This application note discusses the optical and mechanical aspects to be considered for optimal performance by PPG ID recommendations - chest patch wearables.
Introduction to optomechanical design Considerations
The optomechanical integration of light-emitting and light-sensing elements into bio-sensing chest wearables is a fundamental step in the wearable design process. The quality of the signal can be greatly affected by choosing components and geometries that minimize crosstalk and maximize signal-to-noise. Signal-to-noise can be increased by maximizing the signal that has penetrated deep enough into the skin to detect a photoplethysmogram (PPG) signal while minimizing crosstalk, which is the signal on the sensor from sources other than the PPG signal. This document discusses the optical and the mechanical aspects to be considered for optimal performance.
Optical Component Selection – Overview
In general, customers can choose any light-emitting diode (LED) or photodiode (PD) depending on the use case. Alternatively, the customer may refer to Maxim reference design files for optical components or take the following recommendations. Optical wavelength selection is vital sign and body part dependent
|Body Part||SpO2||Heart Rate|
|Fingertip||IR Red||IR, Red or Green|
|Green||Osram Firefly, Osram LT PWSG|
|Ratio* 2:1||Vishay VEMD8080, Osram SFH2703|
|Ratio 1:1 (less recommended)||Vishay VEMD5010, VEMD5080, Osram SFH2704|
Component Selection – LED
Wavelength is a choice dependent on the use case. Scattering of light in the tissue is extremely wavelength dependent. Green wavelengths provide the best heart rate signal while minimizing motion artifacts. Red and IR wavelengths are necessary to provide SpO2 information because of their location on the absorption curve of oxygenated and deoxygenated hemoglobin. We recommend the co-packaged Osram SFH 7016 LED with red (655nm), green (530nm) and IR (940nm) wavelengths or the co-packaged Osram SFH 7015 LED with red (655nm) and IR (940nm) + Osram Firefly green (530nm). For application with Heart Rate only, Osram Firefly or LT PWSG is recommended.
Component Selection – PD
The PD is one of the most critical component selection choices in a wearable heart-rate monitor since it is the first stage in the receiving path of the system. There are many PD options available in the broad market, so it is important to choose one with high responsivity at key operating wavelengths or ranges thereof. For Heart Rate application, a NO IR-cut filter on PD is required due to best-in-class ambient light rejection in Maxim PPG AFE. We recommend Vishay's VEMD8080 or Osram 2703 for its high responsivity in the red, green, and IR wavelengths and optimal sensing area and size for the wearable applications.
Transparent cover can provide a moisture barrier and interface between the optical components and the skin. The transparent cover should consist of a material with high transmission (>90%) in the wavelengths utilized to maximize the light emitted into the skin and signal returning from the skin should be as thin as possible to minimize transmission loss, but thick and robust enough to withstand normal wear and tear. It should have an index of refraction close to that of human skin (~1.5) to minimize transmission losses due to Fresnel reflections. We recommend Gorilla glass, polycarbonate, or acrylic, which have high transmission in red, green, and IR wavelengths, are strong and durable, and have indices of refraction of ~1.5.
Due to the mechanical tolerances, an air gap is needed between the optical components and the bottom of the transparent cover. However, the introduction of an air gap allows light to reflect off the bottom of the cover glass and hit the photodetector. This unwanted light does not traverse the skin and degrades the heart rate monitor's performance. As the air gap increases, the crosstalk increases. Thus, the air gap should be kept to minimum. Additionally, an increase in air gap increases the path length necessary for the signal to reach the sensor and thus decreases the total signal received by the sensor. This is yet another reason the air gap between the sensor/LED's should be minimized. We recommend not to exceed an air gap of 0.8mm to ensure acceptable performance.
Crosstalk-Suppressing Features – Light Barriers
Crosstalk consists of signal incident on the PD that has not traversed through any skin layers. High levels of crosstalk drown out the pulsating heart-rate signal, rendering the wearable monitor incapable of measuring PPG effectively. To maintain a low level of crosstalk, physical absorbing light barriers can be used. The example is shown in Figure 1.
A raised mesa is a commonly used technique to help mitigate motion artifacts by ensuring proper coupling between the device and the skin.
Figure 2 shows the mesa concept with recommended dimensions to ensure the proper skin contact required for PPG detection.
Field of View (FOV) of Optical Components
To increase signal-to-noise ratio, the FOV of the LED and PD can be chosen to maximize the signal ratio that has reached the pulsatile vascular bed vs. total signal. The FOV can be tailored by modifying the air gap and crosstalk suppressing barriers. We have found the optimal FOV when using VEMD8080 PDs and a SFH7016 LED to be +/-60 degrees.
Spacing Between LEDs and Sensor – Overview
One of the major design considerations in building a reflectance heart-rate monitor is determining the optimum separation distance between the LEDs and the PDs. This optimum separation is a balance between power consumption and signal quality. The LED-PD separation should be selected such that PPG signals with both maximum and minimum pulsatile components can be detected, while also not consuming too much power. Larger separations increase the signal quality. We can evaluate signal quality by perfusion index (PI) which is the AC/DC ratio of the detected PPG signal due to the pulsatile blood. Smaller separations decrease the power consumption since more light is reflected/scattered back onto the PD when the LED-PD separation is shorter. We can evaluate power consumption by collection efficiency (CE), which is the incident power on the PD due to a 1W LED output.
Spacing Between LEDs and Sensor – Measured Data
Data is measured at the two positions shown to the right using the device shown in Figure 3, which allows for multiple LED-PD spacings to be analyzed.
General Guidelines - Summary
Maximize Contact Area
- Increases heat capacity available from skin.
- Lowers effective Rskin in the primary heat flow direction.
- Use high thermal conductivity material for the contact to minimize the contact resistance between the sensor and the skin.
Minimize System Thermal Mass