58
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-30, NO. 1, JANUARY 1983
Reductionl of Interference Due to Common Mode
Voltage in Biopotential Amplifiers
BRUCE B. WINTER, STUDENT MEMBER, IEEE, AND JOHN G. WEBSTER, SENIOR MEMBER, IEEE
Abstract-We review how the nonideal properties of biopotential amplifiers can transform common mode voltage into interference. We
then review several design approaches for two- and three-electrode amplifiers, both nonisolated and isolated, that reduce this interference.
We consider the effects of static electricity on the various designs, and
we show how to calculate the optimal values of the circuit components.
INTRODUCTION
BIOPOTENTIAL recordings such as the ECG, EEG, and
EMG are frequently plagued with- interference originating
from nearby power sources. There are four ways in which an
electromagnetic source such as 60-Hz power lines can cause interference in a biopotential recording. 1) A magnetic field
causes an induced voltage in the loop forned by the electrode
leads. 2) An electric field induces into the electrode leads a
displacement current which flows through the patient. This
creates an interfering voltage drop across the electrode impedance. 3) An electric field induces into the patient a displacement current. This current may cause an interference voltage
between the two recording electrodes as. it flows through the
body impedance. 4) The current induced into the patient also
creates a voltage between the two recording electrodes and the
amplifier common. Since this voltage is common to both electrodes, it is referred to as the. common mode voltage v,. A
portion of vu is transformed into an interfering differential
voltage by the nonideal properties of the amplifier.
Huhta and Webster [ 1 ] and Webster [2] present methods for
reducing the interference from these four sources. Magnetic
interference can be reduced by twisting the leads together to
decrease the loop area, and thus the induced voltage. The
effects of induced currents can be minimized by either shielding the cable or incorporating a buffer into the electrode.
Careful electrode positioning avoids recording the voltage
caused by displacement currents flowing through the body
COMMON MODE VOLTAGE
The common mode voltage on a body v, is composed of a
static voltage component vU and a power-line-induced ac component va. Va is caused by a displacement current id which
flows through the stray capacitances shown in Fig. 1. The
sizes of these capacitances are determined by how close the
patient is to power sources and grounded objects. va can be as
small as a few millivolts if the patient is touching a grounded
object or as large as 20 V if the patient grasps an insulated
power cord. Typically, 6a is approximately 1 V. Fig. 1 shows
typical values for the capacitances and displacement current.
Static voltage vs is created by patient movement. Friction
creates a charge that is stored in Cb, the capacitance between
the body and ground. A nurse who is charged in this way can
also induce a static voltage into the patient if he/she moves
close to the patient [3]. Although most biopotential amplifiers
filter out the dc component, a change in v5 will disrupt the
baseline of the recording and may cause the amplifier to saturate [4].
EFFECTIVE COMMON MODE REJECTION RATIO
Common mode voltage v. is transformed into an interfering
differential voltage vi according to the following equation [1:
.1st
term
2nd
term
Vi = VC (1/CMRR + ZdlZc)
(1)
where
Zd = difference between the two electrode impedances
c= common mode impedance
CMRR = differential gain/common mode gain.
The ability of a differential amplifier to reject common
mode voltages and to amplify differential voltages is defined as
impedance.
This paper addresses the reduction of the fourth source of its common mode rejection ratio (CMRR). Ideally,; the CMRR
interference listed above. We first examine how common is infinite, but because of nonlinearities and because compomode voltage is transformed into a differential voltage. We nents can never be exactly matched, typical CMRR's range
then show how this interference can be minimized in noniso- from 60 to 120 dB.
The second term in (1) is due to the voltage divider formed
lated -and isolated amplifiers. We review both two- and threeby the electrode impedances and the amplifier's input impeelectrode amplifiers.
dance as shown in Fig.. 2. If the electrode impedances Z1 and
Z2
differ, then v, will be attenuated more at one of the differManuscript received January 18, 1982.
B. B. Winter was with the Department of Electrical and Computer En- ential inputs than at the other:
gineering, University of Wisconsin, Madison, WI 5 3706. He is now with
the IBM Corporation, Rochester, MN 55901.
J. G. Webster is with the Department of Electrical and Computer Engineering, University of Wisconsin, Madison, WI 53706.
[Z2/(Z2 + Zc) - Z1/(Z 1 + ZA)]
(2)
Z. is the impedance to common presented by stray capaciUd
-
Ve
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59
WINTER AND WEBSTER: COMMON MODE VOLTAGE AND BIOPOTENTIAL AMPLIFIERS
.sour.D
Zi
V
L
Fig. 1. The total common mode voltage on a body vC is composed of a
static voltage component vs and a power-line-induced ac component
Ua. va is typically 1 V and vs can be between zero and several thousand
volts.
