f

Two-electrode biopotential amplifier with current-driven inputs D. Dobrev

I. Daskalov

Centre of Biomedical Engineering, Bulgarian Academy of Sciences, Sofia, Bulgaria Abstract--A circuit was developed for a differentia/ two-electrode biopotential amplifier. Current sources at the amplifier inputs were controlled by the commonmode voltage. This principle is well known in telephony for interfacing the telephone line with analogue-type phones. A l o w impedance of about 1 k,Q was obtained between each input and the c o m m o n point of the circuit. The differential input impedance of 60 Mr2 was obtained with the use of precision resistors. Considerable reduction in the c o m m o n - m o d e voltages of more than 200 times resulted. The circuit can be useful for biosignal acquisition from subjects in areas of very high electromagnetic fields, where high c o m m o n - m o d e voltages could saturate the input amplifier stages. Keywords--Amplifier, Bio-electric amplifier, Differential amplifier, Electromagnetic interference

Med. Biol. Eng. Comput., 2002, 40, 122-127

\ 1 Introduction

MANY EXPERIMENTALand clinical applications connected with biopotential measurement could benefit from the use of only two electrodes, provided adequate signal acquisition would be obtained. Electrocardiogram monitoring in intensive care wards, ambulatory monitors, defibrillators, etc. are among the most obvious examples. One of the main problems in two- and three-electrode differential amplifiers is the transformation of the commonmode interference voltage into a differential signal, owing to disturbed symmetry of the body-amplifier interface (THAKOR and WEBSTER, 1980; WINTER and WEBSTER, 1983; PALLASARENY, 1986; METTINGVAN RIJN et al., 1990). Here, the need to use screened patient cables can be added (WOOD et al., 1995). Other problems can arise in connection with electrostatic potentials, electrode polarisation voltages, electromagnetic interference etc. Modern biopotential amplifiers are highly isolated (floating), as required by regulations and standards for patient safety. Thus the conditions of interference rejection change and require special attention (METTINGVAN RIJN et al., 1991). Another aspect of modern instrumentation is the analogue-todigital conversion of the biosignals, which allows the application of efficient algorithms for power-line interference suppression. Thus the problem of reducing power-line noise is less critical or even practically eliminated (DASKALOVe t al., 1998). However, high-intensity power-line or other types of common-mode voltage could become a more important impeding factor, leading to saturation of the amplifier input stage. Another practical aspect is the need to use non-screened

Correspondence should be addressed to Dr I. K. Daskalov; emaih [email protected] Paper received 3 August 2001 and in final form 30 October 2001 MBEC online number: 20023643 © IFMBE: 2002 122

J wires in some applications, which can also result in an increased common-mode signal and thus create a risk of amplifier saturation. These considerations stimulated us to try and design an amplifier with low impedance of both inputs with respect to the common point, but with adequately high differential input impedance. The amplifier is considered here in the case of electrocardiogram acquisition.

2 Amplifier circuit

The patient-amplifier interface circuit, with and without isolation, and using three or two electrodes, has been investigated by many authors (HUHTAand WEBSTER, 1973; THAKOR and WEBSTER, 1980; PALLAS-ARENY, 1988; WOOD et al., 1995). Therefore the same type of equivalent circuit is used here, and similar designations of the respective quantities are adopted, its well-known configuration is shown in Fig. 1. The following impedances are considered: from power line to patient body = Zp; from body to ground = Zb; skin-electrode = Zel -Ze3 ; from power line to amplifier inputs - Zs; differential amplifier input - Zj; from amplifier inputs to common floating point = Zc; from floating common point to ground = Zg. The power-line voltage Vpl = 220 V/50 Hz; a and b are the amplifier inputs, and r is the amplifier reference voltage point. The principle of the proposed circuit is shown in Fig. 2. The differential amplifier inputs are connected to the reference point using two current generators, driven by the common-mode voltage from the differential pair output. Thus low input-toreference impedances are obtained without a reduction in the differential input impedance. This principle is known in communication engineering, where an impedance-balanced subscriber line helps to reduce noise, and low impedance to earth improves safety (e.g. HARDY, 1986). it was (and still is) used to interface a two-wire telephone line to analogue telephones. The circuit is called a subscriber line interface circuit (SLIC) (e.g. LEGERITYINC., 1999). Medical & Biological Engineering & Computing 2002, Vol. 40

generators driven by the common-mode voltage (V,r + Vbr)/2. in addition, the imbalance of the current sources is simulated by varying their resistor tolerances, as shown below. The voltages V,b, V,~ and Vb~for different values of the circuit elements were obtained. The current sources were implemented as voltage-to-current converters (current stabilisers). We selected the most common type of circuit, shown in Fig. 4a (see the Appendix). Given an 'ideal' operational amplifier, the output current is independent of the load resistance if Zl (Z4 + Z6) - Z2 (Z3 + Zs). Usually, Zl Z2, Z3 - Z4, Z5 - Z6, for simplicity in implementation. Then, the transconductance is

s2

I

Ze1 I

I

Ze2 I

b

!

