Oxygen Saturation (SpO2) has long been considered a vital parameter to be monitored as part of clinical patient care, and has been measured using pulse oximeter devices since the 1970s. Clinical SpO2 monitoring is most commonly done on the finger, with light-emitting diodes (LEDs) positioned on one side of the finger, and a photodiode (PD) on the other side. Such a method is referred to as transmissive pulse oximetry since the light transmitted by the LED passes through the thickness of the finger. The light is absorbed differently by the different components (skin, blood, tissue, and so forth), finally incident on the PD.
SpO2 monitoring is also used in wrist-worn wearable devices like smartwatches, serving as a parameter to asses and monitor the user’s physical condition during exercise. SpO2 monitoring on the wrist uses reflectance pulse oximetry, where the LEDs and photodiode both face the wrist, and the light from the LED is reflected by the skin and various layers below the skin, then incident on the photodiode. SpO2 monitoring on the wrist has also been used to track the quality of sleep and has the potential to detect disorders like sleep apnea. More recently, SpO2 has emerged as a significant parameter, the lowering of which serves as an early indicator of Covid-induced hypoxia.
The AFE4432 is a low-power, high-performance analog front end (AFE) from TI which enables accurate SpO2 monitoring on a wearable device.
Figure 1-1 shows the illustration of an SPO2 monitoring system on a wearable device.
Figure 1-2 shows the reference schematic for an SPO2 system using the AFE4432.
Table 1-1 lists the key specifications for an SPO2 measurement system.
|Multi-sensor support for SpO2,
heart rate monitoring
|4 LEDs, 3 photodiodes (PDs)||A typical SpO2 system can use a green LED in addition to red and infrared (IR) LEDs. The red and IR wavelengths share the same PD, whereas the green wavelength can use a different PD.|
|Sampling rate||1 Hz–1 kHz||A typical SpO2 application can use a sampling rate of a few 100s of Hz|
|Peak SNR||115 dB over 10-Hz bandwidth||Important consideration to achieve good accuracy in low perfusion cases|
|Ambient rejection||> 70 dB up to 160 Hz||Good ambient rejection helps remove the spurious tones caused by ambient light from sources such as indoor lighting.|
Figure 1-3 shows the PPG signal chain in AFE4432 and the interface to external LEDs and PDs.
The LED driver drives up to 4 LEDs with programmable current. Each receiver can interface to up to 3 PDs and comprises a TIA, input DC offset cancellation DACs (for ambient, LED light), and a noise reduction filter with programmable bandwidth. Both receivers share a common analog-to-digital converter (ADC). SpO2 monitoring on wearables is also supported on several other AFEs from Texas Instruments. Recent products include the AFE4950, AFE4960P, and AFE4500.