Download Two-electrode non-differential biopotential amplifier

Two-electrode non-differential
biopotential amplifier
D. Dobrev
Centre of Biomedical Engineering, Bulgarian Academy of Sciences, Sofia, Bulgaria
Abstract—A circuit is proposed for a non-differential two-electrode biopotential
amplifier, with a current source and a transimpedance amplifier as a potential
equaliser for its inputs, fully emulating a differential amplifier. The principle of
operation is that the current in the input of the transimpedance amplifier is sensed
and made to flow with the same value in the other input. The circuit has a simple
structure and uses a small number of components. The current source maintains
balanced common-mode interference currents, thus ensuring high signal input
impedance. In addition, these currents can be tolerated up to more than 10 mA per
input, at a supply voltage of 5 V. A two-electrode differential amplifier with
2610 MO input resistances to the reference point allows less than 0.5 mA per input.
The circuit can be useful in cases of biosignal acquisition by portable instruments,
using low supply voltages, from subjects in areas of high electromagnetic fields.
Examples include biosignal recordings in electric power stations and electrically
powered locomotives, where traditionally designed input amplifier stages can be
saturated.
Keywords—Bio-electric amplifier, Non-differential circuit, Electromagnetic interference
Med. Biol. Eng. Comput., 2002, 40, 546–549
1 Introduction
2 Amplifier circuit
THE USE of conventional unsymmetrical amplifier circuits in
biomedical engineering is very limited, owing to their inadequacy in suppressing interference from the power-line. One of
the patient electrodes being the common reference point of the
amplifier, the interference current flows to this point through
the respective electrode impedance. The voltage drop on this
electrode impedance is amplified and leads to circuit saturation
or to masking of the bio-electric signal.
Many applications connected with biosignal acquisition could
benefit from the use of only two electrodes. Electrocardiogram
monitoring in intensive care wards, ambulatory monitors,
defibrillators etc. are among the most common examples.
Recently, a circuit of a two-electrode differential amplifier was
developed, using controlled current sources at its inputs. Its main
feature was a drastic reduction in common-mode input voltage
(DOBREV and DASKALOV, 2002).
An amplifier circuit is presented here, whose performances
are quasi-equivalent to the ones of the above-cited differential
amplifier. It is of a much simpler and more economical structure.
These two circuits, using current sources at their inputs, are
unable to compensate for electrode imbalance, resulting in
transformation of part of the common-mode voltage to differential signal. However, this is a drawback to all types of
biosignal amplifier, unless very special, but complicated and
not quite efficient, measures are taken (see, for example,
BREDEMANN and SEITZ (1990) and YONCE (2000)).
An equivalent circuit of the body–amplifier interface is
presented in Fig. 1. Part of the interference current flows
through the power-line–body stray capacitance Cp the
body impedance Rbd (presented as resistance for simplification)
and the stray capacitance to ground Cb . The skin–electrode
impedances are Zea and Zeb , (incorporating Rea , Cea and Reb ,
Ceb , respectively), and Cg is the capacitance between the
reference point and ground. Another part of the interference
current ðIa þ Ib Þ traverses the impedances Zea and Zeb and Cg
to ground. The interference current Ib is converted to voltage V1
at the output of operational amplifier A1 , which drives the
potential of input b to the common point. On the other hand,
V1 is used to control the current source, connected to amplifier
input a. It can be seen that Ib Zf b ¼ V1 ; V1 gm ¼ Ia , where gm
is the transconductance of the current source. The circuit is
quasi-symmetric with respect to the interference currents, if
Correspondence should be addressed to Dr Dobromir Dobrev;
email: [email protected]
Paper received 19 February 2002 and in final form 24 June 2002
MBEC online number: 20023711
# IFMBE: 2002
546
gm ¼ I=Zf b
Assuming Zea ¼ Zeb , the voltage drops on them will cancel,
thus cancelling the interference. However, Zea and Zeb are not
equal, and the interference current multiplied by their difference
will result in an unwanted input signal to A2 . As commented
above in Section 1, this is a drawback of most biosignal
amplifiers.
Operational amplifier A1 maintains the potential of input b
equal to the reference point (virtual ground), and thus A2
amplifies the voltage from inputs a and b.
As one of the electrodes is directly connected to the inverting
input of A1 , any capacitance inserted at the input would thus
introduce a phase shift. Thus a potential instability can arise,
which is typical for any potential equalisation circuit involving
connection to the subject’s body with its unknown impedances.
Medical & Biological Engineering & Computing 2002, Vol. 40
Cp
4p
Is
10n
Ia
2k
+
–
Rbd
100
Vpl
+
a
Zea
A2
Ib
Zeb
Zfb
b
2k
Cb
150p
out
–
–
10n
Cg
+
V1
A1
40p
Fig. 1 Equivalent circuit of patient–amplifier interface. Impedances Zea and Zeb include Rea , Cea and Reb , Ceb respectively
The closed-loop transfer function Acl for the circuit of Fig. 2a,
assuming the operational amplifier A1 as ideal is
The problem of ensuring stability has been considered by
LEVKOV (1988), for three-electrode amplification.
