GB2353594A - Capacitive sensing electrode and voltage divider - Google Patents

Abstract

An electric field sensor employs a capacitive pick-up electrode in a voltage divider network connected to a body emanating an electric field. The system is relatively insensitive to variations in the separation gap between electrode and body, reducing sensor motion artifacts in the output signal. The pick-up electrode may be positioned at a "stand off" location, spaced from intimate contact with the surface of the body. This is equivalent to providing low level capacitive values for the capacitive coupling between the pick-up electrode and the body whose electric field is to be monitored. Alternatively, a series limiting capacitor may be provided in the input stage. Human body-generated electrical signals may be acquired without use of conductive gels and suction-based electrodes, without direct electrical contact to the body, and even through thin layers of clothing.

Description

2353594

TITLE: ELECTRIC FIELD SENSOR

FIELD OF THE INVENTION

This invention relates to electric tield sensors in the medical field for the detection of alternati,ng electrical fields originating from within the body to produce electrocardiograms (FCGs) and elec tro-encephal oq rams (EFGs) and the I.ike, as well as hedrt rate monitoring- It also relates to other appiications for sen5ing external electric fields.

BACKGROUND TO THE INVENTION

The detection of electrical potentials occurring on the human body is the basis for ECG/EEC. diagnostic procedures used to assess heart conditions and brain funrtions hereaftor E CCX1 An extensive science has beeri established on the basis of coupling conducti-vt electrodes to the human body to sense the low-level electrical signals that the body is able to generate.

A feature of this technology in the past ha,3 been to foci-is on reducing electr_i.cal resistance at the,9kin/elecL.code interface. For this purpose ECG electrodes are.

often used in conjunction with conductive qels and suctJon cup attachment mechanisms. These arrangements arc uncomfortable for the user, restrict mobility, and have- limited useful life- Dry Electrodes - Prior Art Approach investigations have been made into using capacitive 2 pickups to detect electrostatic potentials on the skin of a patient. F. Yamples in the literature iriclude the text "Introduction to BLo- Electrodes" by Clifford D. Ferris, published by Plenum Press in 1974. In this text the author discusses experiments with insulated, capacil,ive electrodes based upon the configuration (page 184):

"Body surface (skiii)/Dielectric/iiietai/FET"- 1 A shielded single electrode and a Lwro-ealectrode circuit based on such an electrode are depicted on page 185. Electrode capacitance i5 reported a:; 14 uF/cm: at page 187.

The text "Electrodes and Measurement ot 13io-Electric Events" by L.A. Geddes, published in 19?2 by Wiley Interscience discusses "dry electrndes" at pages 98 - 1.0-3). A single electrode circuit based on & insulated anodized electrode and F'ET Lransistor is depicted at page 100. A vaiue, for electrode capacitance is reported at page 102 as hel.rig 3200 picoFarads. Capacitance ranges of 5000-20000 picoFarads/cm are referenced at page 102. In particular, this refcrence reports (page 102) "At present there are attempts to provide ultra thin films of insulaLiriq materials having high.

is dielectric constants and strengths so thaL a high electrode-to -si)bjecL capacitance will be attained... " - This statement recites that obtaining a high level of capacitive coi)piing is ari object-i.ve and neces.bari,ly presumes that such Glectrodes will be placed in inLimaLe cc-ritact with the body of the subject being measured.

In the text "Principles of Applied Biomedicai Instrumentation" 2nd edition, L.A. Ceddes, L.E. Bater pub-1 i shed by Wiley Interscience, 1975, the author observes (at page 217); "To obtain an electrode-subjpct impedance Lhar. is as low as possible, every effort is made to obtain a high capacitance by using a very thin dielectric having a hiqll dielectric ccnstant."

Capacitance values from S,000 pF/cm to 20,000 pF/cin' a-e cited.

