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 common- mode voltage. This principle is well known in telephony for interfacing the telephone line with analogue-type phones. A low impedance of about 1 k,Q was obtained between each input and the common point of the circuit. The differential input impedance of 60 Mr2 was obtained with the use of precision resistors. Considerable reduction in the common-mode voltages of more than 200 times resulted. The circuit can be useful for biosignal acquisition from subjects in areas of very high electro- magnetic fields, where high common-mode voltages could saturate the input amplifier stages. Keywords--Amplifier, Bio-electric amplifier, Differential amplifier, Electromagnetic interference \ Med. Biol. Eng. Comput., 2002, 40, 122-127 J 1 Introduction MANY EXPERIMENTAL and 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 common- mode interference voltage into a differential signal, owing to disturbed symmetry of the body-amplifier interface (THAKOR and WEBSTER, 1980; WINTER and WEBSTER, 1983; PALLAS- ARENY, 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 (METTING VAN RIJN et al., 1991). Another aspect of modern instrumentation is the analogue-to- digital 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 (DASKALOV et 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 ikdas@argo.bas.bg Paper received 3 August 2001 and in final form 30 October 2001 MBEC online number: 20023643 © IFMBE: 2002 122 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 investi- gated by many authors (HUHTA and 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-to- reference impedances are obtained without a reduction in the differential input impedance. This principle is known in com- munication 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 tele- phones. The circuit is called a subscriber line interface circuit (SLIC) (e.g. LEGERITY INC., 1999). Medical & Biological Engineering & Computing 2002, Vol. 40