Signal Processing - November 2016 - 27

IEEE SIgnal ProcESSIng MagazInE

Noisy
PPG
Signal

Homogeneous
Ensemble-Averaging
Signal Processing
Technique

Recovered
PPG Signal

November 2016

Optical
PPG Sensor

FIguRe 2. Analog signal processing for contact-based ECG and PPG acquisition by Toyota and Denso [18].

LED Driver

Current to Voltage
Converter

HPF
-

+
Chrome-Coated
Electrode
ECG Sensor

Palm

Photo Diode

Differential
Amplifier

HPF

0.3 Hz

Amplifier

LPF

30 Hz

ADC

1,000 Hz/
10 bit

ADC

Noisy PPG

ECG

Peak
Detection

ECG
ECG
Signal
1,000 Hz/
10 bit
35 Hz
0.3 Hz

120 dB, and a higher CMRR is obtained by providing feedback
from an electrode on the steering wheel gripped by the driver's
hands, which is illustrated in Figure 3. The noise in the ECG is
minimized by HPF ( fc = 7 Hz) and LPF ( fc = 70 Hz), and
digitized by an ADC with a sampling rate of 1 kHz. The values of Z 1 and Z 2 will be close (balanced) to each other when
the driver's back is in a good contact with the seat. However, if Z 1 and Z 2 lose a balanced state due to the driver's
motion, the CMRR may decrease. Therefore, a multichannel
noncontact-based ECG acquisition technique using a multiplexer is developed to achieve the CMRR value of larger
than 120 dB. The performance of Denso's noncontact-based
ECG acquisition technique is verified in a series of driving
tests on straight roads and curves at a speed of about 70 km/
hour [20]. From the real driving tests, the SNR (the ratio of
the R-peak in the QRS complex to the baseline noise) of the
measured ECG in cruise control is found to be 6.4 dB, and
that under acceleration and deceleration is 5.9 dB.
Ford developed a noncontact-based ECG technique employing six insulated electrodes (tin-coated copper plates of 8 cm
# 5 cm in size) as ECG measurement sensors attached on
the backrest of the seat [15], [16]. The technique was jointly
developed with Aachen University in Germany and the Philips Chair for Medical Information Technology in 2011, and
is applied to Ford's S-Max [16]. Ford utilizes the capacitive
ECG (cECG) method [15] to obtain ECG measurements, even
when the driver wears thick clothing. The impedance depends
on the distance between the driver and insulated electrodes,
which varies irregularly due to the driver's motion. In the technique, the impedance must be kept high, therefore, the active
electrode technique [28] is applied to maintain a high impedance. Moreover, since the noncontact-based ECG acquisition
technique is sensitive to noise; an active shielding technique
[29] is needed to reduce the effect of noise. In the equivalent circuit of the active electrode, as depicted in Figure 4
[15], the overall coupling impedance Z couple between the
driver and the insulated electrode can be computed using
Z couple = 1 (^1 R cloth h + jwC cloth) + 1 (^1 R insh + jwC ins),
where, R ins and R cloth are the resistance of the electrode insulation and the driver's clothing, respectively, while C cloth and
C ins represent the capacitive behavior of the overall contact
impedance [15]. For a sufficient voltage drop at the input of
the active electrode, a high-input impedance Z B is required
in the front end of the operational amplifier. The impedances
Z couple and Z B perform high-pass filtering, therefore, noise
components at the low frequencies of the measured ECG
are removed sufficiently. However, when using the cECG
method, static charges stored in the coupling capacitance
and on the clothing should be removed, for which the bias
resistance ( R Bias) is connected to Z B in parallel. Since Z B
is large, the overall amplifier input impedance ( Z E) becomes
Z E = (R bias Z B R bias + Z B) , R Bias [15]. Since coupled powerline interference occurring from electrodes exists in the ECG
measurements [30], an INA with CMRR of about 115 dB is
applied to remove the interference. In addition, LPF is used
to remove the harmonics of the interference that is 50 Hz or

Table of Contents for the Digital Edition of Signal Processing - November 2016