1995 IEEE-EMBC and CMBEC
Theme 7:Instrumentation
DC rejection and deblocking in multichannel bioelectric recordings.
C.A. Grimbergen, A.C. MettingVanRijn, A.P. Kuiper, R.H. Honsbeek, K. Speijer, A. Peper
Medical Physics Dept., Academical Medical Center, University of Amsterdam,
Measuring & Control Dept., Mechanical Engineering Faculty, Delft IJniversity of Technology,
P.O.Box 22700, 1100 DE Amsterdam/Mekelweg 2, 2628 CD Delft, The Netherlands
&stmt- Multichannel instrumentation for the recordiig of
bioelectric events should have provisions to reject the
relatively large DC voltages which are produced by the
electrodes involved. Active DC rejection is a suitable high
pass filtering technique preserving the common mode
rejection ratio of the preamplifiers. High pass filtering may
lead to very sIow responses of the instrumentation after
overload, which can be handled by deblocldng. A method to
use the DC information of the rejected signal in a
multichannel system is presented.
&ywor&: DC rejection, multichannel Einstrumentation
the instrumentation.
High pass filtering circuits have a slow step response
property. In a system with a 0.16 Hz high pass frequency, it
may last several secondsbefore the signal is within the input
range after overload. A slow response after overload is not
very practical especially fin multichannel systems, where the
need to judge signal integrity is of special importance.
Therefore an automatic deblocking, making the system
response after saturation temporarily faster, has been
designed.
11. CIRCUrrS FOR DC RElECTION AND DEBLOCKING
In bioelectric recordings a high pass filter capability should
be present in the prleamplifiers. Two criteria are of
Bioelectric recordings of physiological signals have specific importance: The commaln mode rejection properties should
not be impaired and large capacitor values should be
problems related to the use of biopotential electrodes.
In this kind of measurements, the electrodes are the avoided in the design. These criteria are met by active DCtransducers which convert the potential differences in the rejection by feedback [4]1. In figure 1 the principle is shown,
body to voltages in the recording instrumentation. which is based on an integrator circuit in the feedback
Electrodes have an intrinsicDC potential and an impedance; circuit of an instrumentation amplifier with a minimal
both entities may be different for every electrode and differ number of parts [5]. The range of DC-rejection is limited in
this design by the voltage range of the integrator and
in every new recording.
High and imbalanced electrode impedances convert the depends on the gain of the amplifier circuit, therefore a
common mode voltages present to differential mode compromise bemeen DC input range and noise figure must
interference. The capacitive coupling of interference sources be made [SI.
to the input cables leads to interference for the same
reasons [l].The "man mode voltages can be reduced by
low capacitance isolation of the front end using fiber optic
signal transmission and a battery voltage suppIy [2]. The
effects of interference can be addressed in addition by active
*S
common mode voltage reduction and by shielding in
combination with guarding, both feedback circuits [l].
R6
In order not to saturate the input amplifiers of the recording
system, the DC potential differences between the electrodes
t
have to be rejected by high-pass filtering. In the design the
common mode rejection properties should not be
jeopardized by the high pass filtering. Active DC rejection
by feedback of the low frequency part of the signal is a
suitablesolution for rejecting DC voltages and exploitingthe
dynamic range of the biopotential amplifier [3]. In this
method the low frequency component is available and can
used for signal analysis giving a DC recording capability to Fig. 1. Principle of active DC-rejection by feedback.
I. INTRODUCTION
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0-7803-2475-7197 $ 1 0.00 0 1 9 9 7 IEEE
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The output voltage of the integrator contains the low
frequency part of the signal, although it will possess in
general a different (i.e. smaller) gain. The integrator output
signal may be used for transferring the low frequency
component of the signal to the signal processing hardware.