DIFFERENTIAL
R2
_
kl
1~~06
0
l_
~~~~~~1
102 104
106
FREQUENCY, Hz
(a)
(b)
Fig. 3. (a) Circuit diagram of a high-input-impedance bootstrap input
buffer that provides input bias current to A4 through RI and R2. ul
connects to the patient. v2 drives the rest of the amplifier. (b) Input
impedance of the buffer as a function of frequency.
Since a third electrode is not used, current cannot flow through
it and the body to provide input bias current for the positive
terminal of the op amp. The bootstrap input buffer of Fig.
3(a) supplies input bias current required by A4 through R1
and R2 and also provides the required high input impedance
[4], [5]:
Zi = R1/(I V3/vO)
-
Fig. 2. Common mode voltage can be transformed into a differential
voltage if the. electrode impedances are imbalanced.
tance and RF capacitors that are present at the input of the
plifier. SinceZc is much greater than the electrode impedanc
V3 = R2 [(V1 - V3)/R1 + (V2 - V3) Cl S]
lGo + TaSlGo).
Combining (5), (6), and (7),
V2= v1/(l +
[Rl (1 + 'rS) +R2](1 + I/Go +STaIGo)
(5)
(6)
(7)
(8)
where Zd = Z2- Z1.
i s2TTaIGo + s(7-1 + 'Ta)/Go + 1 + IlGo
Thus, a low input impedance and a high electrode impeda
where
imbalance will transform vc into an interfering differen
'Ta = 1 /(27rfa), fa = corner frequency of op amp
voltage according to (3). But a low input impedance;
determi
lowers vc. We will show that the optimum Zc is
'Ti =R2C1,
Go = op amp's open-loop gain.
by the degree of amplifier isolation from earth ground. A!
Fig. 3(b) shows a typical frequency response of Zi. For frecommercially available amplifiers that have 50-200 pF cap
below G0/(7-1 + Ta) and below [GoI'ria] 1/2, Z, bequencies
tance from amplifier common to earth ground lower Zc
haves
as
an
inductance in series with a resistance:
betw
high frequencies by placing 100-500 pF capacitors
each of the leads and amplifier common.
Zi= RlR2CIs+R1 +R2(9)
The second term of (1) has the same effect as the first te
This derivation has neglected the common mode input impethey both determine how much of vc is transformed into indance
Zcm of the op amp A4. Equation (9) holds as long as
Re,
terference. We lump the two terms together and call it CMR
«Zm
. With JFET op amps, this is no restriction because
Zo<<Z
the effective CMRR of the system:
Zcm is very large. A bipolar op amp should not be used because
CMRRe = 1/(I /CMRR + ZdIZc).
(4) Zem may be low enough to cause a problem, and thus (9) would
hold only for low frequencies.
INTERFERENCE REDUCTION IN NONISOLATED AMPLIFIEERS
Fig. 4 shows the equivalent circuit of a body that is charged
There are two ways to reduce the interference predictedI by with a static voltage and coupled with the circuit of Fig. 3.
(1). With most two-electrode amplifiers, the approach is tc) in- For a step input, the output will oscillate with a decay time
crease CMRRe so that vi will be within acceptable limits vvith constant 72:
the largest expected vc. The second approach is to add a ti
(10)
T2 =2RR2C1/(R1 +R2).
electrode in order to reduce v,.
If 'T2 is too large, then any static voltage that appears on the
A Two-Electrode Amplifier
body may dissipate slowly, and thus saturate the amplifier or
create unacceptable interference according to (1) [4]. If the
Thakor and Webster [4] have presented the design consii
ations for a two-electrode ECG amplifier. For a nonisol
bootstrap circuit is designed for a short 'T2, however, static
amplifier, a CMRR of 100 dB and a Zc of 1 G&2 are requi
voltage conditions may be tolerable.
Vd = VcZd/Zc
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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-30, NO. 1, JANUARY 1983
60
RIGHT LEG
L =R RI2C1
C~b
B
R= R+R2
EARTH GROUND -
Fig. 4. Equivalent circuit of the buffer of Fig. 4 when it is coupled to a
body that is charged to a static voltage. Cb is the body capacitance to
earth ground.
For Z1 = 1 GE at 60 Hz and r2 100 ms, we choose C1 = 1 pF.
According to (9),
RjR2
=2.6X 1012.
(11)
According to (10),
EARTH GROUND
Fig. 5. A simple grounding circuit that provides a low-impedance path
to ground for currents less than 1 MiA and a high impedance to ground
for currents greater than 1 AA.
guarantee that less than 20 pA will flow if the patient acciden-
R1+R2=52X 106 .
