This blurring of the lines between consumer and medical technology depends on the accuracy and reliability of the data sourced from wearable devices. Fortunately, the measurement performance of wearable devices constantly improves. That’s in large part because of improvements in the optical and electrical components they use.
A smart watch or wristband measures the heart rate and blood oxygenation optically. LEDs emit light into the tissues of the wearer’s arm, and a photodiode measures the amount of light scattered to the detector. Differences in the volume of blood in the arteries, and its oxygenation, are mirrored by variations in the intensity of scattered light detected by photodiodes. This optical measurement technique, called photoplethysmography (PPG), enables a wearable device to track the action of the heart and lungs.
From an optical engineering point of view, the wrist is a challenging place to operate LEDs and photodiodes – it is hard to maintain a stable interface between the optical components and the wearer’s skin. This means that a vital sign monitoring system needs a combination of
- LEDs that emit bright light at specified wavelengths
- Photodiodes that are highly sensitive to these wavelengths
- An analog front end (AFE), an IC which provides the current for the LEDs, as well as amplifies and digitizes the photocurrent signal from the photodiodes
The whole system is optimized for low noise and high signal-to-noise ratio (SNR), while consuming little power so as to extend the device’s battery run-time as long as possible. To meet these requirements, ams OSRAM recently introduced specialized new components for vital signs applications.
The new SFH 7018 is a multi-LED (red/green/infrared) emitter
with an ingenious two-cavity design which minimizes interference between the green light (for heart rate measurements) and the red and IR sources (for SpO2 measurements). The SFH 7018’s red and IR emitters are 40% brighter than the previous generation product, and the green LED is more than twice as bright. This improved brightness allows to reduce the LED current and the power consumption accordingly while keeping the signal quality at the same level.
The TOPLED® D5140, SFH 2202 photodiode
outperforms standard photodiodes that have often been used in wearable devices. Offering higher sensitivity and much higher linearity, the SFH 2202 enables wearable device manufacturers to improve heart rate and SpO2 measurement in challenging ambient light conditions while reducing power consumption.
ams OSRAM also introduced the AS7058, an AFE which drives LEDs
such as the SFH 7018 and processes the signals received from sensors such as the SFH 2202 or the electrodes of an ECG. The striking feature of the AS7058 is its low-noise performance, producing a very high optical signal-to-noise ratio of up to 120dB, and similarly high performance in ECG measurements. Because the wrist is a challenging place to take optical measurements, the optical signals generated by the photodiode are weak: the low-noise AS7058 extracts the greatest possible amount of information from these weak signals, and from the low-voltage output of ECG electrodes, to enable the device’s algorithms to calculate accurate measurements of vital signs.
The AS7058 also provides measurement outputs for additional vital sign parameters: electrodermal activity, for measuring sweat, and – uniquely – body impedance (BioZ), which can be used for approximate measurements of body composition and the body mass index (BMI).