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Technical Note

Two-electrode low supply voltageelectrocardiogram signal amplifier

D. Dobrev

Centre of Biomedical Engineering, Bulgarian Academy of Sciences, Sofia, Bulgaria

Abstract—Portable biomedical instrumentation has become an important part ofdiagnostic and treatment instrumentation, including telemedicine applications. Low-voltage and low-power design tendencies prevail. Modern battery cell voltages in therange of 3–3.6 V require appropriate circuit solutions. A two-electrode biopotentialamplifier design is presented, with a high common-mode rejection ratio (CMRR),high input voltage tolerance and standard first-order high-pass characteristic. Most ofthese features are due to a high-gain first stage design. The circuit makes use ofpassive components of popular values and tolerances. Powered by a single 3 Vsource, the amplifier tolerates �1 V common mode voltage, �50 mA common modecurrent and 2 V input DC voltage, and its worst-case CMRR is 60 dB. The amplifier isintended for use in various applications, such as Holter-type monitors, defibrillators,ECG monitors, biotelemetry devices etc.

Keywords—ECG amplifier, Biopotential amplifier, Low supply voltage amplifier,AC coupled amplifier

Med. Biol. Eng. Comput., 2004, 42, 272–276

1 Introduction

A MODERN tendency in patient diagnosis and treatment involvesthe use of personalised portable biomedical instrumentation. Inaddition to well-known Holter-type ambulatory recorders ofelectrocardiogram (ECG) and blood pressure signals, varioustelemedicine applications require instruments of improveddesign, compatible with modern microcomputers and micro-controllers.

Low voltage and low power are among the most importantrequirements for such instrumentation. Present-day rechargeableor non-rechargeable 3.6V or 3V battery voltages need adequatebiopotential amplifiers. High performance should be obtained inspite of the low supply voltage limitation, especially concerningelectrode polarisation voltage and common-mode input voltagetolerance.

The most widely used circuits for biosignal amplifiers arebased on the three-operational-amplifier configuration, or instru-mentation amplifier, followed by an additional AC-coupledstage (NEUMANN, 1998).

Usually, the ‘classical’ amplifier gain is split between theinstrumentation amplifier and the stage after the high-passdecoupling filter. The first stage gain is set to low values,because of the electrode polarisation potentials. Their voltagedifference can reach up to about 200mV, depending on variousfactors (electrode metal, conductive gel, patient skin etc.), andappears as an input signal DC component (NEUMAN, 1995).

Correspondence should be addressed to Dr Dobromir Dobrev;email: ikdas@argo.bas.bg

Paper received 7 April 2003 and in final form 11 November 2003

MBEC online number: 20043858

# IFMBE: 2004

M

The main performance characteristics of ECG amplifiers canbe summarised as follows:

� frequency band at 3 dB from 0.05 to 100Hz, with first-orderhigh-pass filter

� tolerance of DC input voltage (of level depending on thetype of electrode) without input stage saturation

� overall gain in the range 200–1000 (46–60 dB), with amaximum input signal of about �5mV without outputstage saturation

� differential input impedance >5MO in the entire frequencyband

� common mode rejection ratio (CMRR) >60 dB� for a two-electrode amplifier, the inputs should tolerate at

least 3 mA common mode current per input, without satura-tion of the input stage.

The last requirement corresponds to interference level, whichcommonly occurs in a hospital room environment, according toour previous experience (DOBREV and DASKALOV, 2002;DOBREV, 2002). Even with a battery-supplied amplifier, inputcommon mode currents can often reach 1.5 mA per input.

The idea of setting high gain in the first amplifier stage is wellknown. It allows a high CMRR to be obtained easily. Thesimplest solution is to add a capacitor in series with the gain-setting resistor of the differential amplifier (MCCLELLAN, 1981;PALLAS-ARENY and WEBSTER, 1993), but its value can beinconveniently high, depending on the high-pass cutofffrequency. A version of this circuit, having the same disadvan-tage, was patented by CHEE (2002). In addition, as the first stageis a differential follower, any DC input voltage is amplified bythe second stage. An old solution, using differential high-passfilters at the inputs, has been reconsidered by BURKE andGLEESON (2000). The circuit needs a reference electrode,otherwise the input stage would be saturated even by very

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small common mode currents. Bootstrapped input stages alsosuffer from saturation by relatively low input voltages (THAKORand WEBSTER, 1980).

SPINELLI and MAYOSKY (2000) proposed the use of optocou-plers in photovoltaic mode and an integrator, included in anegative feedback loop, for input DC voltage compensation andhigh-pass filtering. The optocoupler transfer characteristics arenon-linear, and there is a wide variation between specimens ofthe current-to-current transfer ratio (about twice). This leads tolow accuracy of the high-pass cutoff frequency.