There are two solutions to (1 1) and (12):
1)R =50Mn,R2 50kE2
2) R, = 50 kf R2 =50 ME.
-
or
(12) tally touches a 120-V 60-Hz power source, the input circuit
common must have less than 440 pF of stray capacitance to
earth ground.
Two-Electrode Isolated Amplifiers
Fig. 6 shows that v, in isolated amplifiers is determined by
Solution 2) yields a large r1. Equation (8) indicates that to the voltage divider formed by Zc and Z, where Z, is the commaximize the frequency range for which the circuit looks in- mon mode input impedance of the amplifier and Z, E the isoductive, we should minimize r1. Thus, we chose solution (1), lation impedance presented by the stray capacitance C,:
which yielded the predicted time constant of 100 ins.
(13)
UC = Vb [Zc/(Zs + Z)I -VbZclZs
Three-Electrode Amplifiers
Ub = body voltage with respect to earth ground.
The other approach to reducing interference due to common mode voltage is to reduce vc. This is done by placing a
third electrode on the patient, which provides a low-impedance
Equation (13) shows that we can reduce the v. in isolated
path to ground for the displacement current id. This electrode systems to low values by decreasing Z,. The lower limit for
cannot be connected directly to ground, however, because the ZC is dictated by the voltage divider effect of Zc and the elecpatient must be protected from currents of greater than 20 pA trode impedance Z. The measured differential voltage is
[71.
lower than the true voltage by a factor equal to Zcl(Zc + Ze).
One simple way to do this is with the circuit shown in Fig. 5 In order to maintain a 20 percent calibration under worst case
[8] , which makes point c a virtual ground. For currents below conditions (Ze = 100 k2), Zc should not be reduced below
1 pA, diodes D1 and D4 conduct and clamp points a and b 500 k92.
near ground. The impedance from the point c to ground is
Reducing the common mode impedance is not a total soluthen the forward bias resistance of diodes D2 and D3 plus the tion, however. The interference due to impedance imbalance
forward bias resistance of D1 and D4. This is typically around of the electrodes [the second term in (1)] is not reduced be150 k92. For currents above 1 MA, however, diodes D1 and cause it is inversely proportional to Zc. According to (1) and
D3 are reverse biased, and the impedance to ground increases (13),
to 20 MQ.
The most common and most effective way of utilizing the
vi= (vbZclZs)(1/CMRR + ZdlZc)
third electrode is with the driven-right-leg circuit where vc is
(14)
vi= (vb/ZS)(ZC/CMRR + Zd).
actively driven to a few tenths of a millivolt. The driven-rightleg circuit reduces the effective impedance of the third elecWe can reduce both interference terms in (14) by increasing
trode [9].
Z, (decreasing C). If Z, is large enough, as in biotelemetry,
INTERFERENCE REDUCTION IN ISOLATED AMPLIFIERS
then UC is small enough so that a third electrode is not required.
In order to protect the patient from dangerous current flow, For example, (14) shows that if Vb = 1 V and if vi is to be kept
most modern biopotential amplifiers feature an electrical isola- below 10 MV with a 100-k92 imbalance, then Z4 must be
tion barrier between the input circuitry and earth ground. To greater than 10 G12 (Cs < 0.3 pF at 60 Hz).
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WINTER AND WEBSTER: COMMON MODE VOLTAGE AND BIOPOTENTIAL AMPLIFIERS
DIFFERENTIAL
Id
Zc
Vc
Vb
\
61
ICb
I
ISOLATED
COMMON
T
EARTH
GROUND
Cs
-
Fig. 6. The common mode voltage in isolated amplifiers can be reduced
by reducing Rc or by increasing Zs.
Three-Electrode Isolated Amplifiers
EARTH GROUND
*
ISOLATED
+_
COMMON OF A3
In most line-powered instruments, however, such isolation is
7. A two-electrode bootstrapped common amplifier. A 3 drives the
not easily achieved. - Stray capacitances between the amplifier Fig.
isolated common of the input circuitry to the common mode voltage
and ground can be reduced with a negative capacity amplifier
of the patient.
[10], but a few picofarads are typically the limit. Thus, the
third electrode is used to further reduce v,.