A Technical Note entitled "New Technologies tor TnFlrght, Past-eless Bioelectrode3" by D- Prutchiand A.M. Sagi.- Dolev, published in Aviation, Space and Environmental Medicine, Jurie 1993 (page 552) ciiscribes a capacitve, dry bioelectrode for obtaining EEG and ECG signals obtained through a plate anodized with aluminum oxide. Coating thicknesses of 50um and 170 um are referenced- Allowing for a dielectric value of 10 (for aluminum oxide) this thicknet;s would provide an electrode with the ability to develop a capacitance of about 50 pF/czn to 180 pF/cm", if intimately presented to a conducting surface- Accordinqly, the prior art has addressed the problem of capacitive dry electrodes in terms of developing high capacitive values for irit;ulated electrodes placed in intimate contact with the surface being monitored. These prior inve,5tigative efforts have been focused nn maxim-izing the coupling between the electrode and the skin surface carrying the potential to be deLected. This has ied to electrodes that employ thin dielectric surfaces that are capabie of providing capacitive values from about 50-1000 picoFarads/cm' and higher. it Is a necessary adjunct to establishing high caparitive coupling to a body tt&t the electrodes be prossed intimately against the surface being sensed, and that the surface be smooth and free of defects.

A capacitive pickup elertrode- fox- an E;CG system may be designpd t,.-,, have a capacitive value of several hundred picorarads when its insuiated plate surface is laid over a smooth, highly conductivo counter-el ect rode surface, such as a sheet of copper. This is r.he condition foc- maximum capacitance However, whcn placed pruximate Lo the human skin, the dead laYer of the skin acts effectively as an Insulating spacer, removing the plate of the pickup electxodr, further froat the source of the electric field being scn3ed. In such a configurat. ion, the effective value oF the capacitive

3 Coupling between a typical, high capacitance pickup elcctrodc e-g. 100'pF/cm2 and the field source wiLhin thc human body may be on the order of 1-100 piuoFarad:;/cm' depending on the intimacy of contact with the body and the presence of sweat or 5 hair on the skin.

Difficultie, of_Intimate Qoupling The results of prior art endeavcurs have been only moderately successful. one pzoblem that has arisen is the, extensive sensitivity of these capacitive electrodes of prior design to variations in the gap or intimacy of contact between the electrode and the skin. When Atimate contact is the objective, even the presence of hair or sweat can Cause veriations in the value of capacitive coupling being established. The procedure of pressing dry electrodes agdinst the body has presented similar inconveniences to those ari5ing in the use of conductive electrodes, e.g., discomfort and Jimitud mobility due to intimate contact protocols. 1 n particular, prior art systems have never been rcported at; operating through clothing fabric. Oo proposal has been made to obtain alternating elecuical signals of the ECO, EEG type, etc- through use ot dry capacitive electrodes that are nolpositioned at fixed "ccations on the skin surfacc of a subject.

Further difficulLies associated with the use ot dry 25 electrodes pressed into intimate contact with the skin of a person are tribo-electric effect-s - electrical charge5 created by sliding friction and pressure. Tribo-eiectric-effects deli,vcr large, essentially static charqcs, to the Pickup electrode.

Such charges impose a near DC or very low frequency drift in the background Ovel over which the more interesting,

4 higher frequency signals are imposed. To discharge the amplifier input and pickup clectrodc of such capacitively acqui-red charge, the inpuL resistive impedanco of the high impedance first stage amplifi.e-i: shouAd be carefully selected.

Thus a particul-ar concern when sensing alternating signals is the band-pass capabilities of the sensing sy5tem.

Ideally, the pickup elecLrode should drive an amplifier with - L 'CGS i s a complementary input impedance which, in the case of Z able to process low level, e.g. milli-volt, signals in the range 0.05 H, to 150 H, The lower cut-off frequGricy shculd be stable _Lri order to restore the bias value of the driven amplifiers to its normal value in cases, whure the circuit is over-driven by a very low frequency or DC offset signal.

To minimi7e the disruptions caused by very low 1.5 frequency or DC over-driven off-sets, the capacitivo coupling to the body (C) should be matched to the Input impedance of t-he amplifier sonsor (R) via a preferred, turied RC-relati.on. This allows the sensor to have a stable band pass. U_ S. patent 3,744,482 addresses this issue WiLh a r-11ned feed-back loop. However, for the tuni.ng of the sensci- input U) be consistent-, both the resistive -R an,cl capacitive, -C values shcul.d be stable.