In a similar design, JORGOVANOVIC et al. (2001) useddifferential-to-differential amplifiers instead of optocouplers.The circuit is unacceptable for low-power systems, as thesetypes of amplifier, designed for high-frequency operation,consume large amounts of current (20mA or more).

These and other inconveniences of existing solutions stimu-lated us to try and develop a low-voltage, low-power, two-electrode amplifier, satisfying the above requirements.

2 Amplifier circuit concept

The simplified amplifier circuit is shown in Fig. 1a. Thegeneral principle is that the input signal is buffered (two buffersmarked ‘1’) and AC decoupled by the capacitor C and theresistors R3. The second stage consists of two differentialamplifiers Ad. Each of them amplifies half of the differentialinput signal. By summing, the output signal is obtained as

Vout ¼ Ad(Va � Vb þ Vc � Vd)

¼ Ad �s2R3C

1þ s2R3C(VinP � VinN ) (1)

Fig. 1 Basic amplifier circuit concept. (a) Simplified and (b) detailedcircuits

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Here a, b and c, d are the two differential amplifier inputs, and Ad

is the gain. The high-pass cutoff frequency is defined by the timeconstant 2R3C.

The detailed circuit is shown in Fig. 1b. The input stageconsists of operational amplifiers A1, A2, A3 and A4. A1 and A2

are themain gain stages, andA3 andA4 are unity gain buffers. Asthe non-inverting input voltages of A3 and A4 are equal to theirrespective output voltages, resistors R2 and R3 are virtually inparallel. Therefore the ratio of the currents in R2 and R3 isIR2

=IR3¼R3=R2. The current in R1 is the sum of the currents in R2

and R3: IR1¼ IR2

þ IR3¼ (1þR3=R2)IR3

.As mentioned above, the resistors R3 and C form a first-order

low-pass filter, and the AC component on C decreases with 6 dBOct�1 and becomes practically zero for the operating frequencyband. The A1 and A2 amplifiers take one-half of the differentialinput AC signal each. The input DC component is filtered by Cand appears at the A3 and A4 outputs.

The second stage is a unity gain four input adder=subtractorstage. It implements (1), where Ad is as follows:

Ad ¼ 1þR1

R2kR3

with R34R2, Ad ¼ 1þR1

R2

Another solution for the second stage could be by two differ-ential channel analogue-to-digital converters (ADCs), produ-cing a digitised Vout, ready for microcomputer processing. When�5V supply voltage is available, it is possible to obtain Vout bytwo difference amplifiers in a microchip, such as INA2134, forexample.

The first stage has unity common mode voltage gain. Thesecond stage has unity differential mode voltage gain. Theminimum CMRR can be calculated as

CMRR ¼Ad1�4

Acm1�4

�Ad5

Acm5

¼Ad

1�

1

4d=(1þ R4=2R4)

¼ Ad �1:5

4d(2)

where d is the tolerance of the R4 resistors used.If Ad¼ 1000 and d¼ 1%, the theoretically computed

minimum CMRR (assuming ideal operational amplifiers) is91.5 dB, taking opposite signs for the resistor tolerances. WithAd¼ 200, CMRR becomes 77.5 dB. Taking into account realoperational amplifiers (with CMRRmin¼ 75 dB) and withAd¼ 200, the real minimum CMRR is 60.3 dB.

A very important parameter is the operational amplifiers’input offset voltage, especially concerning A3 and A4. The A1

and A2 offsets do not contribute to error, as they are added to theinput signal DC component, which is cancelled by the capacitor C.

The maximum output voltage error due to the operationalamplifiers’ input offset voltage is

Voomax ¼ (VioA3max þ VioA4max) � 1þR1

R2

� �

þ 3VioA5max � 2AdVioA3;4max

Here Viomax are the maximum offset voltages of the corre-sponding operational amplifiers.

When selecting operational amplifiers, the following shouldbe respected: A3, A4 and A5 to be low input offset voltage andhigh CMRR types; A1 and A2 to be of high open-loop gain, highCMRR and high gain-bandwidth product.

3 Body–amplifier interface

As mentioned above, for a two-electrode amplifier, the inputsshould tolerate input common mode currents of at least 3 mA perinput. If the supply voltage is only 3V, this cannot be done usingpassive components, resistors and capacitors. The only solution

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is common mode input impedance reduction by voltage-controlled current sources (DOBREV and DASKALOV, 2002)using negative shunt-shunt feedback.

