Instrumentation & Measurement Magazine 25-2 - 54

Fig. 1. Electrical coupling of skin-electrode interface for wet electrode, dry-contact electrode, and non-contact electrode.
and dry electrodes, the circuit equivalence can be regarded as
a large resistance (MΩ) in the conduction process of ECG from
the human body to the system. Therefore, to ensure the lowfrequency
response of the system, the input impedance of the
AFE circuit needs tens of MΩ or hundreds of MΩ. In the process
of non-contact cECG conduction, due to the existence of
air gap (1 pf-1 nf) and clothes (200 M||20 pf), the cECG signal
source has severe GΩ or even TΩ impedance. What is more,
the coupling capacitance and AFE input resistance form a
high pass filter; therefore, the AFE circuit needs higher input
impedance (GΩ or even TΩ impedance) to ensure the low-frequency
response (<0.5 Hz) and obtain the undistorted cECG
signal. At the same time, the ultra-high input impedance will
increase the influence of power line interference, impedance
mismatch, and motion artifact, which will bring great challenges
to the hardware design. Therefore, when designing
the cECG acquisition system, AFE with high impedance is
needed, and also the conventional driven-right-leg (DRL) circuit
also needs to be improved to increase the CMRR of the
system. In addition, the electrode design, circuit design, and
noise reduction algorithm design need to be optimized to obtain
high-quality cECG.
In a non-contact cECG acquisition system, the physiological
signal is perceived by the capacitive sensor in the form
of displacement current. The clothes or hair is the most common
dielectric. The coupling capacitance Cs and the input
impedance of AFE act as a high pass filter. Therefore, it is necessary
to ensure that the impedance of the back-end circuit is
large enough to ensure the system has an adequate low-frequency
response. However, in actuality, the impedance is
not as high as the theoretical value due to the leakage resistance
of the coupling capacitor and the microclimate on the
human body-electrode surface. Here, we recorded the human
body-electrode impedance (Fig. 2), measured by IM3533
54
high-precision impedance
measuring instrument
(HIOKI, Japan). In the
experiment, a strip fabric
electrode was tested
(35 cm × 4.5 cm). Due to
the capacitive coupling,
the impedance of human
body-electrode decreases
with the increase of signal
frequency, and the electrode
impedance of the
human body is 215 MΩ
when the scanning frequency
is 0.5 Hz (cotton
thickness is about 0.54 mm,
and clothes are relatively
dry). An interesting result
of this experiment is that
the noise performance and
electrode coupling are actually
determined by the
resistance component of the cotton layer rather than the capacitance
[6]. The impedance of the signal source is determined
by the resistance characteristics of the clothing, and the resistance
value depends on the thickness of the clothing, the
humidity of the clothing, and other properties of the clothing.
However, when the clothes have slight humidity, the impedance
decreases to 23.9 MΩ. The slight humidity of clothes can
greatly reduce the signal source impedance and improve the
signal quality. In long-term cECG monitoring, the sweat on the
human body surface can provide humidity and improve the
quality of cECG signal.
When designing high impedance AFE, high impedance
may not necessarily obtain better signal quality because ultrahigh
impedance may bring additional noise and increase cost.
The experimental results show that the slight increase of the
influence of humidity can greatly reduce the impedance, and
this humidity can be provided by human sweat, which can obtain
a higher quality cECG signal.
For the structure of non-contact cECG monitoring, the capacitive
electrode is used to replace the wet electrode, and the
Fig. 2. The change of human body-electrode impedance.
IEEE Instrumentation & Measurement Magazine
April 2022

Instrumentation & Measurement Magazine 25-2

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