I

Ze3 I

I

z

?

Zc~

4~L)

Z3 + Z5

g m - d(v2 - v1) -

zl, z6

where V2 and V1 are the voltages at the positive and negative inputs and IL is the output current (Fig. 4a). A minimum output impedance is obtained for a given resistor tolerance 6, expressed as a fraction of the respective impedance values

Fig. 1 Equivalent cireuit of the patient-amplifier interface

Z6(Z 2 + Z 4 ) _

1

1+

Zo.,i,, ~ (Z 4 ÷ Z6)46 Z1(1+5) I

Z3(1 5)

I ~ ~

I

/1~

/~ -1z5(1~3)

,L '4 Fig. 2 Two-electrode differential amplifier with current sourees, driven by common-mode voltage from difJbrential pair output

Z2(1~:~)

Cp; {cp}

z

Fig. 4 (a) ~ltage-driven current souree. (b) Norton equivalent circuit

Csl ,, parameters {cs}

Rela

oo

i I

100

100

Re2a 311

c~ {cp}

RGND _I_

os

T

Relb 500

Rod

145p

2p

parameters

Ce2ll 20n

100

ZL

a

Cs2I. {cs} i I

Z4(1+5) /L~

The equivalent circuit version corresponding to the twoelectrode configuration is shown in Fig. 3. We simulated it using Design Lab 8.0 PSPICE. Here, the stray impedances to power line and ground are represented with their corresponding capacitances. The body-tissue resistance between the electrodes is Rbd. The amplifier input impedance Z,b is assumed to be of infinite value, being normally much higher than the electrode impedances. The reference electrode is not connected. The body-electrode impedances (an imbalance is adopted) are simulated with resistance-capacitance configurations. The two input-to-reference point impedances are simulated by current

Z6(1+5)

Og Ro

b

30p

lk

Re2b 2k

(V(a)+V(b)V2 ~ Rc2 /(Ro(l+O.Old))~_~] 1G '1 || °g " ~VV

i

]

C4;2i ~ Ccl 4p -

Rsim 1T

:s

r

Rc1 ~(V(a)+V(b)V2/(Ro(1 +O.Old)) 1G

-

o

Fig. 3 Simulation cireuit of the patient-amplifier interface Medical & Biological Engineering & Computing 2002, Vol. 40

123

3 Simulation results The circuit of Fig. 3 was subjected to AC simulation, with the purpose of testing its behaviour in different cases of imbalance of the electrode-skin and input-to-reference impedances, as well as in conditions of varying stray links to power line and ground by the respective stray capacitances. Peak values were taken, starting with the power-line voltage of 311 V (220 V=~s). An example attempting to include a realistic 'worst-case' electrodeskin impedance imbalance was adopted. The respective values were given in the circuit, in addition, the stray capacitances of the body to the power line and of the amplifier inputs to the power line were given several values (2, 10 and 20 pF for Cp and 2 and 8 pF for C~), to assess better the circuit behaviour. The current generator output resistance Ro was simulated with two values: 1 k£2 and 10 M£2. The first value was implemented using the proposed principle. The second one was a commonly accepted value for two-electrode amplifiers in defibrillators and some ambulatory monitors. In addition, a current generator resistor imbalance of -4- 1% was included. Thus a comparison was presented with the 'classic' two-electrode circuit. Given the need for adequate simulation, very large-value resistances were included between reference point and ground and between amplifier inputs and reference point, input capacitances of 4 pF were included too. The results of these simulations are given in Table 1. it can be seen that, as expected, the reduction in the common-mode voltage at the amplifier input, due to the current sources, is considerable, it can reach a factor of more than 1000, but it depends on the values of the stray capacitances to power line and to ground. The simulated current source (Fig. 4a) output impedance Zo, as a function of frequency, shown for VI = V2 = 0 at the amplifier differential input and depending on the current generator resistor tolerance, is presented in Fig. 5. it can be seen that, when 0.1% resistors are used, which are readily available, Zo is about 7 M£2. With expensive 0.01% resistors, Zo can exceed 60 M£2. This impedance is also the differential input impedance of the biosignal amplifier, which is equal to 2Zo. When the current sources are driven with the commonTable 1 Simulation results