Such a circuit is shown in Fig. 2. It makes use of the first stagecircuit shown above. In addition, two bidirectional currentsources are connected to the amplifier inputs. Thus frequency-dependent differential and frequency-independent commonmode input impedances are obtained. If the current sourcetransconductance is gm, it can be seen (Fig. 2) that

Zcm ¼1

2gmZd ¼

2

gm(1þ s2R3C) (3)

Here Zcm and Zd are the common-mode and differential inputimpedances, respectively. Zcm has only resistive character,whereas Zd has resistive (2=gm) and inductive (4R3C=gm)components.

Controlling the amplifier differential input impedance yieldsan advantage: the polarisation potentials’ effect is automati-cally balanced. Owing to the low resistive value of thedifferential input impedance, they recharge and tend to equaliseeach other.

4 Practical amplifier circuit

The two-electrode amplifier design was implemented in apractical circuit shown in Fig. 3. It is powered by a single 3Vsupply voltage. Several operational amplifiers types can be used,e.g. MCP607, OPA2336 or similar. Because of the inputcommon mode voltage range, the signal ground is set to one-third of the supply voltage (U4B). The diodes D1–D4 preventlatch up of the circuit. The inputs are RF noise-protected by theRC networks R7, C4. Its value was derived from the following

Fig. 2 Amplifier with bidirectional current sources connected toinputs

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consideration. With R7 �C4¼ (R1 kR2 kR3)C2�R2C2, the high-frequency zero in the amplifier transfer function is cancelled

Ad(s) ¼Vout

VInP � VInN

¼1

1þ sC4R7

�sC32R3

1þ sC32R3

� 1þR1

R2kR3

� ��1þ sC2(R1kR2kR3)

1þ sC2R1

(4)

Inserting C5 capacitors ensures the circuit stability. The inputimpedances are implemented by two bidirectional modifiedHowland voltage controlled current sources (VCCSs), describedin DOBREV and DASKALOV (2002). The VCCS transconduc-tance can be chosen in the range of 1=20–1=100 kO. Thus a highVCCS output minimum resistance is ensured for a given resistortolerance and signal frequency band. The corresponding inputimpedance (3) differential and common mode resistive compo-nents including the input RF filters are

Rd ¼ 21

gmþ R7

� �¼ 2(R5kR6 þ R7) � 84 kO

Rcm ¼(1=gmþ R7)

2¼

(R5kR6 þ R7)

2� 21 kO

In the signal frequency band, Zd also has an inductive compo-nent (3)

Ld ¼4R3C

gm¼ 461:6MO61 mF6(R5kR6) � 205 kH

The simulated Ad, differential Zd and common mode Zcm inputimpedances for this amplifier (circuit of Fig. 3) are shown inFig. 4.

The frequency band is 0.05–100Hz, as is usual for ECGamplifiers. The circuit tolerates up to 50 mA common mode

Fig. 3 Practical amplifier circuit

Fig. 4 Simulated gain, differential Zd and common mode Zcm inputimpedances of practical amplifier circuit. (uu) Ad, dB; (ss) ZdO; (, ,) Zcm, O

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currents and up to about 2V DC differential signal. The currentconsumption is 150 mA (0.45mW) at 3V supply voltage.

5 Multichannel ground free circuit

Multichannel amplifiers can be built according to the designdescribed above, as shown in Fig. 5. One of the electrodes (REF)is buffered and is common for all channels. The same electrode isconnected to a current source transconductance reciprocalresistor to signal ground (or half of the ADC reference). Theremaining channels have VCCSs connected to their inputs. EachVCCS is driven by the difference between signal ground (or halfof the ADC reference) and a filtered DC component inputvoltage. Each channel amplifies the voltage differencebetween its input, referenced to the common electrode REF.Thus a pseudo-differential multichannel system is achieved.

The output signal can be directly obtained by ADCor referenced to the circuit common point by differentialamplifiers. The amplified differential voltage between input 1and REF is

V (1a)� V (1b)� V (REF)

The REF electrode common for all channels is usually connectedto the left leg. Setting the current source transconductancedepends on the number of channels, and it can be selectedslightly lower than for the single-channel amplifier, for examplein the range of 1=500–1=100 kO.

6 Results and conclusions

A sample recording of an ECG signal acquired using acommercial electrocardiograph* and the proposed ECG ampli-fier is shown in Fig. 6. This type of three-channel electrocardio-graph was selected owing to its abilities to record one lead I ECGsynchronously with two ‘experimental inputs’, where externalunits can be connected. The trace in Fig. 6a was obtained by theelectrocardiograph own amplifier (lead I) and, in Fig. 6b, the

Fig. 5 Ground-free multichannel amplifier principle

*EK53R, Hellige

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signal from the proposed amplifier output is displayed. Standardstick-on disposable ECG electrodes were used, two for the ECGchannel and two for the tested amplifier, at 5 cm distance fromeach other on the arms, plus a third one for the ECG unit, whichrequired a reference electrode. The two signals were identical,except for a small difference in channel sensitivities. Low-amplitude electromyogram signals can be observed in bothtraces.

