Two-electrode biopotential amplifier with current-driven inputs

Transcription

1 Two-electrode iopotentil mplifier with current-driven inputs D. Dorev I. Dsklov Centre of Biomedicl Engineering, Bulgrin Acdemy of Sciences, Sofi, Bulgri Astrct A circuit ws developed for differentil two-electrode iopotentil mplifier. Current sources t the mplifier inputs were controlled y the commonmode voltge. This principle is well known in telephony for interfcing the telephone line with nlogue-type phones. A low impednce of out ko ws otined etween ech input nd the common point of the circuit. The differentil input impednce of 60 MO ws otined with the use of precision resistors. Considerle reduction in the common-mode voltges of more thn 00 times resulted. The circuit cn e useful for iosignl cquisition from sujects in res of very high electromgnetic fields, where high common-mode voltges could sturte the input mplifier stges. Keywords Amplifier, Bio-electric mplifier, Differentil mplifier, Electromgnetic interference Med. Biol. Eng. Comput., 00, 40, 7 Introduction MANY EXPEIMENTAL nd clinicl pplictions connected with iopotentil mesurement could enefit from the use of only two electrodes, provided dequte signl cquisition would e otined. Electrocrdiogrm monitoring in intensive cre wrds, multory monitors, defirilltors, etc. re mong the most ovious exmples. One of the min prolems in two- nd three-electrode differentil mplifiers is the trnsformtion of the commonmode interference voltge into differentil signl, owing to distured symmetry of the odymplifier interfce (THAKO nd WEBSTE, 980; WINTE nd WEBSTE, 983; PALLAS- AENY, 986; METTING VAN IJN et l., 990). Here, the need to use screened ptient cles cn e dded (WOOD etl., 995). Other prolems cn rise in connection with electrosttic potentils, electrode polristion voltges, electromgnetic interference etc. Modern iopotentil mplifiers re highly isolted (floting), s required y regultions nd stndrds for ptient sfety. Thus the conditions of interference rejection chnge nd require specil ttention (METTING VAN IJN et l., 99). Another spect of modern instrumenttion is the nlogue-todigitl conversion of the iosignls, which llows the ppliction of efficient lgorithms for power-line interference suppression. Thus the prolem of reducing power-line noise is less criticl or even prcticlly eliminted (DASKALOV et l., 998). However, high-intensity power-line or other types of common-mode voltge could ecome more importnt impeding fctor, leding to sturtion of the mplifier input stge. Another prcticl spect is the need to use non-screened Correspondence should e ddressed to Dr I. K. Dsklov; emil: ikds@rgo.s.g Pper received 3 August 00 nd in finl form 30 Octoer 00 MBEC online numer: # IFMBE: 00 wires in some pplictions, which cn lso result in n incresed common-mode signl nd thus crete risk of mplifier sturtion. These considertions stimulted us to try nd design n mplifier with low impednce of oth inputs with respect to the common point, ut with dequtely high differentil input impednce. The mplifier is considered here in the cse of electrocrdiogrm cquisition. Amplifier circuit The ptientmplifier interfce circuit, with nd without isoltion, nd using three or two electrodes, hs een investigted y mny uthors (HUHTA nd WEBSTE, 973; THAKO nd WEBSTE, 980; PALLAS-AENY, 988; WOOD et l., 995). Therefore the sme type of equivlent circuit is used here, nd similr designtions of the respective quntities re dopted. Its well-known configurtion is shown in Fig.. The following impednces re considered: from power line to ptient ody = p ; from ody to ground = ; skinelectrode = e e3 ; from power line to mplifier inputs = s ; differentil mplifier input = d ; from mplifier inputs to common floting point = c ; from floting common point to ground = g. The power-line voltge V pl = 0 V/50 Hz; nd re the mplifier inputs, nd r is the mplifier reference voltge point. The principle of the proposed circuit is shown in Fig.. The differentil mplifier