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THERMAL NOISE MODEL OF CAPACITIVE
ACTIVE ELECTRODE FOR INDIRECT-CONTACT ECG
Yong Gyu Lim
Department of Oriental Biomedical Engineering, Sangji University, Wonju, Republic of Korea
Keywords: Indirect-contact ECG, Active electrode, Electrode impedance, Background noise of ECG, Cloth impedance.
Abstract: The indirect-contact ECG (IDC-ECG) shows large background noise in comparison with conventional ECG
measurement. To improve the signal quality, close study of the background noise is necessary. This study
was carried out to investigate how much the thermal noise influences the background noise in IDC-ECG. To
do so, the thermal noise model was built for the active electrode. And then, the parameters which determine
the thermal noise were estimated by measuring the gain of the active electrode. Finally, the level of thermal
noise was estimated and compared with actual background noise. The results show that the thermal noise is
the dominant component of background noise and the intrinsic noise of the preamp’s active devices is
negligible.
1 INTRODUCTION
The need for physiological signal measurement at
home is increasing due to the interest in the early
disease detection and the continuous or frequent
prognosis monitoring. In addition, the need for
health monitoring in daily life is increasing for the
sake of improving life quality. For the above stated
uses, we introduced an ECG measurement method
adequate for daily long-term measurement (Lim,
2006). Using the introduced method, ECG was
measured through usual clothes without direct
contact between the skin and instrument.
In comparison with conventional ECG, the
background noise of IDC-ECG is large, and its
characteristics vary widely according to the
measurement conditions. To improve the signal
quality, an analysis of the background noise is
necessary.
In this study, we carried out an experiment to
determine the gain of IDC-ECG measurement for
some sample cloth, cotton. And based on the
measured gain, the thermal noise level was
estimated. By the comparison between the estimated
thermal noise and the actual background noise of
IDC-ECG, we investigated how much the thermal
noise influences the background noise.
2 METHODOLOGY
2.1 Indirect-contact ECG
Measurement
Indirect-contact ECG (IDC-ECG) is a method that
enables ECG measures without direct contact
between the electrodes and bare skin. The method is
composed of two components. One is a high-input-
impedance capacitive and active electrode that
enables ECG measurement through the high
impedance of the clothes. The other component is
the indirect-contact grounding through the clothes.
Figure 1: Diagram of the IDC-ECG measurement system
showing the electrode impedance (Z
CLTH
), input
impedance of the preamp (Z
B
), and impedance between the
body and ground (Z
G
).
Figure 1 shows the diagram of the whole
measurement system. The ECG generated in the
subject’s body is sensed by the two active electrodes
through cloth impedance Z
E
. The difference between
the outputs of the two electrodes is acquired by an
110
Lim Y..
THERMAL NOISE MODEL OF CAPACITIVE ACTIVE ELECTRODE FOR INDIRECT-CONTACT ECG.
DOI: 10.5220/0003720901100113
In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2012), pages 110-113
ISBN: 978-989-8425-91-1
Copyright
c
2012 SCITEPRESS (Science and Technology Publications, Lda.)