PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011
1598
New Techniques to Reduce the Common-mode Signal in
Multi-frequency EIT Applications
Mohamad Rahal1 , Ibrahim Rida1 , Muhammad Usman1 , and Andreas Demosthenous2
1
Department of Electrical Engineering, University of Hail
P. O. Box 2440, Hail, Saudi Arabia
2
Department of Electronic and Electrical Engineering, University College London
London, WC1E 7JE, UK
Abstract— Bio-impedance voltage measurements suffer from many potential errors. One of
the key errors that affect the accuracy in medical impedance imaging and bio-impedance measurements is the common-mode error. In electrical impedance tomography (EIT) applications,
where current is injected into the subject to make the measurements, the major problem is that
the interference does not occur at the power supply frequency (50 or 60 Hz) but at the working frequency. Traditional techniques such as filtering or screening have little effect in reducing
this common-mode interference. In this paper, we present a common-mode feedback topology
which reduces these errors for use in EIT systems (10–200 kHz current injection frequency). A
frequency-selective feedback network is described which reduces the common-mode voltage due
to electrode impedance mismatch at the input of the differential amplifier. The circuit was designed and implemented in CMOS technology dissipating about 20 mW. Measured results show
that the common-mode signal is reduced by 85%, 75%, 70% and 65% at 10 kHz, 50 kHz, 100 kHz
and 200 kHz, respectively.
1. INTRODUCTION
Electrical Impedance Tomography (EIT) is an imaging technique that estimates the complex conductivities of the interior of an object from measurements made on its surface. In the medical field
EIT has the potential for use in various applications such as breast, brain and lung imaging [1].
In particular, EIT has the ability to become a clinical tool for studying regional lung ventilation
in particular for neonate applications, which can only be viably assessed through EIT due to the
difficulty of applying traditional techniques such as X-ray and MRI. Although EIT suffers from
poor resolution compared to traditional imaging techniques; it has advantages compared to these
in terms of simplicity, cost, non-invasive, and the absence of ionizing radiation [3]. EIT data are
collected via an array of electrodes (multiple electrodes) attached on the surface of the subject
where the induced voltages are measured [1]. Fig. 1 shows a typical EIT application where a small
alternating current (AC) (< 1 mA) at 50 kHz is commonly injected through one electrode pair and
voltages differences are recorded from the remaining electrodes. Then, the current is injected using
the next pair of electrodes until all the available electrodes have been switched. The speed of the
switching determines the frame rate, with real time imaging requiring frame rates from a few Hz
up to 30 Hz. Recent studies have shown that injecting multiple frequencies at the same time will
produce better tissue characterization. In addition, injecting multiple frequencies simultaneously
reduces the measurement time.
The sources of errors in bio-potential recording have been extensively studied in the recording
of signals such as the electrocardiogram (ECG) or electromyogram (EMG). In addition, sources
of errors in EIT have been discussed in various studies [2, 5, 6]. It was indicated in [5] that the
common-mode errors in EIT measurement are due to the changes in stray capacitance and electrode
impedance mismatches. Furthermore, the common-mode (CM) voltage is usually higher than the
Electrode
Current Source
@ f1
Human Body
Differential
Amplifier
Output to
+
demodulator
Z?
−
Figure 1: Simple EIT injection/recording system.
Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, Mar. 20–23, 2011 1599
differential voltage (DV) at the recording electrodes and since this CM is of the same frequency
as the DV, suppression of the CM by the differential amplifier is limited, in particular at high
frequencies. Various studies [4, 8] have suggested the use of common-mode feedback (CMFB) to
reduce the CM but no measured results are given. In [8], it was shown that using an extra electrode
the use of common-mode feedback has the potential to obtain a significant improvement at 10 kHz,
although it was suggested a large phase margin is required to guarantee the stability of the feedback
system. However, no measured result was presented. In this work, we present an integrated topology
for CMFB that is suitable for multi-frequency EIT. After this introductory part, Section 2 describes
the theory and the operation of common-mode feedback. Section 3 presents measured results and
conclusions are suitable drawn in Section 4.