inputs re connected to the reference point using two current genertors, driven y the common-mode voltge from the differentil pir output. Thus low input-toreference impednces re otined without reduction in the differentil input impednce. This principle is known in communiction engineering, where n impednce-lnced suscrier line helps to reduce noise, nd low impednce to erth improves sfety (e.g. HADY, 986). It ws (nd still is) used to interfce two-wire telephone line to nlogue telephones. The circuit is clled suscrier line interfce circuit (SLIC) (e.g. LEGEITY INC., 999). Medicl & Biologicl Engineering & Computing 00, Vol. 40

2 Fig. V pl ody p e e e3 g s d c Equivlent circuit of the ptientmplifier interfce ln P ln N I I V cm differentil mplifier r s c out genertors driven y the common-mode voltge (V r þ V r )/. In ddition, the imlnce of the current sources is simulted y vrying their resistor tolernces, s shown elow. The voltges V, V r nd V r for different vlues of the circuit elements were otined. The current sources were implemented s voltge-to-current converters (current stilisers). We selected the most common type of circuit, shown in Fig. 4 (see the Appendix). Given n idel opertionl mplifier, the output current is independent of the lod resistnce if ( 4 þ 6 )= ( 3 þ 5 ). Usully, =, 3 = 4, 5 = 6, for simplicity in implementtion. Then, the trnsconductnce is g m dði LÞ dðv V Þ 3 þ 5 ; 6 where V nd V re the voltges t the positive nd negtive inputs nd I L is the output current (Fig. 4). A minimum output impednce is otined for given resistor tolernce d, expressed s frction of the respective impednce vlues 6ð þ 4 Þ ð 4 þ 6 Þ4d g m 4d þ 4 V ( δ) I 3 ( δ) I I 5 ( δ) IL I 6 ( δ) Fig. Two-electrode differentil mplifier with current sources, driven y common-mode voltge from differentil pir output The equivlent circuit version corresponding to the twoelectrode configurtion is shown in Fig. 3. We simulted it using Design L 8.0 PSPICE. Here, the stry impednces to power line nd ground re represented with their corresponding cpcitnces. The ody-tissue resistnce etween the electrodes is d. The mplifier input impednce is ssumed to e of infinite vlue, eing normlly much higher thn the electrode impednces. The reference electrode is not connected. The odyelectrode impednces (n imlnce is dopted) re simulted with resistncecpcitnce configurtions. The two input-to-reference point impednces re simulted y current V Fig. 4 I ( δ) gm.( V V ) I O I 4 ( δ) O () Voltge-driven current source. () Norton equivlent circuit I L I L L L V pl AC=3 C p {cp} od 00 e 00 e 00 C e e C e 00n 500 0n k e C s {cs} C s {cs} prmeters o p p oo 45p os p prmeters o g 30p o k d C p {cp} ( V ( ) VV ( ) /( o(0.0 d)) c G C c 4p C c 4p c G ( V ( ) VV ( ) /( o(0.0 d)) GND C g {cg} sim T 0 r Fig. 3 Simultion circuit of the ptientmplifier interfce Medicl & Biologicl Engineering & Computing 00, Vol. 40 3

3 3 Simultion results Tle Simultion results C s,pf C p,pf U s(ko), mv U s(0mo),mv k U s (U r þ U r )/; k U s(0mo) /U s(ko) o MΩ Fig % (50, 60M Ω) 0.% (50, 6.9M Ω) % (50, 688.6K Ω) frequency, Hz Current source output impednce s function of frequency nd its dependence on resistor tolernce The circuit of Fig. 3 ws sujected to AC simultion, with the purpose of testing its ehviour in different cses of imlnce of the electrodeskin nd input-to-reference impednces, s well s in conditions of vrying stry links to power line nd ground y the respective stry cpcitnces. Pek vlues were tken, strting with the power-line voltge of 3 V (0 V rms ). An exmple ttempting to include relistic worst-cse electrode skin impednce imlnce ws dopted. The respective vlues were given in the circuit. In ddition, the stry cpcitnces of the ody to the power line nd of the mplifier inputs to the power line were given severl vlues (, 0 nd 0 pf for C p nd nd 8 pf for C s ), to ssess etter the circuit ehviour. The current genertor output resistnce o ws simulted with two vlues: ko nd 0 MO. The first vlue ws