Variance in Capacitance A pickup electrode may be of:!Duch a design as to permit it to achieve high val.ue capacitive coupling, as for example inaximum values of 50-100+ pi. co Farads /cm2 when placed on a conductive plate. This can be effected through use of thin or high dielectric value insulative layers. A dif f icu.1 ty arises, however, in ensuring thatUlle frequency cut-off of the JC network at Uhe, input.sLage is appropriately tuned when 'the pickup electrode is capable of high capacitance coupling. This difficulLy arises front the fart th,-:it a p-i_ckup electrode witti potentially high capacitance w.ill exhibit varying actual capacitive coup] ing valuos when p] aced adjacent- to the bo,,Jy generating the etectric field, parti.cular.l.y when an aLLempt is made to place such an electrode in intimate contact with the 5 skin of the human body being sensed.

By example, the actual capacitive coupling value may range over an order of magnitude or more commonly by several hundred percent if the eleeLrc-, de is pressed very tightly against skin wetted with body swea.t. In this si.tuat5on, since capacitance Varies inversely with the gap separating with 1-he capacitor electrodes, the system is operating in the separati.orl--s-ens.itive region of a graphic plot of capacitance v.s Separation. Distance (cf Figure 5).

when the effective capacitance of the p-jckl-,p electrode var.Les subs tantially, the cutoff value of the RC filte.r arrangement will vary correspondingly. This wi.-Ll reduce Llie performance of the RC combination as a well-tuned, high-pass, low frequency cut-off filter. Settling tinies for low frequency signal artifacts will be lengthened as the capacitive value of C is doubled or tripled.]3ackqround--Nc)ic?.. Rejeclion A major source of noise for electronic systems is ambient 60 Hz signaLs (in North America) arisi ng from tho power system- lt is known that sixty hertz background noise

2) 5 can be eliminated or greatly reduced through the use Qf a dtf(-, rential amp.-].if-ier arrangement. However, for maxiraum rejecLion of common mode noise to be achieved, the inputs to both branches of thP differential amplifier should be fully balanced. If tho inputs are not balanced improper signal differencing will occur and the output will be disturbed by the imbalance. Tn the case of ECG balance woul.d ideally be achieved by having two separalle ECG, pickup 6 electrodes couple to the source body originating the electrical field with the same degree of capacitive coupling.

Where intimate-contact, high capacitance electrodes are empioyed, Lhis balancing is hard to maintain. A need exists for a more stable system to be enpioyed For these types of applications- The invention hercin addresses Lhis need.

Tn su=ary, a need exists in the medical field to provide an electrical field sensor for detecting alternating signals that is less demanding in terms of electrodo/body coupling. In non-medical fields, useful applications may also arise where the measurement of an oscillating surface charge is to be effected without contact arising between the charged surface and the electrical sensor. The imiention herein addresses such needs.

1.5 The invention in its general form wi.il first be described, and then its implementaLion in terms of specific embodiments will be detailed with reference to the drawings following hereafter- These embodiments are intended to demonstrate the principle of the invention, and the manner of A5 implementation. The invention in its broadest and more specific forms wiil then be further described, and defined, in

Claims (1)

1. each of the individual Claims which concludc this

   Speci f i ca t i on.

   3MMARY OF THE INVENTION According to one aspect of the invention the signal pickup procedure for obtaining an electrLc field or ECG signal and the like is carried-out under a configuration wherein the effective capacitance coupling the electrical field source to a high impedance sensing amplifier is relatively insensitive to variations in the separation betwoen the body that servts as a field source and the pickup eiectrode.

   7 According to the invention in one aspect, an electric field sensor is provi-ded that includes a first pi--kup electrode for placement next to a surface whose electrical field is to be sensed through capacitive coupling. This pick- up electrode is not operated, as in the past, to achieve hi.qh capacitive coupling values for such el ectrodes, i. e. operating in Lhe separation- sensitive region of a Capacitance vs Separation Distance qraph (as per Figure b) - Rather, by 1he arrangements of the present invention, the value of th,-- capacitive couplings between the source field and the sensing kept small. i. e. under 40 pi. cc Farads ampl if ier j, 5 preferably 20 pi caFarads/cirL', more preferably, 1 - 1 picoFarads/cm'. This may be achieved by avoiding iritmate contact with Lhe body e.g. by positioninq the plate of the pickup, electrode aL a "stand-nFf" location that reduces the.,(2nsitivity of the incasured output Lo motion effects i.e. variations in the separation of the pic-up elertrode froirt the surface of the body being sensed. And it may be achieved by placing a I.imitinq capacitor in spries with the. input to the 22-0 sensin.g amplifier.