In the case of a two-electrode amplifier, an appropriate
selection of the feedback impedance Zf b is to be considered.
The equivalent circuit of the current-to-voltage converter is
shown in Fig. 2a. The interference current is represented by
the current source Ipl , and its output impedance is represented
by Co , with Co being the equivalent of the series connection of
Cg , Cb þ Cp and Ceb , which is the capacitance component
of Zeb . Practically, Co Cg for non-screened patient leads.
Acl ¼
1 þ sðCf b þ Co Þ Rf b
1 þ s Cf b Rf b
with a zero for wz ¼ 1=ðRf b ðCf b þ Co ÞÞ and a pole for
wp ¼ 1=ðRf b Cf b Þ.
Cfb
33p
Rfb
300k
–
–
+
Co
40p
Ipl
+
A1
a
160
Cfb=0
Az
Ω, dB
120
dB
+
+
+
+
+
+
Acl, dB
80
+
Cfb=33pF
Cfb=0
40
+
Aol, dB
0
–20
Cfb=33pF
10–2
10–1
100
101
102
103
104
105
106
Hz
b
Fig. 2 Potential-equaliser amplifier: (a) equivalent circuit; ( b) gain-frequency characteristics by simulation. Vertical scale is in dB. ‘I-V gain’ is
current-to-voltage transverter gain (O) (or transimpedance); Aol and Acl are open-loop and closed-loop gain, respectively, with and
without feedback capacitor Cf b .
Medical & Biological Engineering & Computing 2002, Vol. 40
547
The transimpedance transfer function Az (again assuming A1
as ideal) is
Az ¼ are precisely matched. The output impedance Zo depends on the
common-mode rejection ratio (CMRR) of U1
Rf b
1 þ s Rf b Cf b
Zo ¼ Rgm 10CMRR=20
where CMRR is expressed in dB. For example, the CMRR of
INA105=BB is 80 dB, yielding a very high value of Zo .
The operational amplifier U2 was selected for a low input bias
current (MOSFET or JFET input stage) and a high open-loop
gain, to minimise the input error voltage. In this case, the popular
TL072 is used.
The amplifier inputs InP and InN can tolerate serial resistors,
for protection and=or as part of a low-pass filter. The voltage
drops across such resistors will cancel and not be amplified
by U3. Additionally, the circuit stability is increased.
An example of the amplifier performance is demonstrated in
Fig. 4. The circuit is supplied by two 9 V batteries and connected
with non-screened wires to a pair of chest electrodes located
about both axillae of a subject under test, who was positioned
about 50 cm from a power-line cable collector. A batterypowered oscilloscope was connected to the output of U3B.
With Rgm disconnected (Fig. 4a), the acquired signal was
extremely noisy. Restoring the circuit by reconnecting Rgm
allowed recording of an electrocardiogram that was virtually
free of interference.
The circuit was also tested with a power line-powered
oscilloscope. Under the above-described conditions, the result
for the proposed circuit was as follows: measured commonmode currents of 0.8 mA per input, leaving a large reserve up to
saturation. With a differential amplifier, having 2610 MO input
resistances to reference point and CMRR ¼ 80 dB, full saturation was obtained. It needed 12 V supply voltage to start
operating slightly below saturation. The residual 50 Hz interference level in these conditions did not differ for the two types
of amplifier and was about 1 mV referred to the input.
This result can be compared with that of a previous test under
the same conditions, again using a differential amplifier, with
10 MO resistors between each input and the floating reference
point as a reference circuit (see Fig. 7a of DOBREV and
DASKALOV (2002)). The advantage of the proposed circuit is
its power to tolerate common-mode currents of about 10 mA
and also has a pole for wp
Taking into account now that the operational amplifier is not
ideal, especially with respect to the frequency dependence of the
open-loop gain Aol , we obtain
Az ðsÞ ¼ Rf b
Aol
1 þ sCf b Rf b 1 þ sCo Zf b þ Aol
When we evaluate the circuit stability by the loop gain, if
Cf b ¼ 0, the phase margin is 1.73 , and oscillations will
appear. With Cf b ¼ 33 pF, the phase margin becomes 75 ,
meaning stable functioning.
The circuit of Fig. 2a was simulated using OrCAD 9.2
PSPICE. The results are shown in Fig. 2b, for Cf b ¼ 0 and
Cf b ¼ 33 pF. The A1 open-loop and closed-loop gains are
shown, together with the transimpedance gain (‘I–V gain’, or
current-to-voltage converter gain), as a function of frequency. It
can be seen that, for Cf b ¼ 0, there is a peak of Acl and Az ,
showing that the circuit would oscillate about the corresponding
frequency. With Cf b ¼ 33 pF, there are no peaks in the Acl and
Az characteristics, and the circuit will be stable. However,
a higher value for Cf b is not recommended. It can be seen
from Az that this would reduce the frequency band of the currentto-voltage converter and thus increase the phase shift between
Ia and Ib .
