IEEE Circuits and Systems Magazine - Q3 2018 - 15

Motion
Artiface

Noise Source
(Body Motion)

y=x+m

PPG
Sensor

Adaptive Noise Cancellation
Signal Source
(Body Volume
Changes)

Accelerometer
(Noise Reference)

calculations for the SVM, SVR, and
ANN algorithms and show higher
error than the other methods using both ECG and PPG signals. The
last three methods [45], [46], [60]
are only based on the PPG signal.
They use linear regression and
non-parametric models that do
not need as much training and
calculation as the SVM methods.
However, they do require calibration and have the highest error.

Restored
Signal

Estimated
Motion
Artifact
Adaptive
Filter

C. Motion Artifact (MA)
Cancellation
Figure 11. Block diagram of adaptive noise cancellation (ANC) to remove MA. The
noise reference is generated by an accelerometer to sense the movement [27].
MAs introduce a disturbance to the
PPG signal. One of the examples is
as shown in Fig. 10. The time domain parameters like pulse amplitude, pulse length, pulse
rate, valley depth difference, and peak height difference,
LED
PD
Blood
od
will vary too much at strong MA events. A wearable BP deVessel
sel
vice must be able to reduce/cancel artifacts generated by
Motion Artifact
movements. The high-frequency MA components can
M
Motion Artifact
Sensor
Signal
be canceled by a 5 Hz LPF, but the in-band MA noise canu (n
(n)
LED
not be removed by a simple filter.
BVP
Normal PPG
Component
Advanced signal processing techniques are required to
d
(n)
Sensor
deal with MA disturbances. If the motion-induced noise sigPD
nal is known, the desired noise-free signal can be obtained
Skin Tissue
by subtracting the MA noise from the noisy output of the
PPG sensor, as shown in Fig. 11. The adaptive filters require
Figure 12. MA cancellation using a secondary LED/PD to
a MA reference noise signal that has a strong correlation to
generate the MA noise [63].
the MA and is uncorrelated with the MA-free PPG signal. A
synthetic MA noise reference signal can be generated from
the MA-corrupted PPG signal [61]. Reference MA noise strong voltage. The main requirements of a PPG preamsignals can be obtained from an additional transducer at- plifier are high sensitivity, high gain, wide dynamic range,
tached to sense the movement (accelerometer) [27], [62] or and the ability to reject ambient light (DC photocurrent).
by employing an additional reflectance-type optoelectronic A low power consumption is essential to prolong the batsensor, as shown in Fig. 12 [63]. Fig. 12 shows the principle of tery life for portability. As the transmitted optical power is
MA cancellation using a secondary LED/PD to generate the limited by safety regulations, the photodiode current can
MA noise. The main sensor LED/PD contacts the skin to de- be as small as a few tens of nano-amperes. A high sensitect the blood volume pulse (BVP) component (inevitably tivity (low noise) optical receiver is required to deal with
MA affected). The secondary LED/PD does not contact the this weak light signal. Depending on the relative positions
skin (7.5 mm air gap) to detect only MA [63]. Then, the MA of the receiver and the LED and the sensing location, the
is canceled by a non-linear adaptive canceler filter using a signal strength can vary dramatically from a few nA to several n A. A wide dynamic range is a key parameter in order
recursive least squares (RLS) algorithm.
to accommodate the different signal levels. The light
penetrates through the human tissues and is absorbed
IV. PPG Sensing Circuitries
The PPG receiver receives the weak optical signal, con- by the blood, and the light is then reflected after it has
verts it, and amplifies it to a strong electrical signal. The been modulated by the blood volume change. The moduoptical signal is converted to electrical current by means lation depth is very small, and the AC to DC ratio can vary
of a large-area photodiode (a few mm2) to collect most of from 0.1% to 3% depending on skin color, thickness, sensthe PPG optical signal. A low-noise, high-gain preampli- ing position and the light wavelength. A DC photocurrent
fier follows the PD to amplify the weak photocurrent to a rejection is needed to prevent PPG receiver saturation.
ThIrD qUArTEr 2018

IEEE CIrCUITs AND sYsTEMs MAGAzINE

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