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sponsored FeATUre u Cover story


Optical integratiOn withOut cOmprOmises


By János Pálhalmi, Ph.D., CEO at DataSenseLabs Ltd and Jan-Hein Broeders, business development manager at Analog Devices, Inc.


P


hotoplethysmography (PPG) is a common technology for measuring oxygen saturation (SPO2) levels in


blood. Light is sent by a light emitter into the body, and the amount of reflective or unabsorbed light is measured with a photoreceiver. Depending on the ratio between two wavelengths, the amount of oxygenated hemoglobin can be measured. Comparable technologies are also used to measure heart rate with an optical technology or heart rate variability. All these systems require one or more photoemitters, which need to be controlled, and a photodetector to measure the amount


of photocurrent as a measure for the received light. This receive signal finally needs to be amplified, conditioned, and digitised. Such an optical system might sound straightforward; however, with a missing dose of optical knowledge, it is very easy to retrieve an optical signal, which does not have anything to do with the signals the user is looking for. To help companies achieve their optical objectives, a new, fully integrated optical module has been introduced. It has been tested and compared to a well-proven discrete optical system with outstanding results. You will read more about the results and methodologies behind this exercise.


Figure 1. Classical block diagram of an optical HRM/HRV system.


THEOry AnD InTrODuCTIOn TO PPG MEASurEMEnT With an increased focus on home health, wellness, and prevention, a new market has been born around smart devices for tracking several vital parameters. Initially this started with chest straps—monitoring heart rate using a biopotential technology—but the last five to eight years have brought a big shift toward optical systems, making use of photoplethysmography (PPG). The big advantage of this technology is that we can measure at a single spot on the body, whereas biopotential systems require at least two electrodes that measure across the heart. This is not very user friendly, and, for that reason, interest in optical heart rate monitoring (HRM) and monitoring heart rate variability (HRV) has increased drastically. While designing a system like this, there are


several questions that need to be answered. What is the end application, at what location on the body do you want to measure, and how much time do you have for developing the system? Depending on the answers to these questions, the designer might follow a certain design route. There are two different principles for


measuring PPG. You can send light through a part of the body, such as finger or earlobe, and measure the amount of received or not absorbed light at the opposite side; or light is sent at the same side of the body where the amount of reflected light is being measured. Measuring by transmission through the body gives you roughly 40 dB to 60 dB more signal compared to a reflective system; however, in a


8 April 2021 Instrumentation Monthly


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