SpO₂
Measures changes in red and infrared light transmitted through, or reflected from, tissue. A complete calibrated system uses the pulsatile optical signals to estimate oxygen saturation and pulse rate. A raw sensor or educational module does not establish clinical accuracy.
The system alternates illumination and measures photodiode current. A transimpedance amplifier converts current to voltage; timed LED and ambient measurements help separate the useful signal from background light. Processing relates the pulsatile components at the two wavelengths to a calibration model. Motion, ambient light and the optical arrangement can affect the result.
In plain terms
Like reading the colour of tomato sauce by holding it up to a torch — bright red oxygen-rich blood absorbs IR, dark red deoxygenated blood absorbs visible-red, and the ratio reveals the mix.
Why designers use it
- Provides an optical approach to pulse and oxygen-saturation monitoring in validated instruments.
- Supports transmissive or reflective optical arrangements.
- Integrates portions of the signal chain: AFE4400 includes LED-driving electronics, a receiver and ADC, but requires external LEDs and a photodiode.
Best for
- Wearables
- ICU monitors
- Spot checks
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- As a substitute for an appropriately validated medical instrument or professional assessment.
- When optical placement, calibration and interference have not been evaluated.
- When a specific sensor's operating conditions or application requirements cannot be met.
Common mistakes
- Assuming an integrated LED driver means the LEDs themselves are inside the IC.
- Treating raw ADC readings as a calibrated oxygen-saturation percentage.
- Ignoring ambient light, motion, sensor placement or component calibration.
- Using an unvalidated educational sensor for diagnosis or treatment decisions.
Where you will find it
- TI's AFE4400 examples connect external red/infrared LEDs and a photodiode to an electronic front end. That front end provides timed acquisition and ADC data to a host; the complete instrument also needs suitable optics, processing and validation.
A short history
ADI's pulse-oximeter design tutorial describes the optical probe, low-noise signal chain and calibration requirements. TI's AFE4400 illustrates integration of the electronic front end while leaving the optical LEDs and photodiode external. Integration reduces circuit complexity; it does not remove the need for complete-system validation.
Good to know
- An LED driver is circuitry that controls an LED; it is not the light-emitting component itself.
- The photodiode receives ambient light as well as the deliberately timed illumination.