Instrumentation & Measurement Magazine 25-2 - 58

Table 1 - Typical high impedance circuit
Type of AFE
Structure
Voltage follower
Charge amplifier
with the cECG signal and caused an amplitude of up to 80% of
the peak-to-peak cECG amplitude, which leads to the cECG
signals being invisible. The methods to improve the system's
anti-EMI mainly include the design of a high CMRR cECG acquisition
system and shielding and guarding structure.
For improving the CMRR of the system, the DRL circuit in
the conventional ECG measurement system, which can improve
CMRR, has been proved. Many kinds of research show
that the DRL is also suitable for capacitive electrodes. The
method of cDRL is to feedback the common-mode signal to
the human body through negative feedback. Unlike the conventional
DRL circuit, the capacitive electrode is not in direct
contact with the human body, so the effect of common-mode
suppression is not obvious. Therefore, increasing the negative
feedback gain and the area of the driven-right-leg electrode
can achieve a better common-mode suppression effect. However,
larger feedback gain will destroy the stability of the
front-end circuit, and also the larger coupling area has caused
inconvenient use.
Active shielding and guarding are also effective ways to
Instrument
amplifier
reduce EMI [9]. The path of cECG transmission needs attending,
including the cable, electrode, and sensing surface. The
parasitic capacitance of the transmission path can be reduced
by active shielding. The guard contains the same signal as the
input but has a low resistance to the ground. This guard can
effectively reduce the influence of parasitic capacitance and
external EMI on the original cECG signal. The structure of active
shielding includes a guard ring and guard cover; the active
shield is connected to the same signal as the input signal.
Bootstrap circuit
bias paths. The diode works in the cut-off state. The advantage
of this scheme is that the diode can provide a large impedance
(more than 50 GΩ, determined by the selected diode characteristics)
and very low input leakage current (pA, fA). At the
same time, the influence of motion artifacts can be effectively
reduced by using the volt-ampere characteristics of the diode.
Design of EMI Suppression
The capacitive electrode obtains the biological potential
through capacitive coupling. In the cECG transmission path,
parasitic capacitors exist among the human body, electrode,
cable, and signal ground that affect the signal quality of noncontact
cECG, and the displacement current through these
capacitances generates differential mode and common-mode
voltage. Generally speaking, 50 Hz or 60 Hz common-mode
voltage with a bandwidth of less than 1 Hz caused by the
power line interference is one of the biggest EMI sources. The
power line interference has a similar frequency characteristic
58
Design to Reduce Electrostatic Interference
The triboelectrification effect is closely related to the humidity
of the surrounding environment. In a dry environment,
electrostatic charge is stored on the surface of the human body
and clothes. However, when the humidity is relatively high,
the electrostatic charge of the human body will drop sharply.
For electrode design, a polymer embedded fabric electrode is
used to solve this problem. The electrode that has hygroscopicity
can reduce the influence of electrostatic build-up. What is
more, reducing the relative movement between the electrode
and the human body is also effective in decreasing the effect of
electrostatic interference. The flexible electrode can better fit
the human body and avoid the relative movement between the
clothing surface and the electrode. For the hardware circuit, a
reasonable electrostatic discharge path can also alleviate the
impact of electrostatic [10] interference.
Design of Reducing Motion Artifacts
There are two parts in the process of motion artifact generation:
one is caused by respiratory movement, and the other
is caused by interference with the movement of the human
body. The interference caused by respiration can be filtered by
a high pass filter, and the respiratory signal can be obtained
[11]. Motion artifacts can be optimized through two aspects,
by improving the electric structure design and the hardware
circuit design.
IEEE Instrumentation & Measurement Magazine
April 2022
Instrumentation & Measurement Magazine 25-2

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