IEEE Circuits and Systems Magazine - Q3 2018 - 9

A. The Selection of PPG Sensing Mode
As mentioned earlier, there are two modes for PPG measurement. The transmission mode requires a higher LED
light intensity and works well for thin body parts, whereas reflectance mode shows a higher detected signal level
compared to transmission mode at low transmitted light
intensity in thicker body parts. In general, reflectance
mode can give the same signal quality as transmission
mode using lower LED power [18]. As the probability is
much higher for weak light photons to be reflected than
transmitted through thick tissue, reflectance mode can
be used in any part of the human body, while transmission mode is limited to peripheral parts such as the fingertip, foot, and earlobe. For example, PPG signals from
the front of the head and chest can only be detected in
reflectance mode. It is worth noting that fingertips and
earlobes have limited blood perfusion such that these
sites are susceptible to low ambient temperatures; addiThIrD qUArTEr 2018

tionally, fingertip sensors are affected by daily activities.
Thus, the reflectance signal from the wrist or forehead
would be a better solution for wearable devices. However, the signal received from reflectance mode is easily
affected by MAs and pressure disturbances, which limit
the measurement accuracy [17], [18].

LED

LED PD
(b)

(a)

Figure 2. Position of light-emitting diode and photodetector
in different photoplethysmography modes: (a) transmission
mode and (b) reflectance mode.

Light Source
Other Tissues
Venous Blood
Non-Pulsatile
Component of
Artery Blood
Systolic
Phase

Diastolic Time
Phase

PPG
Waveform

ac: Pulsatile
Part
dc: Steady
Part
Time

Figure 3. DC and AC components of the PPG signal due to
variation in light absorption from blood volume and tissue [17].

100
Tp

80
Amplitude

cNIBP and PC/smartphone via serial port/wireless so
that the ECG and PPG signals and BP values from the
user can be collected by the phone/PC for monitoring
and storage. The smartphone can transmit the collected
data and report it to a medical care unit.
A PPG sensor is required to sense the light signal
modulated by the BP wave traveling through the arteries. The PPG sensor consists of two components, a light
emitted diode (LED) as a light source and an optical
receiver. There are two modes for PPG sensing, transmission mode and reflectance mode as shown in Fig. 2.
Light passing though human tissue can be absorbed
by skin, bone, and arterial and venous blood, as shown
in Fig. 3. The optical receiver detect the changes in the
reflected light intensity due to the change in the blood
flow volume. The incident light is amplitude-modulated by the changes in blood volume during the cardiac
cycle, with maximum blood volume occurring in the
systolic phase and a minimum blood volume at the diastolic phase. The systolic peak (primary peak) is coming from the direct pressure wave traveling from the
left ventricle to the periphery of the body, the diastolic
peak (secondary peak) is the effect of the pressure
wave reflections by arteries of the lower body [15], as
shown in Fig. 4. The received optical signal has both AC
and DC components. The AC component has an amplitude that depends on the amount of light absorbed by
the change in the blood volume and has a fundamental
frequency related to the heart rate. The other detected
optical signal component has a constant value (DC) depending on the tissue structure thickness, skin color,
and the average blood volume. This fixed tissue structure absorbs a constant amount of light and does not
change with time [16].

Primary Peak

Secondary Peak

First
40 Pulse
Foot
20
0

0.2

Dicrotic Notch
0.4

0.6
0.8
Time/s

1.2

1.4

Figure 4. PPG waveform with basic BP features indicated.

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Table of Contents for the Digital Edition of IEEE Circuits and Systems Magazine - Q3 2018