   Tr.) ensuro the first arrangement is achieved,an insulating layer ma 1 ectrode to y be provided over the e. separate it from. a body by a gap thaL ensures that capacitive coupling does not vary sf-.nsitvity with separaLion. In soine cases, _signals can be obtained by placing scrisors of the invention over proLective layers already present on Lhe 1Cody.

   The objective in designing the sen--.r in accordance with this cciterion is to en2ure that the overall, effective capacitance formed betweeil the pick-Lip electrode and any surface that may be presented to the outer f'ace of the pick-up electrode will always have a value irl the region of a plot. of capacitance value versus iRr--,r)aration distance wherein, upon 8 displacement of the electrode by a s:tandard amount, the capacitance is varied by a linlited percentage value.

   r In pa.,r:-ticular, and preferably, when Llie separation of the electrode fruiti the surtace varies tly 0.1 mrn or less, the capacitance value of the coupling between the body and the pick-up electrode varies by less than SO%- More preferably the capacitive vallue varies by less than 20%..

   By providing an ECG pickup with an insulative layer. that precluctes the pickup electrode from achieving capacitance values of higher than a specific value, e.g. 40, 20 or rnore, preferably 10 picoFarad.s/cm2, an ECG system so equipped wil.1 be inherently suited for operation in the preferred, socond, sepa ration- insens i tive region (as per Figure 5). The presence of such a capacitance-limiting insulative la_yer will preclude an electrode from operating in the firsL, separation seTisiti.vc-, zone it is preferable for thc- insulating layer to have a th.i.-kness wh'ch is equal to, or greaLer than, the size of surface irregularities of the body bei.ricj measured, and equal tr, er greater than the variati-ons in trie sensor-to-body separatIon gap.

   T h...L s is completely counter-i ntuitive to the methodologies applied by the prior --:irt experiments with capacitive, "dry" electrodcs which employ extremely thin 211 dielectric layers and then proceed to place the sensor in intimate contact with the surface of the body being sensed.

   Thus, the presenL invention, in one aspect, employs a dielectric layer for the pick-up electrode Ltiat ensures that sens-ing is occurring at a stand-oft location which is insensitive to ininor motion and/or surface irregularities.

   The instability arising from the vari-ations in t.he coupling capacitance of the pi.ckup olectyode can be addre-3:sed 9 in a further manner, namely by inserting into the input of the high impedance sensing ampliLier that -receives gignals from the pickup electrode a scries capacitor of fixed and Lintited -value. TIiis limiting capacitor should preferable have a b minimum vaiue that is greater than the input capacitance of the amplifier stage that is driven by L-he signal received from the body througn both the pickup electrode and the limiting capacitor. As & preferred upper iimit, the. limiting capacitcr may have a value that is less than the offective coupling capacitance beLween Lhe pickup eiectrode and the body. Vai ue3 for this limi L.ing capacitor outside this prefer-rcd range may atso be adopted. The inclusion of such a series capaciLor has the same effect in constraining variations in the effective, overall capacitance value of the coupling between the electrical field source and the input ampl:i,fier as the "standoff" variant of the invention referenced abovewhen this alternate procedure for rendering the input amplifier relaLively insensiti.vo to Lhe electrode/body separation distance, P_(j. placing a limiting capacitor in 2 0 series at the input to the firsL b-tage amplifier, is employed, use of a series limiting capacitor of appropr-iaLe value, o-g. 40 picoFarads, will set an upper limit c;n the capacitance coupling between the field source and the inpuL ampl.ifier. As the pickup electrode i-9 in series W. LLII the limiting capacitor, Lhe combined capacitan(.,-e of Lhe two cannot exceed the value of the limiting capacitance- Because the value of the limiting capacitor is fixed, the RC value for the high paz3s filter at the input stage is stabilized. Even if the pickup elecLrode ias a relatively higti maximum possible capacitance, e-g. over WOO picoFarads, because it is in series with the limiting capacitor, it cannot absorb -a substantial static charge. Viewed alternately, if the pick.up clectrode were to d(:hjeve in fact, a very high level of capacitance coupling to the body, at a value greatly exceeding the capacitance value of the limiting capacitor e.g. 10:1, then we may treat it as having a minimal, or transparent impedance contribution to the combined series capacitance of the amplifier's input. This will still leave the limiting capacitor, e.g. 40 pf as dominating the capacitive coupling between the field source and the input amplifier.