To this end the integrator output signal is low pass filtered
with a comer frequency identical to that of the DC rejection
circuit (see figure 2). Because of the low bandwidth of this
signal component, the transfer can be performed with proportionally less samples. The acquisition of this low
frequency component gives the instrumentation a DC
recording capability. In the signal processing hardware this
information may, after appropriate scaling, also be combined
to a pseudo DC-measurement, in which the signal to noise
ratio of the low frequency component will be lower than
that of the rest of the signal.
In any high pass filter there is a slow response to large
disturbances in the signal like movement artifacts or other
electrode related artefacts and connecting the inputs. The
response time is dependent on the design but is typically 5
to ten times the time constant of the high pass filter
(integrator time constant in the DCrejection circuit).
Especially in a multichannel setup this may lead to
impractical delays during electrode application or occasional
electrode problems. To solve this a deblockingprovision has
been designed which makes the @ne constant temporarily
smaller during overload (see figure 2). The deblocking
circuit is controlled by the output voltage being out of
range, but control may also be manual. When the signal has
reached the limits of its output range the time constant is
switched to its original low value and the signal will adjust
to the signal with DC-suppression. The deblocking is
maintained for an additional hundred milliseconds to make
the amplifier to start with a zero level at is output.
t
I
between different electrodes. In general the DC-voltagedoes
not possess information of the signal to be measured and
wiII limit the range of the recording. Filtering the DC
component will allow a larger gain to be used and
consequently the AC-signal will be recorded with a better
accuracy leading to a higher signal to noise ratio.
Active DC rejection is a suitable implementation for high
pass filtering in bioelectric recordings because common
mode rejection properties are preserved and the use of large
capacitors may be Circumvented. An additional advantage is
that the low frequency component of the signal is also
available and can be recovered from the DC rejection
CirCuitIy.
An important drawback of high pass filtering with a large
time constant is the slow response of the recording circuitry
to disturbances and after connection of the input signal. A
deblocking provision may solve this problem and allow for
a nearly continuous recording of valid data. Overload in any
channel will be detected and displayed as a constant signal.
High pass filtering may be avoided by increasing the dynamic
range of the system to level at which both the DC and the
AC component of the signal can be recorded with the same
accuracy. Improvements in the quality of the analog and
digital components may increase the dynamic range in the
near future enabling the development of general recording
hardware for DC recording of any electrophysiologicalsignal
[6J.
REFERENCES
A. C.MettingVanRijn, A. Peper and C k Grimbergen, "High quality
recording of bioelectric events. I: interference reduction, theory and
practice," Med & BWL En& & Comput, vol. 28, pp. 389-397,1990.
k C MettingVanRijn, A. Peper and C. k Grimbergen, "The
isolation mode rejection ratio in bioelectricamplifiers,"lEEETruns.
Biomed &g, BME-38,1154-1157,1991.
G.H. Hamstra, k Peper and C. k Grimbergen, "Low-power,low-noise
instnunentationamplifier for physiological signals.'' Med & BioL Eng
& Comp. VoL 22, pp 272274,1984.
A. C. MettingVanRijn, k Peper, C. k Grimbergen, ''High-quality
recarding of bioelectric events. Part 2. A low-noise low-power
multichannel ampmer design."Med & BWL Eng. & Cornpur)VoL29, pp
433-440,1991.
A. C MettingVauRijn, k Peper, CA. Grimbergen, "A high
pformance bioelectric amplifier with a minimal number of parts.",
Med & BWL Ens & Comput VoL 32, p305-310,1994.
C k Grimbergen, k C MettingVanFUjn, k Peper, "System
configurations with e/D convmion for multichannel bioelectric
recordings!', prom?& &r 16thAnn I . . Coni W E Eng m Med & BWL
SOC,B-e,
996-997,1994.
OTHER CIV"ELS
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Rg. 2. DC-rejection circuit with deblocking circuit and DC output.
111. DISCUSSION
In bioelectric recordings high pass filtering is necessary in
view of the potentially large DC-voltage which may exist
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