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Cover story t SponSored FeATUre


reflective system you are free to choose where you place the sensor. Because most users prefer sensor comfort


over performance, reflective measurement became more popular. For that reason, this article will focus on reflective measurement only. During the time span of a heartbeat, the flow


and volume of blood in a cardiac system changes, resulting in a scattering of the amount of reflected light received. The wavelength of the light source that is used for measuring optical HRM/HRV depends not only on the location on the body where you are measuring, but also on the relative perfusion level, as well as the temperature and colour tone of the tissue. In general, for devices worn at the wrist, where arteries are not located on top of the wrist, you need to pick up pulsatile components from veins and capillaries just under the skin surface. Green light in this situation gives the best results. At places where you have enough blood flow, like the upper arm, temple, or ear canal, red or infrared light might be more effective as they penetrate deeper into the tissue, resulting in a stronger receive signal.


ADPD188 Is GAme ChAnGInG


When trade-offs such as sensor position and LED wavelength are being made, you need to select the most appropriate optical solution. There are many choices in terms of analogue front ends, either discretely built or fully integrated, but there is also a wide offering of photodetectors and LEDs that can be selected. It is key to place the transmitter and receiver in such a way that you get the maximum amount of receive signal for every milliampere of


Instrumentation Monthly April 2021


transmit current used. This is called the current transfer ratio and is usually given in nA/mA. What is also important in an optical system is the modulation index, which is the amount of ac signal in relation to the optical dc offset. Modulation index improves when you increase the spacing between the photosensor and the LED. There is a certain sweet spot along the distance between photodetector and LEDs, and this is also dependent on the LED wavelength. In an improperly designed mechanical system, LED light can reach the photosensor directly without having penetrated the tissue. This results in a dc offset that will have a negative impact on the modulation index. It shows up as optical crosstalk, also known as internal light pollution (ILP). To minimise design efforts and to shorten time to market, especially for companies with minimal optical knowledge, ADI built a fully integrated optical subsystem for reflective measurement. This is called ADPD188GG and contains everything needed to run an optical measurement. In Figure 2, a picture of this module is shown. The ADPD188GG is a completely newly designed optical module with different dimensions compared to earlier generations. The form factor is almost square, 3.98 mm by 5.0 mm, with a total thickness of 0.9 mm. The biggest improvement is to the photodetector, which is rotated 90° from its predecessor. In relation to the LEDs, this sensor position provides a better sensitivity. The photosensor itself has been split into a size of 0.4 mm2 0.8 mm2


and . This provides flexibility to either


increase the overall photodiode surface for more sensitivity or use a smaller size detector to prevent the sensor from saturation. The photodiode is placed on top of the analogue front end. ADI is making use of the standalone ADPD1080 AFE. It has four input channels, each designed around a transimpedance amplifier with selectable gain (25k, 50k, 100k, and 200k), an ambient light rejection block, and a 14-bit


Figure 2. ADPD188GG optical subsystem.


SAR ADC. The ambient light rejection is done in the analogue domain and is superb compared to other solutions on the market. Finally, the two green LEDs are controlled by integrated current sources, capable of driving currents up to 370 mA, and pulses as narrow as 1 μs, to reduce overall average current. The package is designed in such a way that transmitted LED light can hardly reach the photosensor without penetrating the tissue. This prevents optical crosstalk and provides the user the best modulation index, even when the sensor is placed under a glass or plastic window. This is a great feature when designing an optical reflective system. For those applications where transmissive measurement is preferred, the ADPD188GG can be used with externally connected LEDs and by having the internal LEDs bypassed.


ComPArIson to A Proven solutIon


Before you start a new optical design, it is important to determine the target market and required specifications for the final product. An optical system with medical grade performance in general has higher specifications compared to a device intended for the sports and wellness market. The ADPD107 is an analogue optical front end designed for discrete optical systems. It is


Table 1. Optical Module Comparison Between the ADPD188GG and the Gold Standard ADPD107


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