Impedance Measurement System for Determination

of Capacitive Electrode Coupling

IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS, VOL. 7, NO. 5, OCTOBER 2013

Benjamin Eilebrecht, Johannes Willkomm, Antje Pohl, Student Member, IEEE, Tobias Wartzek, and Steffen Leonhardt, Senior Member, IEEE

Adviser: Ji-Jer HuangPresenter: Hou-Yu ChouDate: 103/3/19

Outline

1. Introduction2. Materials and Methods3. Results4. Discussion5. Conclusion & Outlook

Introduction

ECG:– Advantages:

better signals– Disadvantages: 1. skin irritations2. specific preparation before

measurements 3. limitations related to the

connecting cables

http://140.134.32.60:83/news/test/biotech/bei/bei.html 102/12/9

Introduction

Capacitive ECG : Advantages:– does not need contact– make continuous – an office chair, a bed, a toilet seat , automobile

Disadvantages: – still suffer from strong artifacts

Introduction

• This paper presents a combined impedance and ECG measurement system based on capacitive electrodes,which allows simultaneous measurement of the ECG and the coupling impedance of the electrodes.

Introduction

• Common ECG-systems measure the impedance between two electrodes in order to warn against a removal of electrodes.

http://www.macaodaily.com/res/1/20131217/49051387214448609.jpg 103/2/26

Introduction

• The combination of an impedance measurement unit and a capacitive biosignal measurement could be beneficial– the coupling impedance

can easily result in the range of some GΩ

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 58, NO. 5, MAY 2011

MATERIALS & METHODS

• Two units: one to measure the ECG and the other to measure the impedance

• The signal separation can be done with a passive high and an active low pass filter with cut-off frequencies of 1500 and 100 Hz Bioimpedance(BIS)

MATERIALS & METHODS

MATERIALS & METHODS

• MUX– Switch between the

different electrodes serve as a reference

for the remaining electrodes

3-dimensional construction

MATERIALS & METHODS

• Driven right leg circuit, with a negative amplification of 1000. – the common mode

rejection ratio is markedly enhanced

MATERIALS & METHODS

• The impedance measurement unit generally consists of a current source for injecting a current into the human body and a voltage measurement circuit to detect the voltage drop over the segment

BIS Measurement

Inject current

Detect voltage drop

Impedance

MATERIALS & METHODS

• Determine both the coupling and the bioimpedance, – allowing to switch

between the different electrodes was necessary

MATERIALS & METHODS

• Four-pointmeasurement– a current is injected by

two electrodes, whereas the voltage drop is measured by two different electrodes

• Two-pointmeasurement– the same electrodes for

both the injection of the current as well as for measurement of the voltage drop

MATERIALS & METHODS

• EL₁ and EL₂ uses Z₁,in

and Z₂,in for the current injection

• Z₁,meas and Z₂,meas to measure the voltage drop

MATERIALS & METHODS

MATERIALS & METHODS

• The ring electrode– the outer ring was used

to measure the voltage drop

– the inner area for current injection

MATERIALS & METHODS

• Both the resistance and the capacitance can be calculated from the real and imaginary part of the coupling impedance

MATERIALS & METHODS

• Before applying the system to human, the system was tested :– The bioimpedance unit

was tested separately on a dummy circuit and with a galactose polymer.

– calculated for different clothing materials and thicknesses http://spinoff.nasa.gov/spinoff2002/t_2.html 103/2/26

MATERIALS & METHODS

• The distance between the electrodes was also tested to establish

Results

• The first channel shows a flat line

• In the other channels, the characteristic ECG waveforms with the representative R-peak can be seen

Results

• Frequencies between 25 kHz and 500 kHz

• Measurements at frequencies below 25 kHz result in significant deviations

Results

Results

Results

• The four-point measurements of the galactose polymer– dummy resulted in

impedances between 2kΩ and 4kΩ ,

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103/2/26

Results

• From a combined two and four-point measurement, the real and imaginary part of the coupling impedance, using a single cotton layer as insulation material, could be calculated as:15k Ω

Results

• With the electrode area A=16.6 cm² , textile thickness d = 0.5mm , dielectric constant ϵᵣ= 1 (almost air) and electrical conductivity ρ=10⁷Ωcm.

• Applying these values to(1)and(2)

Discussion1. According to the theory, such doubling should halve

the coupling capacitance, which was not observed.

2. The results were achieved under controlled conditions and will be degraded in real scenarios

3. Even if the galactose-polymer model was the best choice for mimicking a real skin-electrode interface—show much lower resistive fractions of the coupling impedance than expected.

Conclusion & Outlook

1. The model assumption of a capacitive coupling with a resistive fraction were in the order as expected.

2. An extension of the system to spectrometric measurements is required and will be explored in the future.

3. Increasing the number of electrodes in the system will increase the order of the system of equations used to calculate the impedances

Thanks for your attention

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