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Consumer Peripherals

Space-Saving Photodetector Targets Fitness Trackers

27 December 2016

The new IR Topled D5140 photodiode from Osram Opto Semiconductors requires considerably less PC board space than before, allowing for more compact sensors to be embedded in a design. The spectral sensitivity of the D5140 has been optimized for visible light and allows precise heart rate measurements using red or green light. Osram considers the component ideal for use in fitness armbands as well as for use as an ambient light sensor.

Optimized for heart rate measurements using red or green light, Osram's IR Topled D5140 photodiode offers packaging improvements and a smaller physical size compared to its predecessor. Image source: Osram Opto Semiconductors.Optimized for heart rate measurements using red or green light, Osram's IR Topled D5140 photodiode offers packaging improvements and a smaller physical size compared to its predecessor. Image source: Osram Opto Semiconductors.The IR Topled D5140 is a further development of the SFH 2440 photodiode, which Osram recently introduced for fitness sensors. A pre-molded package soldered to the bottom of the overall package forms the basis of the new component, whereas earlier packages had to be soldered to side legs. Because these legs are no longer part of the new design, the Topled D5140 is approximately 1.4 mm shorter, now measuring 5.1 mm.

The electrical and optical properties of the new photodiode are identical to those of its counterpart, the SFH 2440. Along with the short switching time of 90 nanoseconds (typical), the spectral sensitivity optimized for visible light is particularly noteworthy, with a maximum sensitivity of 620 nanometers. However in the infrared spectral range, the sensitivity of the two photodiodes is greatly suppressed.

Precise heart rate measurements

Just like the SFH 2440, Osram rates the IR Topled D5140 ideal for pulse rate measurements on the wrist, for example. The device works by shining visible light on the surface of the skin; some of the light is absorbed, and some is reflected to the detector. Because arterial blood absorbs more light than the surrounding tissue, the strength of the detector signal changes with the volume of blood through which the light passes. The periodicity of the signal then indicates the heart rate. However infrared light that shines onto the measuring point from the surroundings and disperses in the body also reaches the photodiode.

The Topled D5140 offers an outstanding signal-to-noise-ratio because it registers the reflected visible light particularly well while at the same time suppresses the infrared light. Thanks to its short switching time, the light signal modulated with the heart rate can be time-resolved perfectly.

Ambient light sensor

Applications that have been using large-area photodiodes, such as the SFH 2440 or SFH 2430, as ambient light sensors can also be made smaller with the new component. For its automotive customers, Osram plans to offer a photodiode with broadband spectral sensitivity in the new package.



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