The supply voltage of A3 must be large enough to accomThe simplest approach would be to connect the third elec- modate the largest expected common mode voltage according
trode directly to the isolated common. Fig. 6 shows that a to the following equation [11 ]:
small resistance between the patient and the isolated common
Vp >vc(Zs+Rr)IZs
(15)
results in a small v, Since the electrode resistance Ze can be
as large as 100 kQ, the driven-right-leg circuit is usually used
Rr should have a resistance of at least 10 Mi2 to isolate the
to reduce its effective impedance by a factor of 100 [9].
patient from earth ground. For a stray impedance of 10 MQ
(Cs = 200 pF), (15) shows that the voltage supply to A3 would
A Two-Electrode Bootstrapped Common Circuit
have to be greater than twice the largest common mode voltThe preceding analyses show that we can reduce vC in isolated age expected.
amplifiers by: 1) increasing the isolation so that most of vb is
dropped across the isolation impedance, or 2) by adding a third
CONCLUSIONS
electrode that passively or actively provides a low-impedance
The reduction of interference due to common mode voltage
path from the body to the amplifier common. We will review is accomplished in one of two ways. 1) The amplifier's effecone last approach that combines the convenience of two elec- tive common
mode rejection ratio is improved so that only an
trodes and the low common mode voltage of a driven-right-leg acceptable amount of is
v, transformed into interference. 2)
system.
is
or
actively
passively
reduced before it is transformed into
v,
Fig. 7 shows the principle of a bootstrap circuit that is interference.
The second approach can be done by 1) increaspatented by the Hewlett-Packard Corporation [I1]. It drives ing the isolation of the amplifier, 2) using the third electrode
the isolated common of the input amplifier to the body volt- to equalize the
voltage between the patient and the amplifier,
age. Unlike the driven-right-leg circuit, the third electrode is or
earth
3)
using
ground as a reference to drive the voltage of
not needed to drive the body potential. This circuit is similar
the amplifier's isolated common to the voltage on the patient.
to the circuit shown in Fig. 3(a). The common mode voltage
We have reviewed the equations and circuits used to impleis buffered by a unity gain amplifier A3 and fed back to the
ment these approaches.
isolated common end of the input impedance so that the voltage on both ends of the input impedance is nearly the same.
REFERENCES
This simulates a high input impedance according to (9), and
[1] J. C. Huhta and J. G. Webster, "60-Hz interference in electrocarthus reduces the interference due to the second term of (1).
diography," IEEE Trans. Biomed. Eng., vol. BME-20, pp. 91-101,
In addition, since the circuit of Fig. 7 also drives the isolated
Mar. 1973.
common of the differential amplifier, the amplifier will see
[2] J. G. Webster, "Interference and motion artifact in biopotentials,"
in IEEE Region 6 Conf. Rec., 1977, pp. 53-64.
very little common mode voltage. This reduces the effect of
[31 D. H. Gordon, "Triboelectric- interference in the ECG," IEEE
the first term of (1).
Trans. Biomed. Eng., vol. BME-22, pp. 252-255, May 1975.
[4] N. V. Thakor and J. G. Webster, "Ground-free ECG recording
A second isolated power supply powers A3, which drives the
with two electrodes," IEEE Trans. Biomed. Eng., vol. BME-27,
isolated common through C1. C1 blocks the dc offset voltages
pp. 699-704, Dec. 1980.
of the op amps so that the positive feedback will not saturate
[51 R. P. Betts and B. H. Brown, "Method for recording electrocardiograms with dry electrodes applied to unprepared skin," Med.
the op amps. R3 is included so that static voltages will be disEng., vol. 14, pp. 313-315, 1976.
charged through the R, -R3J-Rr impedance path with a time [6] BioL
M. R. Neuman, "Biopotential amplifiers," in Medical Instrumen=
constant given by (1 1) where R2 Rr + R3. As before, we use
tation: Application and Design, J. G. Webster, Ed. Boston, MA:
Houghton Mifflin, 1978.
(9) and (10) to calculate the values of R1, R2, and C1.
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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. BME-30, NO. 1,
62
"Electricity in patient care areas of hospitals," Nat. Fire Protection Ass., Quincy, MA, NFPA-76B, 1980.
8811A Bioelectric Amplifier Service Manual, Med. Electron. Div.,
Hewlett-Packard Corp., Waltham, MA, 1971, p. 5.
B. B. Winter and J. G. Webster, "Driven-right-leg circuit design,"
this issue, pp. 62-66.
78213 Neonatal Heart Rate Module Service Manual, Med. Electron. Div., Hewlett-Packard Corp., Waltham, MA, 1977, p. 3.2.
A. Miller, "Coupling circuit with driven guard," U.S. Patent
4 191 195, 1978.
0 X B. Winter (S'80) was born on July 8,
lBruce
1958. He received the B.S. degree in electrical
engineering from Montana State University,
Bozeman, in 1980, and the M.S.E.E. degree
k
a
from the University of Wisconsin, Madison, in
1981.
