IEEE Circuits and Systems Magazine - Q3 2018 - 21

With a modulated light source, the receiver can effectively
suppress the interference from ambient and 1/f noise.

low bandwidth (5 Hz), higher frequency additive noise can any in-band transmitted noise reducing the SNR. Therebe removed with low-pass filtering. However, static (DC) fore, low-noise and low-ripple LED driver is required.
offsets and low frequency noise (also known as fixed pat- For wearable/mobile devices, the battery lifetime is a
tern noise), which occupy the same spectrum as the PPG critical issue. The battery lifetime can be extended by
signal, require different rejection techniques. One solution reducing the LED ON time; also, the LED current should
is to modulate the light source to shift it to a higher spec- be reduced as much as possible, as shown in Fig. 27. The
trum range where there is less noise content, as shown sampling rate needs to be high enough not to miss the
in Fig. 25. With a modulated light
source, the receiver can effectively
suppress the interference from ambient and 1/f noise. Demodulation
Modulation
Demodulation
envelope detection is attractive
due to the simplicity of its implementation and low power use, but
+
+
+
+
it is prone to distortion by addiLPF
Va
Vin
Vb
Vout
tive noise. The system illustrated
-
-
-
-
+
in Fig. 25 uses decimation by 4 FIR
filters for the demodulation. DownAggressors
sampling the signal by a factor M re(a)
sults in placing a copy of the signal
Va
Vb
Vout
Vin
Signal
Aggressores
back in the baseband, completing
the demodulation [74]. For demodulation using multiplications inside
the MCU [88], as shown in Fig. 26,
1 2 3 4
1 2 3 4
1 2 3 4
1 2 3 4
f/fch
f/fch
f/fch
f/fch
the MCU spent much time in full(b)
power processing mode, which led
to an average high power consumpFigure 25. (a) Block diagram and (b) frequency-domain illustration of the chopper [74].
tion. To reduce power consumption
in the MCU, the percentage of time
that the processor core is in a
sleep or power-down state should
be increased. Processing time can
Analogue Domain
Digital Domain
be reduced by choosing an alterLocal Oscillator
native MCU core that supports
(1 + m [n])/2
LED
PWM
hardware multiplication [88].
Driver
m [n]
Even though light modulation
Homodyne
Bias
Output
will save the MCU and LED transMidrange
LPF
Stabilisation
Setpoint
mitter power dissipation, the detecFifth Order
+
Product
tion with the pre-amplifier requires
LPF
DAC
Baseband
Detector
First
Order
a higher bandwidth and hence
-
Output
LPF
higher bias current than if modulaADC
Fifth Order
Vout[n]
tion was not employed.
Photodide
Amplifier

E. LED Driver
A key characteristic for the PPG
LED driver circuit is its low noise;
the optical receiver will amplify
ThIrD qUArTEr 2018

Figure 26. A block diagram illustration of a single channel PPG homodyne detection
system with current offset compensation [88].

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