implemented using the proposed principle. The second one ws commonly ccepted vlue for two-electrode mplifiers in defirilltors nd some multory monitors. In ddition, current genertor resistor imlnce of % ws included. Thus comprison ws presented with the clssic two-electrode circuit. Given the need for dequte simultion, very lrge-vlue resistnces were included etween reference point nd ground nd etween mplifier inputs nd reference point. Input cpcitnces of 4 pf were included too. The results of these simultions re given in Tle. It cn e seen tht, s expected, the reduction in the common-mode voltge t the mplifier input, due to the current sources, is considerle. It cn rech fctor of more thn 000, ut it depends on the vlues of the stry cpcitnces to power line nd to ground. The simulted current source (Fig. 4) output impednce o, s function of frequency, shown for V V 0 t the mplifier differentil input nd depending on the current genertor resistor tolernce, is presented in Fig. 5. It cn e seen tht, when 0.% resistors re used, which re redily ville, o is out 7 MO. With expensive 0.0% resistors, o cn exceed 60 MO. This impednce is lso the differentil input impednce of the iosignl mplifier, which is equl to o. When the current sources re driven with the commonmode voltge, s shown in Fig., the synthesised input impednce for common-mode signls is e considerly reduced, without the differentil impednce eing degrded. 4 Prcticl mplifier circuit The circuit of n mplifier uilt ccording to the proposed principle is shown in Fig. 6. The inputs of the two current sources UA nd UB re tken from the mplified (U3B) commonmode output signl of the differentil pir. The negtive feedcks of UA nd UB mke use of two resistors in series (0 k nd 47 k) to fcilitte low-tolernce mtching. The use of mplifiction, insted of just uffering y U3B, llows the use of high-vlue resistnces etween the current genertor outputs nd the mplifier inputs (0 ko in this cse), so tht higher differentil input impednce is otined. This cn e seen from the well-known reltionships for the differentil mplifier common-mode voltge V cm V r þ V r differentil voltge V d = V V common-mode current for ech input I cm I þ I differentil current I d = I I currents for ech input (common-mode nd differentil) I I cm þ I d ; I I cm I d common-mode current for oth inputs I cm() I þ I I cm. The common-mode impednce cm cn e derived s cm V cm V cm V cm I cmðþ I þ I I cm V cm g m A cm V cm g m A cm where A cm is the mplifiction coefficient of U3B nd g m is the trnsconductnce. The remining prt of the circuit is clssicl differentil mplifier with n AC coupled output stge. The mplifier sturtes when one of the electrodes is disconnected. This cn e used for detection of detched ptient led. A prcticl ppliction of the circuit is demonstrted in Fig. 7. The mplifier ws supplied y two V ccumultor tteries nd connected with non-screened wires to pir of chest electrodes locted out oth xille. The suject eing tested ws positioned t out 50 cm from power-line cle collector. First, clssicl mplifier ws used, otined y replcing the current genertors of Fig. 6 y two 0 M O resistors. The upper chnnel of ttery-supplied oscilloscope ws connected to pin of UA. The lower chnnel took the output signl (pin of U4A) through 50 Hz rejection notch-filter. It cn e seen tht U ws nerly sturted (first trce of Fig. 7). The output signl ws distorted ECG, s the QS complexes tended to emerge from the ner-sturtion level. Introducing the current genertors UA nd UB, the 50 Hz interference voltge t the output of UA dropped from 0 V pp to less thn 0. V pp (Fig. 7, upper trce). The output signl ws non-distorted ECG, s seen in the lower trce of Fig. 7. However, this circuit cnnot prevent trnsformtion of prt of the common-mode interference signls with respect to erth into n unwnted differentil signl, resulting from impednce imlnce t the inputs. 4 Medicl & Biologicl Engineering & Computing 00, Vol. 40

