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Application Report
SLAA655–March 2015
How to Design Peripheral Oxygen Saturation (SpO
2
) and
Optical Heart Rate Monitoring (OHRM) Systems Using the
AFE4403
Sang-Soo Oak, Praveen Aroul
ABSTRACT
Pulse Oximeters in clinics have a finger clip type probe that has an LED on one side and a photo detector
on the other side. The light emitted from one side of the finger travels through tissue, venous blood and
arterial blood and is collected in the detector. Most of the light is absorbed or scattered before it reaches
the photo detector in the other side of the finger. The flow of blood is heartbeat induced, or pulsatile in
nature so the transmitted light changes with time. Red and infrared lights are used for pulse oximetry to
estimate the true hemoglobin oxygen saturation of arterial blood. Oxyhemoglobin (HbO2) absorbs visible
and infrared (IR) light differently than deoxyhemoglobin (Hb), and appears bright red as opposed to the
darker brown Hb. Absorption in the arterial blood is represented by an AC signal which is superimposed
on a DC signal representing absorptions in other substances like pigmentation in tissue, venous, capillary,
bone, and so forth. Cardiac-synchronized AC signal is approximately 1% of the DC level. This is referred
to as the perfusion index %. The ratio of ratios ‘R’ is approximated in Equation 1. % SpO
2
is calculated as
follows:
R = (ACrms of Red / DC of Red) / (ACrms of IR / DC of IR) (1)
The standard model of computing SpO
2
is defined as shown in Equation 2. This model is often used in the
literature in the context of medical devices. However, accurate % SpO2 is computed based on the
empirical calibration of the ratio of ratios for the specific device.
% SpO
2
= 110 – 25 × R (2)
Figure 1. Red and Infra Red Modulated by Cycling Blood Figure 2. Red/Infrared Modulation Ratio
This document does not go into the details of the SpO
2
calculation, instead, it discusses the key
considerations on how to obtain a good quality PPG signal. The PPG signal can also be used to extract
the heart rate information. Advanced applications with the PPG signal include vascular age, arterial
stiffness index, and so forth. Irrespective of the target application, understanding the characteristics of light
traveling through human body tissue is very important before one designs an LED lighting and photo
detector module fitting into a given space and location. With mobile phones or wearable applications,
there are also special constraints in space.
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1
SLAA655–March 2015 How to Design Peripheral Oxygen Saturation (SpO
2
) and Optical Heart Rate
Monitoring (OHRM) Systems Using AFE4403
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