   "Itage Divider Network In detecting electric field signals through thccapacitive pickup arrangoment of the invention, the signal being sensed by the input amplifier is essontiaily being taken from acrow a voltage divider network defined by the pickup electrode, the limitinq capacitor (if present), -the input capacitance of the aMlificr and the reffLaining electrical coupling (either resistive or capacitive or both) at the other end of the voltage divider network which is connected to the body which in the source of the electric f ield. Assuming this last connection in of relatively low imp- edance-, the signal strength seen at the input to the amplifier depends on the ratio of the input capacitance of the amplifier to the other capacitors in the series chain- If the input capacitance of the amplifier is small, then most of the signal strungth will appear across this capaciLance, and be sensed by the amplifier.

   Tn actual use, the effective capaciLive %ralue of the pickup electrode may be on the ordec of the value of the limiting capacitor. In Lhis case, its impedance contribution will becaae significant. For example, the pickup electrode effective coupling capacitance being equal in value to that of the limiting capacitance --e.g. 40 picoFayads- then t-he combined, net capacitanco of these twu elements in series 11 would drop to half V their dndividual capacitance values e.g. 20 picoFarads. This will riot, however, have a Seri(,)us deleterious ef%ct on the signal detecLion performance of the overall syztem so long as the input capacitance to the high 5 impedance amplifier is small e.g. 2-5 picoFarads- DifferentiaL AmplifieUpual inputs As is done in the case of conductive clecLrodo FCC systems, two pick-up sensors may bc applied at two disLinct locations on the ski By taking the differonce in the output signals from two locations on the body the beneEits of nommon mode noise rejection may be obtained. The objective of minimizing variations in such capacitance values is also important for this special case arrangement in ECG-measuring systems: tho use of dual input different-.!a! amplIfiers to obtain rejection of common mode noise.

   Differential amplifiers used to reject common mocie noise, e.g. ambient 60 Hz, faii to achieve full rejection when the Was levels of the amplifiers are inbalanced or it the amplifiers have unequal RC characteristics. To maximize the prospects that these levels are balanced, both branches of Lhe pickup elements should have similar settling times when disrupted by an off- setting, very low frequency signal. This requires thaL the effective capacitance of the couplings within both branches between the sensed body and amplifier inputs be similar. The invention addresses means for achieving this last criterion.

   Clothing-Supported-Arrav_ On the foregoing basi5, this invention provides a moans Lor detecting electrical fields present on the surface of a hody without the use of conductive gp.',s and suction-ba3ed appliances- useful signals atay be obtairted based on Lhk- 12 k combination of multiple electrodes assembled in a f.j.xed, preformatod array. Thus, mult-tple electrodes, e.g. 4 or itture, maV be carried by a clothing-type of support as, an array that can be teadily donned or removed with minimal inconvenj.ence.

   This provides considerable freedom for the tele-monitoring of patients while they engage in daily routines. Freedom f rorti the.1 i rnitati ons oF conventional tele-fticinitoring arrangements repre-sents a valuable advance in this field.

   The foregoing summarizes the, princi.pal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preterred cnibodiments, in conjunction with the drawings, which now follow.

   BRTEE' DESCRIPTI7ON OF THE DRAWINGS is rigure 1A is a combined pictorial/electrical schematic depiction of a single pick-up of the invention i.n position adjacenL to a body whose electrica] field is to he sensed. The voltage divider network is capacitively coipled to Lhe body at both ends and drives an operational amplifier.

   b'-igijre 1B is a conventional electrical schematic corresponding Lo the input portion driving the amplifier of Figure 1A_ Figures 1C and ID a.Le tho schematics of Figure 1A drid 16 with the added presence of a series capacitor in the amplifier input.