2. METHODS
2.1. CMFB
Figure 2 shows the basic topology for the common mode feedback to suppress the common-mode
error due to electrodes mismatches with the feedback path applied to one of the injecting electrodes.
According to [8], there are two other alternatives, a) by applying feedback to an extra electrode, but
this might change the current distributions, b) by using resistors/capacitors, feedback is applied to
both terminals but this might reduce the output impedance. For the topology shown in Fig. 2 the
initial CM is higher. In order for the feedback path to support multi frequency, it is desirable for
the feedback network to be frequency selective and tunable.
Figure 3 shows the basic topology of the feedback network [4]. The CM is multiplied by the
I/Q at the locking frequency, then its low-pass filtered. For example if the CM is in phase with the
I signal, then the output from the low-pass filtered in the I path will give a maximum DC signal,
whereas the output from the Q path will give a minimum. After that the outputs from the low-pass
filter stages are then multiplied by the corresponding I/Q signals, then the outputs are summed,
amplified and applied to the injecting electrode in a negative feedback loop. For single frequency
injection the overall expression for the feedback network can be expressed as:
AF B (s) = −
1
1 − jw1 τ + sτ
(1)
where τ represents the time constant of the low pass filter and w1 is at the locking frequency. The
magnitude of the above expression can be expressed as:
|AF B (s)| = −
s2 τ 2
1
+ 2τ s + w12 τ 2 + 1
(2)
The frequency response of the feedback network has a sharp peak at the locking frequency (w1 ).
A small time constant causes inadequate phase margin (< 60 degrees), which causes ringing in the
Electrode
I(f n)
I(f 1)
+
…..
Human
Body
Diff.
Amp
Differential
+ ± CM signal
−
Vfb
+
+
CM signal
+
Feedback Network
@f1
.
.
.
Feedback Network
@fn
Figure 2: Multi-frequency CMFB to reduce the CM signal.
PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011
in-phase signal I
VI (f1)
X
To injecting
electrode
VCM
X
VQ (f1)
LPF
X
−k
+
V FB
LPF
X
1600
quadrature-phase signal Q
Figure 3: Common-mode feedback network.
Figure 4: CM signal reduction as a function of the locking frequency for the RC model with different injection
frequencies.
step response. Large time constant increases the phase margin; however the suppression of the CM
will take longer.
3. RESULTS
An integrated solution was designed using a 5 V, 0.35-µm CMOS process technology. The simulations and layout were carried out using the Cadence design kit provided by the foundry. The closed
loop response network was tested for various input frequencies when the locking frequency of the
I/Q signals was varied from 10 kHz to 1MHz and the results is shown in Fig. 4. The output has
as a band-reject filter response centred at the locking frequency. The bandwidth of the feedback
network is about 250 kHz. The closed loop response was carried out using a realistic RC model of
the electrodes [7]. The average reduction over 10 chips at 10 kHz, 50 kHz and 100 kHz were 86%,
63% and 45%, respectively.
4. CONCLUSIONS
In this paper, we have presented a practical implementation of a feedback technique to suppress
the CM voltage present in EIT measurements. The topology used is frequency-selective and the
feedback is applied to one of the injecting electrodes. Experiments were carried out using a RC
model of the electrodes to demonstrate the suppression of the CM signal. Future work will concentrate in reducing the time constant of the feedback network in order to support high frame rates,
in particular when large common-mode signals are present.
ACKNOWLEDGMENT
We would like to acknowledge the support of the UK Engineering and Physical Sciences Research
Council (EPSRC) under grant numbers EP/E031633/1 and EP/E029426/1.
Progress In Electromagnetics Research Symposium Proceedings, Marrakesh, Morocco, Mar. 20–23, 2011 1601
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