High-speed PIN photodiode delivers enhanced sensitivity for visible light

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 company. Anything that provides greater accuracy would be useful.

Vishay said that 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, said Vishay.

The enhanced sensitivity that 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% 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 hypoxia).

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 1,100 nm (deeper into the infrared).

The surface of the device measures 4.8 x 2.5 mm. It is housed in a 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 eight 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.