4 6 47k 47k 6 0k 6 0k V EE 4 V V V CC 8 UA TL07/30/TI 3 C5 5.6p ln P ln N 4 47k 5 4.7k k 47k UA 3 8 V CC TL07/30/TI C7 V 33p V C 3.3n 4 V 4k EE C8 33p 6 4 V EE C V 3.3n 4k 3 4k V TL07/30/TI 5 8 V CC UB 4 0k 6 0k 0k 5 0k U3A TL07/30/TI 3 8 V CC V V 4 V EE 7 0k 3 40k C3 u 8.5MEG U4A 3 8 V CC TL07/30/TI V V 4 V EE 9 39k 0 k C4 3.3n out 34 47k 36 0k V CC 8 5 C6 V UB 5.6p V TL07/30/TI k V EE k 0k 4 V EE 6 V V 5 8 V CC U3B TL07/30/TI 33 47k 3 47k Fig. 6 Prcticl mplifier circuit Similr results re otinle y implementing the circuit with the well-known integrted differentil mplifiers AD60; this produces, in ddition, higher signl-to-noise rtio. Fig. 7 (0V/div) (V/div) (0.V/div) (V/div) (00ms/div) (00ms/div) Electrocrdiogrm nd interference cquired from suject ner power-line cle collector: () with conventionl mplifier; () with the proposed circuit Another version of the mplifier (Fig. 8) is n exmple of it eing uilt s n integrted circuit. Very precise mtching of the respective elements is possile, so tht higher performnce chrcteristics of the entire mplifier re otined. Such n integrted circuit would e suitle for multichnnel pplictions, s shown in Fig. 9, lthough it cn lso e implemented using seprte chnnels of the type shown in Fig Discussion nd conclusions The use of current sources t the mplifier inputs requires considertion of the ptient uxiliry currents in norml nd possile fult conditions. The current injected into the ptient circuit in norml opertion ws ssessed to e less thn ma. It only depends on imlnces of the current source circuits (input offsets voltge nd is current of the opertionl-mplifier nd resistor mismtch). No AC component of the uxiliry ptient current rises from the function of the current source to mintin low impednce etween the inputs nd the common reference. AC current in the ptient circuit rises only from power-line interference. A possile fult condition would result, for exmple, in the ppernce of the supply voltge (þ5vor 5V) t the current source output, yielding mximum uxiliry current of 00 ma. This current cn e reduced considerly, for exmple y the use of higher-vlue resistors in the current source circuits. The mplifiction of U3B should e selected depending on the desired output resistnce vlue. However, independent of the use of the current sources, higher ptient uxiliry current would rise in fult condition of the input iosignl mplifier. A more direct pproch would e to increse, if pplicle, the vlue of the input filter resistnces, which could limit possile fult condition current oth from the current sources nd the iosignl mplifier. Medicl & Biologicl Engineering & Computing 00, Vol. 40 5

5 V CC ln P c 0 differentil mplifier out c ln N V EE Fig. 8 Version of mplifier uilt s integrted circuit using precisely mtched elements L F I I I V cm Fig. 9 Use of voltge-driven current sources in multichnnel configurtion A two-electrode iopotentil differentil mplifier is presented tht uses lnced current sources t its inputs with respect to the common point. The circuit llows drstic lowering of the common-mode voltges t the inputs, thus prcticlly eliminting the risk of mplifier sturtion in conditions of high-intensity electromgnetic interference. Such conditions cn occur when io-electricl signls hve to e tken from suject, for exmple, in n electric power sttion, electric locomotive etc. (KAN et l., 000). This mplifier does not increse the sensitivity to skin electrode imlnce, which results in trnsformtion of common-mode interference signls t the inputs into unwnted differentil signl. Appendix The following rief circuit nlysis is sed on the circuits of Figs 4 nd with the corresponding designtions, ssuming n idel opertionl mplifier. For the circuit of Fig. 4, the Kirchoff voltge lw is pplied to three closed loops ð þ 3 þ 5 ÞI þ 6 ði L I Þþ L I L V 35 þ 6 ði L I Þþ L I L ð þ 4 ÞI þ L I L V 4 I þ L I L ð 3 þ 5 ÞI 4 I 6 ði L I Þ 35 I For compctness, sums of i ove nd elow re presented using indexes, for exmple þ þ 3 = 3 ; þ 4 = 4 etc. I is derived y tking I from () nd sustituting it in (3). Further, the expressions otined for I nd I re sustituted in (), yielding the following reltionship: 46 þ 4 35 V 35 ; V 4 þ þ þ L 4 I L 0 ð4þ ðþ ðþ ð3þ 6 Medicl & Biologicl Engineering & Computing 00, Vol. 40

