Enables slim wearable designs with 0.48-mm profile

By Brian Santo, contributing writer


Health and fitness
trackers remain popular, and nobody likes getting punctured, twin incentives to
continuously increase the accuracy of noninvasive medical and health monitoring
technology. Some of these devices rely on clever optical techniques to make
their measurements. For these applications, Vishay
Intertechnology Inc.

has introduced a high-speed silicon PIN photodiode with enhanced sensitivity
for visible light.

 The ability to
monitor heart rates is migrating from single-purpose devices made by companies
like Fitbit and Xiaomi to general-purpose smartwatches (e.g., the Garmin
Vivoactive 3). Smartwatch sales are expected to grow vigorously over the next
few years, according to Gartner. 

Vishay’s improved
PIN photodiode, the VEMD8080, can be used to improve two different
measurements that rely on two different wavelengths of light: heart rate (which
uses green) and blood oxygenation (which uses red). 

Optical heart
rate monitoring relies on a technique called phytoplethysmography (PPG). Microvascular blood volume
changes as the heart pumps blood in pulses through tissue. The basic concept of
PPG relies on the observation that when you shine light on a tiny area of
tissue, the amount of light that gets scattered changes depending on blood
volume, and blood volume increases and diminishes during each heartbeat. The
goal, then, is to continuously measure the amount of light scattered to get a
heart rate.   

PPG is difficult
to do accurately, especially in wearables, according to one medical sensor
. Anything that
provides greater accuracy would be useful. 

Vishay said its new
PIN photodiode, when used with green LEDs (and not coincidentally Vishay has
one: the VLMTG1400), is appropriate for use in optical
heart beat detection in wearables. The VEMD8080’s rectangular shape allows
several light emitting diodes to be placed close to the radiant sensitive area
to maximize the signal, Vishay said.

The enhanced sensitivity
the company claims is associated not only with ganging together more LEDs, but
also with detecting the scattered light. The company said that for green LEDs,
the VEMD8080’s increase in sensitivity represents a 30 percent improvement over
the standard technology in previous-generation solutions. 

As for blood
oxygen rates? Roughly 10 years ago, noninvasive blood oximeters started
becoming common. The oxygen content of blood is now, as one emergency medical
technician and blogger put it, a “sixth vital sign.” 

Oximeters measure peripheral capillary oxygen saturation, referred
to as SpO2. The goal is to get an estimate of the percentage of oxygenated
hemoglobin in the blood. Oxygenated hemoglobin is red, unoxygenated blood is
blue. Different levels of oxygen saturation will naturally lead to subtle variations
in the color of the blood. 

Oximetry works by emitting light and measuring the variation of
the light passing through tissue (the monitor is typically attached to a
finger). Medical professionals can use this information to gauge whether a
patient needs oxygen or not (an insufficient level of oxygen is called

Vishay said that its new photodiode, when combined with 660 nm
(red) and 940 nm (near infrared) dual color emitting diodes, is “ideal” for
SpO2 measurement in medical monitors. The device’s full spectrum range is 350 nm
(ultraviolet) to 1100 nm (deeper into the infrared). 

The surface of
the device measures 4.8 mm x 2.5 mm. It is housed in surface-mount package with
a 0.48-mm profile. The company claims that makes the PIN photodiode 0.37 mm
lower than its nearest competitors. 

In addition, the VEMD8080 “offers fast switching
times and low capacitance of 47 pF for precise signal detection in wearable devices
and medical applications,” said Vishay. Other specifications include a
radiant-sensitive area of 4.5 mm², high radiant sensitivity with a reverse
light current of 28 µA and dark current of 0.2 nA. 

Samples of the
VEMD8080 are available now. Production quantities will be available in March
2018 with lead times of 8 to 10 weeks for large orders. Pricing for U.S.
delivery starts at $0.55 per piece. A
product demo board is available from MikroElektronika and Vishay distributors.