Cs, pF 2 8

Cp, pF

gs(lkO) , g V

2 10 20 2 10 20

48 107 175 135 188 248

k

Us(lOMO),mV

486 1060 1730 1450 1860 2450

~ 10 4

~ 10 4

U s - (U.~+ Ub~)/2; k-- Us(lOMg2)/Us(lkg2)

102.

o o1o(5O, 1012; 100.

lO 1

lO 2

1 Vo (50, 688.6KU2)

\

\ 1'oO

1'02

1'o4

;06

frequency, Hz Fig. 5

124

Current souree output impedance as" a function of J?equency and its" dependence on resistor tolerance

mode voltage, as shown in Fig. 2, the synthesised input impedance for common-mode signals is be considerably reduced, without the differential impedance being degraded.

4 Practical amplifier circuit The circuit of an amplifier built according to the proposed principle is shown in Fig. 6. The inputs of the two current sources U2A and U2B are taken from the amplified (U3B) commonmode output signal of the differential pair. The negative feedbacks of U2A and U2B make use of two resistors in series (20 k and 47k) to facilitate low-tolerance matching. The use of amplification, instead of just buffering by U3B, allows the use of high-value resistances between the current generator outputs and the amplifier inputs (20 k£2 in this case), so that a higher differential input impedance is obtained. This can be seen from the well-known relationships for the differential amplifier • common-mode voltage

V~m--

V.r+Vbr 2

• differential voltage V a - V, Vb • common-mode current for each input

Icm --

Ia + Ib

2

• differential current la - I, Ib • currents for each input (common-mode and differential)

l"=l~m+ ~,Ib=lLm

I~2

• common-mode current for both inputs 1cm(,b) 1, + l b 2]cm •

The common-mode impedance Zcm can be derived as Zcm

--

V~.m V~.m - 1Lm(~,b) 1,, + 1b

V~.m 2.1~ m

V~m

1

2 " g m ' A c m " Vcm

2.gin.Acre

where Acre is the amplification coefficient of U3B and gm is the transconductance. The remaining part of the circuit is a 'classical' differential amplifier with an AC coupled output stage. The amplifier saturates when one of the electrodes is disconnected. This can be used for detection of a detached patient lead. A practical application of the circuit is demonstrated in Fig. 7. The amplifier was supplied by two 12 V accumulator batteries and connected with non-screened wires to a pair of chest electrodes located about both axillae. The subject being tested was positioned at about 50 cm from a power-line cable collector. First, a 'classical' amplifier was used, obtained by replacing the current generators of Fig. 6 by two 10 M£2 resistors. The upper channel of a battery-supplied oscilloscope was connected to pin 1 of U1A. The lower channel took the output signal (pin 1 of U4A) through a 50 Hz rejection notch-filter. it can be seen that U1 was nearly saturated (first trace of Fig. 7a). The output signal was a distorted ECG, as the QRS complexes tended to emerge from the near-saturation level. Introducing the current generators U2A and U2B, the 50 Hz interference voltage at the output of U1A dropped from 20 Vppto less than 0.1 Vpp (Fig. 7b, upper trace). The output signal was a non-distorted ECG, as seen in the lower trace of Fig. 7b. However, this circuit cannot prevent transformation of part of the common-mode interference signals with respect to earth into an unwanted differential signal, resulting from impedance imbalance at the inputs.

Medical & Biological Engineering & Computing 2002, Vol. 40

R26 47k

R21 47k

R26 20k

U2A TL072/a01FI C5

R26 20k

5.6p R22 47k

R24 47k R14 47k

, Vcc

TL072/301/TI

~ 33p

R5 20k

R4 20k

TL072/301/TI C3

R2 24k

U4A

NCC TL072/301/TI

o

R7 20k

2 c8 R15 4.7k "~33p

Rll 20k U1B

R32 47k

R34 47k

R13 140k

86~c

R36 20k

R10 lk

R12 20k

C6 U2B

5.6p

TLO72/ao1/TI

U3B

R35 20k R33 47k

R31 47k

Fig. 6 Practical amplifier cireuit

Similar results are obtainable by implementing the circuit with the well-known integrated differential amplifiers AD620; this produces, in addition, a higher signal-to-noise ratio.

JUUUUUUUUUUUUUIUUUUUUUU UIUUIIUI UNU II IUUIUI

I I .... I.... I .................. (100ms/div) a

tv~wu. .

!ln°~l'"4;V" A A A , .v--v

(2V/div) ~ A

~AAAtAA^^Jt~.,t~t vvv "~v-" ~Nv

¢[~,tL*.l~aAAJAln^~_^. "~v-.'L¥~.

