a clean lathed digital output. Alternatively, the MAX7359 (146-1302) key switch controller is suitable for managing microcontroller with up to 64 key switches without external components. The controller supports auto-sleep and auto wake features for reducing power consumption in battery-powered applications.
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