Test & measurement
systems is a single green, red, and IR LED array surrounded by multiple PDs, as seen on the ADI VSM watch in Figure 5. PD to LED spacing has been optimised to reduce backscatter and the baffle design reduces LED to PD crosstalk. Multiple prototypes of the ADI VSM
watch were trialed to verify the most efficient PD to LED spacing for our HR PPG and SpO2 measurement.
Motion Artifacts Motion ar tifacts provide one of the greatest design challenges to a PPG measurement system. When motion is present, the width of the ar teries and veins change due to pressure. The amount of light absorbed by the photodiode changes and this is present on the PPG signal because photons are absorbed or reflected differently than when a body is at rest. For an infinitely wide photodiode area
Figure 4. LED-PD configuration. Reflective PPG configurations are chosen
when the PD and LED must be placed next to each other for practicality, such as with wrist- or chest-worn devices.
Sensor Positioning and Perfusion Index Positioning on the wrist and chest require greater dynamic range in the PPG AFE as the DC signal is greatly increased due to the depth of the arteries below static reflective components such as skin, fat, and bone. Greater resolution in the PPG
measurements will reduce the uncertainty in
the SpO2 algorithm. With a typical PI of one to two per cent for wrist-worn SpO2 sensors, the goal of pulse oximeter design is to increase the
PI through mechanical design or to increase the dynamic range. The spacing of the LED to PD will have
a major effect on the PI. Too little spacing will increase LED to PD crosstalk or backscatter. This will appear as a DC signal and saturate the AFE. Increasing this spacing reduces the effect of
both backscatter and crosstalk but also reduces the current transformer ratio (CTR), which is the LED output to PD return current. This will affect the efficiency of the PPG system and require greater LED power to maximise the AFE dynamic range. Rapidly pulsing one or multiple LEDs has the
benefit of reducing the 1/f noise contribution to the overall signal. Pulsing the LEDs also makes it possible to use synchronised modulation at the receive side to cancel out ambient light interferers. Integrating multiple pulses increases the PD signal amplitude and lowers the average current consumption. Increasing the total PD area also increases CTR as more of the reflective light is captured.
20 For hear t rate PPG measurement, a
combination of a single large PD and multiple power efficient green LEDs has been adopted by many HR device manufacturers to be used on places where there is limited blood flow. Green LEDs are chosen due to their high rejection of motion ar tifacts. However, this comes at the cost of power. Green LEDs have a higher forward voltage than red and IR and high absorbance in human tissue, meaning a higher LED power is required to return meaningful cardiac information. As SpO2 requires multiple wavelengths and
most systems still incorporate high efficiency green LEDs for the HR PPG, the most common configuration for HR and SpO2 PPG
covering an infinitely long deep tissue sample, all photons will eventually be reflected to the photodiode. In this case, no artifact due to motion will be detected. This, however, cannot be achieved; the solution is to increase the photodiode area while taking capacitance into account—lowering AFE and providing filtering for motion artifacts. The normal frequency for a PPG signal is
between 0.5Hz to 5Hz while motion artefacts are typically between 0.01Hz to 10Hz. Simple band-pass filtering techniques cannot be used to remove motion artifacts from the PPG signal. To achieve high accuracy motion cancellation, an adaptive filter needs to be supplied with highly accurate motion data. For this purpose, Analog Devices has developed the ADXL362 3-axis accelerometer. This accelerometer provides 1mg resolution with up to 8g of range while consuming only 3.6μW at 100Hz and is available in a 3mm × 3mm package.
Figure 5. ADI VSM watch V4, baffle, and LED DP array November 2021 Instrumentation Monthly
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