6 I L will not depend on L if þ ; yielding ð5þ If (5) is true, then 46 þ ð6þ The trnsconductnce g m is derived from (4) g m I L 3 þ 5 V V 6 (4) cn lso e written in the following form: I L þ L I L ðv V Þg m ð7þ The current source cn e presented y its Norton equivlent circuit of Fig. 4. Sustituting ( L I L ) with (I o o ) in (7), where o is the output impednce, we otin I o I o o o ð8þ To otin the minimum output resistnce for given resistor tolernce d (expressed s dded nd sutrcted frctions to simulte worst-cse condition), ð þ dþ 4 4 ð þ dþ 6 6 ð þ dþ ð dþ 3 3 ð dþ 5 5 ð dþ re sustituted in the denomintor of (8) 6 4 ðþdþ 46 ðþdþ ð dþ 35 ð dþ ð9þ By respecting the rule of (5), = nd 46 = 35. Then cn e expressed s ððþdþ ð dþ Þ 6 4 ð0þ 46 4d It cn lso e presented using g m 4 g m 4d g m 4d þ 4 ðþ eferences DASKALOV, I. K., DOTSINSKY, I. A., nd CHISTOV, I. I. (998): Developments in ECG cquisition, preprocessing, prmeter mesurement, nd recording, IEEE Eng. Med. Biol., 7, pp HADY, J. K. (986): Electronic communiction technology (Prentice Hll, New York), p. 34 HUHTA, J. C., nd WEBSTE, J. G. (973): 60-Hz interference in electrocrdiogrphy, IEEE Trns. Biomed. Eng., 0, pp. 90 KAN, K. G., USS, W., DEILE, S., BIBETHALE, P., JACK, M., nd MUTSCHLE, W. (000): Electromgnetic comptiility of utomted externl defirilltors, esuscittion, 45, p. S6 LEGEITY INC. (999): Am790 Dt Sheet, # Legerity Inc. METTING VAN IJN, A. C., PEPE, A., nd GIMBEGEN, C. A. (990): High qulity recording of ioelectric events. Prt : Interference reduction, theory nd prctice, Med. Biol. Eng. Comput., 8, pp METTING VAN IJN, A. C., PEPE, A., nd GIMBEGEN, C. A. (99): The isoltion mode rejection rtio in ioelectric mplifiers, IEEE Trns. Biomed. Eng., 38, pp PALLAS-AENY,. (986): On the reduction of interference due to common mode voltge in two-electrode iopotentil mplifiers, IEEE Trns. Biomed. Eng., 33, pp PALLAS-AENY,. (988): Interference rejection chrcteristics of iopotentil mplifiers: A comprtive nlysis, IEEE Trns. Biomed. Eng., 35, THAKO, N. T., nd WEBSTE, J. G. (980): Ground-free ECG recording with two electrodes, IEEE Trns. Biomed. Eng., 7, WINTE, B., nd WEBSTE, J. G. (983): eduction of interference due to common mode voltge in iopotentil mplifiers, IEEE Trns. Biomed. Eng., 30, 586 WOOD, D. E., EWINS, D. J., nd BALACHANDAN, W. (995): Comprtive nlysis of power-line interference etween two- or threeelectrode iopotentil mplifiers, Med. Biol. Eng. Comput., 33, 6368 Authors iogrphies DOBOMI DOBEV otined his MSc in Electronic Engineering from the Technicl University of Sofi in 994. He specilised in medicl electronics with diplom thesis on filtering nd mplifiction of iosignls. He hs worked in the Institute of Medicl Engineering of the Medicl Acdemy s reserch ssistnt nd since 997 hs een with the Centre of Biomedicl Engineering of the Bulgrin Acdemy of Sciences. His recently otined PhD is in the field of neontl monitoring. The study of nlogue circuits, including design nd integrtion of iosignl mplifiers nd filters, electricl impednce mesurement circuits nd trnsient processes in mplifiers re mong his present reserch interests. IVAN DASKALOV is grdute of the Fculty of Electricl Engineering in Sofi Technicl University. He hs PhD in Electricl Stimultion nd Dr Med Sc in the nlysis of clinicl physiologicl signls. He hs een Professor in Biomedicl Engineering t the Medicl Acdemy of Sofi since 976 nd Director of the Centre of Biomedicl Engineering of the Bulgrin Acdemy of Sciences. He is one of the founders of the Bulgrin Society of Biomedicl Physics (collective memer of the IFMBE) nd its president of mny yers. His current reserch interests include electricl defirilltion, stimultion, nd iomedicl signl nlysis. Medicl & Biologicl Engineering & Computing 00, Vol. 40 7