   Figure 2A. is Figure 1A. wit-.h the sub-,titution of a resistive, conductive coupling 1,o the body at one end of the voltage divider network- A smaller parallel capacitive coupling remains present as well.

   Figure 2B is a conventional electrical schematic.

   corresponding to Figure 2A.

   13 Figure 3 is an electrical schematic for a dual pJ(-jup e3.ectrode configuration, based on the pick-up of Figure 1A, with signals being fed to a dtfferential amplifier, but with dual, parallel. Schotkey diodes as input leakage res-[.stors.

   Figure 4 is an expanded electrical schematir. of thf'circuit of Figure 3 with the additional presence o an amplifier and optical coupler to provide L.1p;Lrical isolation.

   Pigure 5 is a graph showing the change of capacitance of pickup electrodes with various surface areas as a function of separation distance for the electrodes.

   Figure 6 is a graph showing the perceriLage change in capaci.tance for a 0.1 nun change in electrode-to-body gap distance as a function of noininal electrode-to-body gap distance over a range of 0. 0 to J. 0 mm, assuming. the body act-n as a -perfect electrode.

   Figure 7 is a plaii view of an electrical circuit corresponding to Figure 4 laid-out in a belt to be worn over the chest of a patient.

   Figure 8 is a pictorúal depiction Qf the belL of E'iqure 7 in Placc over the chesL of a patient.

   'igilre 9 is a pictorial version of a garment worn by a patient that carries four pi.ck-up electrodes- Figure 10 is a graph of tota.1 effcctive c-oupliilrg capacitance between the sensed body and the input to the amplifiei. of the sensor, plotted as a function of the separation distance of the electrode from the surface beinq sensed. Three curves are shown, twn with a lintiting secies capacitor pcesent and one with no limiting capacitor present.

   Figure 11 is similar to fi.gure 10 but with Lhe vert:ical scalo for Lhe input capacitance increased by a factor of ten and showing one curve wth and one. curve- wilhour. a limiting capacitor present.

   14 OESCRIPTION OF THE PREFERRED EMBODIMENT In Figure A a Pictorial schematic is shown of an electrical Amnsor system incorporating a pick-up electrode 1 in the form of a flat conductive surface placed adjacent a first location 2 on a body 3 where an electrical signal is to be sensed nriginating frpm an electrical signal generator 4 within the body 3 that provides a source volLage V,.. The pick-up electrode 1 develops a capacitive coupling to the body 3 through an intervening dielectric layer separating it frorn the body 3. This capacitive coupling for the pickup electrode 1 is represented schematically by the capacitor Cp.

   The electrode 1 is connected to the input of an operational amplifier IC1A, or its equivalent such as a field effect transistor- input resistance R, connected between the amplifier i nput and circuit ground has a resistance value of on the order of 100 ohms and serves to discharge the input of DC offsets and restore proper voltage input levels while accepting signals of the desired frequency.

   The outpul V,.. from the voltage divider network which drives the operational amplifier ICIA is mea-sured across inpuL resistor R, thaL extends between the input of the operational amplifier ICIA through circuit ground to a refezence capacitnr C, that is coupied to the body 3 at- a second, separate location 5. This locaLion 5 may be separated. from the first location 2 in obtaining conventional ECG signals. The locations 2, 5 may also be proximate, e.g. adj acenL, at certain body locations and sti12 provide useful Rignals- Capacitive coupling C. is effected by means of an electrode (not shown in Figure jA) that is separated from the hody 3 by a nen-conducting material that acts as a dielectric. Conveniently, the casn for an on-board battery holder can serve as this electrode, as shown futther below.

   For th.e present invention the nature of the diGlectri-e material haS little, effect when the pick-up electi.ode5 are p] aced in 'casual' mechanical contact with the body being scrised as in the case of ECG pickup on hairy skin or over clothing. Satisfactory values of dielecrcic constant have been found in the range 1 to 10.

   Tnside the body 3, the si..grial generator 4 i.s seen as being subject to internal resisLarice R, within the body The input portion of circuit of Figure 1A i., redrawn as Figure IB in more conventional forin- In tigure 1B, trie capacitance 2, ari-scs from the combined input capaci.Lance of the operational amplifier 1C1A and the input resisto.r Rt. The r-otal apparent input resistance of this amplifiec 1.5 represented by R,,, including the resistive, value of the input resistor R,- Collecti-vely, the capacitances C,., C,., C, act as a voltage divider neLwork whereby the output vultage V,, is proportional to the source voltage V.