While at the University of Wisconsin, he was a
Fellow and Teaching Assistant in instrumentation and logic design. Presently, he is working
with large-scale integration at the IBM Corpora-
tion, Rochester, MN.
JANUARY 1983
John G. Webster (M'59-SM'69) received the
B.E.E. degree from Cornell University, Ithaca,
NY, in 1953, and the M.S.E.E. and Ph.D. degrees from the University of Rochester, Rochester, NY, in 1965 and 1967, respectively.
He is a Professor of Electrical and Computer
Engineerig at the University of Wisconsin,
Madison. In the field of medical instrumentation, he teaches undergraduate, graduate, and
short courses, and does research on electrodes,
impedance plethysmography, and portable arrhythmia monitors.
Dr. Webster is Associate Editor, Medical Instrumentation, of the
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, and is a member of
IEEE-EMBS Administrative Committee. He is coauthor, with B. Jacobson, of Medicine and Clinical Engineering (Englewood Cliffs, NJ: Prentice-Hall, 1977). He is Editor ofMedical Instrumentation: Application
and Design (Boston, MA: Houghton Mifflin, 1978). He is coeditor,
with A. M. Cook, of Clinical Engineering: Principles and Practices
(Englewood Cliffs, NJ: Prentice-Hall, 1979), with W. J. Tompkins, of
Design of Microcomputer-Based Medical Instrumentation (Englewood
Cliffs, NJ: Prentice-Hall, 1981) and, with A. M. Cook, of Therapeutic
Medical Devices: Application and Design (Englewood Cliffs, NJ: Prentice-Hall, 1982).
Driven-Right-Leg Circuit Design
BRUCE B. WINTER,
STUDENT MEMBER, IEEE, AND
Abstract-The driven-right-eg circuit is often used with biopotential
differential amplifiers to reduce common mode voltage. We analyze
this circuit and show that high loop gains can cause instability. We
present equations that can be used to design circuits that minimize
common mode voltage without instability. We also show that it is
important to consider the reduction of high-frequency interference
from fluorescent lights when determining the bandwidth of the drivenright-leg circuit.
INTRODUCTION
Wa rHEN a differential amplifier records biopotentials, the
voltage of the patient with respect to the amplifier's
common is called the common mode voltage vc. Since v, can
be transformed by the amplifier into an interfering differential
signal [1], it is desirable to minimize Vc by attaching a third
electrode to the patient. This electrode provides a low-impedance path between the patient and the amplifier common so
that v, is small. Connecting the electrode directly to the common is undesirable for two reasons. 1) If the circuit is not
isolated, dangerous currents could flow through the third electrode. 2) A poor electrode contact may present up to 100 k92
of resistance between the patient and the common.
The most common and effective use of the third electrode
W
Manuscript received January 18, 1982; revised July 23, 1982.
B. B. Winter was with the Department of Electrical and Computer
Engineering, University of Wisconsin, Madison, WI 53706. He is now
with the IBM Corporation, Rochester, MN 55901.
J. G. Webster is with the Department of Electrical and Computer
Engineering, University of Wisconsin, Madison, WI 53706.
JOHN G. WEBSTER,
SENIOR MEMBER, IEEE
is to connect it to a driven-right-leg circuit [2], [3]. This
circuit overcomes both of the problems listed above. It reduces the effective electrode resistance by several orders of
magnitude, and it allows only a safe amount of current to flow
through the third electrode. Although the circuit is used
extensively in modern biopotential amplifiers [4]-[6], little
has been written on optimal design technique. We present a
design approach that results in the minimization of v,. However, a nonoptimal driven-right-leg circuit may seem to work
as well as one that minimizes vc because: 1) the nonoptimal
circuit reduces the interference to a level that is below the
sensitivity of the recorder, or 2) other sources create more
interference than vc [7], [8].
CIRCUIT DESCRIPTION
Fig. I shows the components of the driven-right-leg circuit.
Resistors Ra and Ra average the voltage of the differential
electrode pair to sense vc. A3 amplifies and inverts this voltage and feeds it back to the body via the third electrode. For
ECG systems, the third electrode is commonly applied to the
right leg; hence, we have the name driven-right-leg circuit.
Fig. 2 shows an equivalent circuit of the system shown in
Fig. 1. The displacement current id that flows into the body
via stray capacitances to nearby power lines divides between
the current that flows directly to ground id, and the current
that flows back to ground through the driven-right-leg circuit
1d2:
'd2
= idCsl(CS + Cb).
0018-9294/83/0100-0062$01.00
1983 IEEE
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(1)