3 Practical amplifier circuit
The circuit of an amplifier built according to the proposed
principle is shown in Fig. 3. The current source U1 is of the most
common type and can be built around an available operational
amplifier. Using an integrated circuit INA105=BB, AMP03=AD, or similar type, can be very practical, as the resistors
U1
INA105 or similar
Vcc
25k
25k
–
25k
25k
+
Rgm
300k
InP
Vcc
+
–
Vee
U2A
TL072
Vee
24k
Cfb
33p
InN
–
+
Rfb
300k
U2B
TL072
Vcc
+
–
15n
Vee
U3A
TL072
24k
2u
+
out
15n
1.5M
1k
U3B
TL072
–
39k
10n
1k
Fig. 3 Practical amplifier circuit
548
Medical & Biological Engineering & Computing 2002, Vol. 40
1V per division
200 ms per division
a
1V per division
The high value of the resistor between the amplifier input and
the current source keeps possible auxiliary patient current below
the safety standard limits. The circuit can accept high-value
input filter resistances, which can also be a security measure
against possible fault conditions.
This amplifier cannot prevent the transformation of commonmode interference voltage into unwanted differential signal
owing to skin–electrode imbalance. As is well known, this
type of transformation is typical for biosignal amplifiers, and
its prevention involves complicated circuits, whose performances usually do not merit the investment of material and
effort for their implementation. On the other hand, modern
software methods for power-line interference suppression and
even elimination are very efficient (DASKALOV et al., 1998),
provided the pre-amplifier does not become even momentarily
saturated (for example, during interference or baseline drift plus
peak amplitudes of the signal). In this sense, it should be
remembered that modern battery-equipped instruments, such
as Holter-type recorders and automated external defibrillators,
tend to use relatively low supply voltages, for example from
3 V to no more than 5 V, which makes them more sensitive
to saturation.
This point suggests that avoiding or reducing the possibility
for saturation, preferably at the amplifier input, becomes an
important issue when obtaining biosignals in conditions of
strong electromagnetic fields. Examples might be Holter-type
recordings from drivers of powerful electric machines, electric
power station operators etc. Especially sensitive cases could be
ones where defibrillators are to be used in such locations.
References
200 ms per division
b
Fig. 4 Electrocardiogram and interference acquired from subject
near power-line cable collector: (a) with conventional nondifferential amplifier; (b) with proposed circuit
per input, at a supply voltage of 5 V. With the 2610 MO
amplifier, only 0.5 mA per input could be tolerated.
BREDEMANN, M., and SEITZ, F. (1990): ‘Differential amplifier’. Patent
Number EP0380976
DASKALOV, I. K., DOTSINSKY, I. A., and CHRISTOV, I. I. (1998):
‘Developments in ECG acquisition, preprocessing, parameter measurement and recording’, IEEE Eng. Med. Biol., 17, pp. 50–58
DOBREV, D., and DASKALOV, I. (2002): ‘Two-electrode biopotential
amplifier with current-driven inputs’, Med. Biol. Eng. Comput., 40,
pp. 122–127
LEVKOV, Ch. (1988): ‘Amplification of biosignals by body potential
driving. Analysis of the circuit performance’, Med. Biol. Eng.
Comput., 26, pp. 389–396
YONCE, D. (2000): ‘Input impedance balancing for ECG sensing’.
Patent Number WO00=45699
4 Discussion and conclusions
Author’s biography
Combining a conventional non-differential amplifier with a
current source driven by the common-mode interference signal
and with a potential-equalising circuit, a biosignal amplifier was
built with added protection against saturation. It uses three
integrated circuits and a small number of passive components.
Its performance matches that of a differential amplifier, but with
added tolerance of high common-mode input currents. It is
convenient for use in biosignal acquisition instrumentation to
be operated in a high electromagnetic interference environment
and where the number of electrodes may be a critical factor.
DOBROMIR DOBREV obtained his MSc in Electronic Engineering from
the Technical University of Sofia, in 1994. He specialised in medical
electronics, with a diploma thesis on filtering and amplification of
biosignals. He has worked in the Institute of Medical Engineering of
the Medical Academy as a Research Assistant and, since 1997, has
been with the Centre of Biomedical Engineering of the Bulgarian
Academy of Sciences. His recently obtained PhD is in the field of
neonatal monitoring. The study of analogue circuits, including the
design and integration of biosignal amplifiers and filters, electrical
impedance measurement circuits and transient processes in amplifiers,
are among his present research interests.
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