~o.,---~.,...

kAtlll~AAtttl ~1 ' V ~ ' v

~..~.-~,,~ r

~

(100ms/div)

b

Fig. 7 Electrocardiogram and interference acquired from subject near power-line cable collector." (a) with a conventional amplifier; (b) with the proposed cireuit

Medical & Biological Engineering & Computing 2002, Vol. 40

Another version of the amplifier (Fig. 8) is an example of it being built as an integrated circuit. Very precise matching of the respective elements is possible, so that higher performance characteristics of the entire amplifier are obtained. Such an integrated circuit would be suitable for multichannel applications, as shown in Fig. 9, although it can also be implemented using separate channels of the type shown in Fig. 6.

5 Discussion and conclusions

The use of current sources at the amplifier inputs requires consideration of the patient auxiliary currents in normal and possible fault conditions. The current injected into the patient circuit in normal operation was assessed to be less than 1 gA. it only depends on imbalances of the current source circuits (input offsets voltage and bias current of the operational-amplifier and resistor mismatch). No AC component of the auxiliary patient current arises from the function of the current source to maintain low impedance between the inputs and the common reference. AC current in the patient circuit arises only from power-line interference. A possible fault condition would result, for example, in the appearance of the supply voltage (+5 V or - 5 V) at the current source output, yielding a maximum auxiliary current of 200 gA. This current can be reduced considerably, for example by the use of higher-value resistors in the current source circuits. The amplification of U3B should be selected depending on the desired output resistance value. However, independent of the use of the current sources, a higher patient auxiliary current would arise in a fault condition of the input biosignal amplifier. A more direct approach would be to increase, if applicable, the value of the input filter resistances, which could limit a possible fault condition current both from the current sources and the biosignal amplifier. 125

nn

Fig. 8

Version o f amplifier built as integrated circuit using precisely matched elements

,

<-w->

%

C-v->

A

$1

,LI

]R

$/

.'3

Fig. 9

Use o f voltage-driven current sources in multichannel configuration

A two-electrode biopotential differential amplifier is presented that uses balanced current sources at its inputs with respect to the common point. The circuit allows a drastic lowering of the common-mode voltages at the inputs, thus practically eliminating the risk of amplifier saturation in conditions of high-intensity electromagnetic interference. Such conditions can occur when bio-electrical signals have to be taken from a subject, for example, in an electric power station, electric locomotive etc. (KANZ et al., 2000). This amplifier does not increase the sensitivity to a skinelectrode imbalance, which results in transformation of common-mode interference signals at the inputs into unwanted differential signal.

Appendix The following brief circuit analysis is based on the circuits of Figs 4a and b with the corresponding designations, assuming an 'ideal' operational amplifier. For the circuit of Fig. 4a, the Kirchoffvoltage law is applied to three closed loops 126

( Z 1 Jr- Z 3 Jr- Z 5 ) - I 1 + Z 6 ( I L --

12) + Z L . I L

= V1 = Z135 + Z 6 ( I L - 12) + Z L . I L

( Z 2 -]- Z 4 ) -

12 + Z L . I 6 = V2 = Z24 - 12 + Z L . I 6

( Z 3 + Z 5 ) - I 1 = Z 4 - 12 - Z 6 - (I 6 - 12) = Z35 - I 1

(1)

(2)

(3)

For compactness, sums of Zi above and below are presented using indexes, for example Z1 + Z2 + Z3 - Z123; Z2 + Z4 - Z24 etc. 11 is derived by taking 12 from (2) and substituting it in (3). Further, the expressions obtained for 11 and 12 are substituted in (1), yielding the following relationship: Z 1 -Z46 -]-Z 4 -Z35 Z35, Z24

v2-vl +(1-i

-]-[z6-Z135Z

Z35

6

Z6 Z24

Z135"-Z46~Z

]

Z35. Z24 ]

Lj

-IL = o

(4)

Medical & Biological Engineering & Computing 2002, Vol. 40

( 1 -~ 22426 Z1350 , 2224 Z46~ 3 5] = .

yieldingZ2.Z35=Zl.Z46

(5) i f (5) is true, then Z1 " Z46 ÷ Z4 " Z35 =

1

(6)

Z35 - Z24

The transconductance gm is derived from (4) IL gm --

Z3 + Z 5

V2 __ V1 - -

Z1

" Z6

(4) can also be written in the following form: I L -~

Z 1 -Z46 -

Z 2 -Z35

(7)

Z L . I L = ( V 2 - V1)g m

Z 1 - Z 6 - Z24

The current source can be presented b y its Norton equivalent circuit o f Fig. 4b. Substituting (ZL.I D with (lo.Zo) in (7), where Zo is the output impedance, we obtain Io =