The measured CMRR was 60 dB, using 1Vpp 50Hz commonmode voltage. The measurements were extended for thefrequency range of 3–129Hz, yielding the same value. Inaddition, this value includes common mode input voltage andinput current simultaneously, owing to the low common modeinput impedance (21 kO). The common mode input current was48 mApp.

Eliminating the two current sources at the amplifier inputsproduced CMRR¼ 66 dB. Obviously, the price for the commonmode input impedance reduction (which prevents saturation by ahigh level of common mode noise) was the loss of 6 dB CMRR,mainly owing to non-ideal resistor matching in the currentsources.

The following advantages of this circuit should be pointed out:

(i) the overall gain is ensured by the first stage; thus a highCMRR is obtained without the use of high-precisionresistors in the second stage

(ii) additional input buffers are avoided by connecting the lowfrequency determining RC network to the inverting inputsof op amp pair which amplifies the input signal

(iii) implementing different common mode and differentialmode input impedances achieves two goals:

– improved tolerance to input common mode cur-rents, thus avoiding saturation even with low supplyvoltage;

– low resistive differential impedance component,helping to minimize and equalize electrode polar-isation potentials difference

(iv) low supply current and power consumption: 150 mA,0.45mW

(v) acceptable input common mode currents (<50 mA) andinput DC differential voltage (2V)

Acknowledgments—The author thanks Professor I. K. Daskalovfor the useful discussions and help.

Fig. 6 Lead I electrocardiogram of volunteer taken simultaneouslyby (a) commercial electrocardiograph and (b) the amplifier ofFig. 4

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References

BURKE, M. J., and GLEESON, D. T. (2000): ‘A micropower dry-electrode ECG preamplifier’, IEEE Trans. Biomed. Eng., 47,pp. 155–162

CHEE, J. (2002): ‘Low-frequency high gain amplifier with high DC-offset voltage tolerance’. US patent, US6396343 B2

DOBREV, D. P., and DASKALOV, I. K., (2002): ‘Two-electrode biopo-tential amplifier with current-driven inputs’,Med. Biol. Eng. Comp.,40, pp. 122–127

DOBREV, D. P. (2002): ‘Two-electrode biopotential amplifier’, Med.Biol. Eng. Comp., 40, pp. 546–549

JORGOVANOVIC, N., PETROVIC, R., DOSEN, S., and POPOVIC, D.(2001): ‘A novel AC amplifier for electrophysiology: active DCsuppression with differential to differential amplifier in the feedbackloop’. 23rd Ann. Internat. Conf. IEEE EMBS, CD-ROM, paper441.pdf

MCCLELLAN, A. D. (1981): ‘Extracellular amplifier with bootstrappedinput stage results in high common-mode rejection’, Med. Biol.Eng. Comp., 19, pp. 657–658

NEUMAN, M. R. (1998): ‘Biopotential amplifiers’ in WEBSTER, J. G.(Ed.): ‘Medical instrumentation: applications and design’, 3rd edn(John Wiley & Sons, New York, 1998), pp. 262–264

NEUMAN, M. R. (1995): ‘Biopotential electrodes’, in BRONZINO, J. D.(Ed): ‘Biomedical engineering handbook’ (CRC Press, Boca Raton,1995), pp. 745–757

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PALLAS-ARENY, R., and WEBSTER, J. G. (1993): ‘AC instrumentationamplifier for bioimpedance measurements’, IEEE Trans. Biomed.Eng., 40, pp. 830–833

SPINELLI, E. M., and MAYOSKY, M. A. (2000): ‘AC coupled three op-amp amplifier with active DC suppression’, IEEE Trans. Biomed.Eng., 47, pp. 1616–1619

THAKOR, N., and WEBSTER, J. (1980): ‘Ground-free ECG recordingwith two electrodes’, IEEE Trans. Biomed. Eng., 27, pp. 699–704

Author’s biography

DOBROMIR DOBREV obtained his MSc in electronic engineering fromthe Technical University of Sofia, in 1994. He specialized in medicalelectronics, with a diploma thesis on filtering and amplification ofbiosignals. He has worked in the Institute of Medical Engineering ofthe Medical Academy as a Research Assistant and, since 1997, hasbeen with the Centre of Biomedical Engineering of the BulgarianAcademy of Sciences. His PhD is in the field of neonatal monitoring.The study of analogue circuits, including the design, simulation andintegration of biosignal amplifiers and filters, electrical impedancemeasurement circuits and transient processes in amplifiers, are amonghis present research interests.

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