Page 35 - EngineerIt August 2021

MEASUREMENT

Where: EHbO 2,red = extinction coefficient of HbO 2 at 600 nm,
E HbO2,ired = extinction coefficient of HbO 2 at 940 nm

E RHb,ired = extinction coefficient of RHb at 940 nm, E RHb,red =
extinction coefficient of RHb at 600 nm

However, the Beer-Lambert law cannot be used directly as
there are a number of variable factors in every optical design
that cause variations to the RoR to SpO 2 relationship. These
include mechanical baffle design, LED to PD spacing, electronic
and mechanical ambient light rejection, PD gain errors, and
many more.
To obtain clinical grade accuracy from a PPG-based SpO 2
pulse oximeter, a lookup table or algorithm must be developed for
the correlation between RoR and SpO 2.

Calibration Figure 4: LED-PD configuration
Calibration of the measurement system is required to develop
a high accuracy SpO 2 algorithm. To calibrate an SpO 2 system, Reflective PPG configurations are chosen when the PD and
a study must be completed where a participant’s blood oxygen LED must be placed next to each other for practicality, such as
levels are medically reduced, monitored and overseen by a with wrist- or chest-worn devices.
medical professional. This is known as a hypoxia study.
The SpO 2 measurement system can only be as accurate as Sensor positioning and perfusion index
the reference. Reference options include medical grade finger Positioning on the wrist and chest require greater dynamic range
clip pulse oximeters and the gold standard co-oximeter. The in the PPG AFE as the DC signal is greatly increased due to the
co-oximeter is an invasive method of measuring the oxygen depth of the arteries below static reflective components such as
saturation of blood that yields high accuracy, but in most cases is skin, fat and bone.
not convenient to administer. Greater resolution in the PPG measurements will reduce the
The calibration process is used to generate a best fit curve uncertainty in the SpO 2 algorithm. With a typical PI of 1% to 2%
of RoR value calculated from the optical SpO 2 device to the co- for wrist-worn SpO 2 sensors, the goal of pulse oximeter design is
oximeter SaO 2 measurement. This curve is used to generate a to increase the PI through mechanical design or to increase the
lookup table or equation for calculating SpO 2. dynamic range.
Calibration will be required for all SpO 2 designs as RoR is The spacing of the LED to PD will have a major effect on
dependent on a number of variables such as LED wavelength the PI. Too little spacing will increase LED to PD crosstalk or
and intensity, PD response, body placement and ambient light backscatter. This will appear as a DC signal and saturate the AFE.
rejection, which will differ with each design. Increasing this spacing reduces the effect of both backscatter
An increased perfusion index and, in turn, a high AC dynamic and crosstalk but also reduces the current transformer ratio
range on the red and IR wavelengths will increase the sensitivity (CTR), which is the LED output to PD return current. This will
of the RoR calculation and, in turn, return a more accurate SpO 2 affect the efficiency of the PPG system and require greater LED
measurement. power to maximise the AFE dynamic range.
During a hypoxia study, 200 measurements equally spaced Rapidly pulsing one or multiple LEDs has the benefit of
between 100% and 70% blood oxygen saturation need to be reducing the 1/f noise contribution to the overall signal. Pulsing
recorded. Subjects are chosen with a variety of coloured skin the LEDs also makes it possible to use synchronised modulation
tone and an equal spread of age and gender. This variation in
skin tone, age and gender accounts for differing perfusion index
results from a spread of individuals.
The overall error for transmissive pulse oximeters must be
≤3.0% and ≤3.5% for reflective configuration.
DESIGN CONSIDERATIONS
Transmissive vs. reflective
A PPG signal can be obtained using a transmissive or reflective
LED and PD configuration. A transmissive configuration measures
the non-absorbed light passed through a part of the body. This
configuration is best suited to areas such as the finger and
earlobe where measurement benefits from the capillary density
of these body locations, which make the measurements more
stable, repetitive and less sensitive to variations in placement.
Transmissive configurations achieve a 40 dB to 60 dB increase in
the perfusion index. Figure 5: ADI VSM watch V4, baffle, and LED DP array

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