Z 1 - Z46 -

Z 2 - Z35

Io . Z o

Z1 - Z 6 -Z24

Zo =

Z 1 - Z 6 - Z24

Z 1 -Z46 -

Z 2 - Z35

(s) To obtain the minimum output resistance for a given resistor tolerance 6 (expressed as added and subtracted fractions to simulate a worst-case condition), Z 1 = Z 1 -

2 2=2

(1 + 6)

J. K. (1986): 'Electronic communication technology' (Prentice Hall, New York), p. 342 HUNT& J. C., and WEBSTER, J. G. (1973): '60-Hz interference in electrocardiography', IEEE Trans. Biomed. Eng., 20, pp. 91-101 KANZ, K. G., Russ, W., DEILER, S., BIBERTHALER,P., JACK, M., and MUTSCHLER, W. (2000): 'Electromagnetic compatibility of automated external defibrillators', Resuscitation, 45, p. S16 LEGERITY INC. http://www.legerity.com (1999): Am7920 Data Sheet, © Legerity Inc. METTING VAN RUN, A. C., PEPER, A., and GRIMBERGEN, C. A. (1990): 'High quality recording of bioelectric events. Part 1: Interference reduction, theory and practice', Ned. Biol. Eng. Comput., 28, pp. 389-397 METTING VAN RUN, A. C., PEPER, A., and GRIMBERGEN, C. A. (1991): 'The isolation mode rejection ratio in bioelectric amplifiers', IEEE Trans. Biomed. Eng., 38, pp. 1154-1157 PALLAS-ARENY, e. (1986): 'On the reduction of interference due to common mode voltage in two-electrode biopotential amplifiers', IEEE Trans. Biomed. Eng., 33, pp. 1043-1046 PALLAS-ARENY, e. (1988): 'Interference rejection characteristics of biopotential amplifiers: A comparative analysis,' IEEE Trans. Biomed. Eng., 35, 953-959 THAKOR, N. T., and WEBSTER, J. G. (1980): 'Ground-free ECG recording with two electrodes', IEEE Trans. Biomed. Eng., 27, 699-704 WINTER, B., and WEBSTER, J. G. (1983): 'Reduction of interference due to common mode voltage in biopotential amplifiers', IEEE Trans. Biorned. Eng., 30, 58-62 WOOD, D. E., EWlNS, D. J., and BALACHANDRAN,W. (1995): 'Compaxative analysis of power-line interference between two- or threeelectrode biopotential amplifiers', Ned. Biol. Eng. Cornput., 33, 63-68 HARDY,

IL will not depend on ZL if

Z 4 =

Z4(1 ÷ c~) Z

2 -(1-c~)2 3 =23(1-c~)2

6 =

Z6(1 ÷ c~)

5=25(1-c~)

are substituted in the denominator o f (8)

Authors" biographies

20mi n z

Z 1 - Z 6 - Z24

21 - (1 ÷ 6)- 246- (1 ÷ 6) - 2 2 - (1 - 6)- 235- (1 - 6) (9) By respecting the rule o f (5), Z1 - Z2 and Z46 - Z35. Then Zo.,,,, can be expressed as Zo , = ""

Z 6 - Z24

Z6 - Z24

246- ((1 + 6) 2 - (1 - 6) 2)

-- -

-

246- 46

(10)

it can also be presented using gm Z°"*" -- Z1 g i n

1 46 -- g i n 4 6 1

+

(11)

References I. K., DOTSINSKY, I. A., and CHRISTOV, I. I. (1998): 'Developments in ECG acquisition, preprocessing, paxaxneter measurement, and recording', IEEE Eng. Ned. Biol., 17, pp. 50-58

DASKALOV,

Medical & Biological Engineering & Computing 2002, Vol. 40

DOBROMIR DOBREVobtained 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 Phi) is in the field of neonatal monitoring. The study of analogue circuits, including design and integration of biosignal amplifiers and filters, electrical impedance measurement circuits and transient processes in amplifiers axe among his present research interests. IVAN DASKALOVis a graduate of the Faculty of Electrical Engineering in Sofia Technical University. He has a PhD in Electrical Stimulation and a Dr Med Sc in the analysis of clinical physiological signals. He has been a Professor in Biomedical Engineering at the Medical Academy of Sofia since 1976 and Director of the Centre of Biomedical Engineering of the Bulgarian Academy of Sciences. He is one of the founders of the Bulgarian Society of Biomedical Physics (collective member of the IFMBE) and its president of many years. His current research interests include electrical defibrillation, stimulation, and biomedical signal analysis.

127