   In Figures 2A and 2B, the coup ling to the body 3 at the end of the voltage divider network opposLc- to the pick-up electrode 1, js. effected principally by a direct, conductive (-ontacL- The resi-stance of the interface is _Lndicated by Rn. NGcessarily, some slight capacitance coupling is also stj.IL-1 present, indicatod by C',- The output sianal of the sensor is extracted by measuring the voltage difference across an electrical component in the voltage divider network that i. connected to the subject electrical source. This shotild be done Ltirough a Pligh impedance, low capacitance circuit or sensing Yncans to minifitize jignal toss. A field effecL transistor or operational ampLifier having ari input impedarice of on the order of 1011 ohms and ari input capacitance of abouL.3 picoFarads has been found to be satisdctory when the other 16 capaciter(s) in the voltage d.i.vider network have values of on the order of 10 picot'arads. Used in conjunction with a pick,up electrode having an area of on the order of one to ten square cent.imetres, dielectric media havIng a total effective dielectric constant of 1-10 and a body-to-surface gap distance of on the order of 0.1 to 4 millimetres, signal. values of the order of 1 millivolt or less may he detected fr.om the skin surface of the human body.

   With this type of sensor configuraLion useúul 10 signals may be obtained with tho plate of the pick-up electrode separated from the skin or sensed body by a gap) that allows the pick-up to qualify as a "stand-off" electrode. As L.he gap varies, the strength of the output signal will varY. But by operating the sensor in the capacitance/gap separation region specified by the criterion of the invention,!:uch variations will. not detract inordinately fro'n tho valile, af the signals being obtained- A pi(,-kup electrode that is removed somewhat from Lhe electrical field source -is able to supply a satisfactory signal by reason of the mathernatical relationship that exists between Lhe value of capacitance and the separation distance existing between capacitor plates or L-lectrode.s. Since capaciLance varies inversely with separation, th.e maLtiemat-ical forTri of a curve for capacitance value plotled agains t.

   separation distance i, i.ii Lhe shapt- of a hyperbola. rhis means thaL the capacitance performance of a pickup eleeLrc)de can operate in two distinct regions:

   1) a first- region wherein the separatl?-)ri distance is small and the curve is steep,, corresponding to the situation where the rapaciLance value wLll vary highly, wiLh great sensiLivity, in response to small. changes in the separation and a second region wherein the separation is greater, the curve is relat.i. vely flat, and the capac i Lance value varies relatively inscrisitively with si.rrt-'Llar changes in the.-3ep<lration disLance.

   For the purposes of the present invention, the prefer-red region of operation accordinq to one variant of the invetrition is in the second, separation-insensitive zone.

   In rigure 5 a grdphic plot Js depicted of the variation of capacitance C with a variation in the separation di.stance d at various separation distances d, based upon the theoretical formula: C =k.A d where: C is the effective capacitance of, for example C,, d Ls the separation di.st-.,ince of the electrode-- plate is from the body giving rise to the capacItance, A is the area, or effective area, c-" the pick-up electrode 1; and k is a proportionality constant affected by the dielecLric material in the separation gap- 22 0 In Figure 5 the value of the dielectric constant is assumed to be that of air, i.e. 1.0 and the plates forming the capacitance are assumed to be fully conductive. This is therpfore an idealized variant on the case of coupling Lw the human body- True. Effective Capacitance It is believed that all of the capac-itive values cited in the prior art references are based on the piemise that cited capacitance values are for the maximum capacitance Lhat an insulated electrode can develc)p when pressed against a conductive surface.

   18 Four curves are shown in Fiaure 5 for pick-up el-ectrodes 1 having surface areas, as follows.

   a = 1 cm, c 5 0 cr,,' b 10 (jtxt' d 100 cm2 Each capacitance curve can be,separated into two important regions: region 6, in which the capa-itance changes relatively rapidly with a given change in separation distarlu(-,; and region 8 in which the capacitance changes relatively slowly with a similar given change in separation distance.

   These regions are generally separated on Figure 5 by boundary line -7.

   For a capacitor with an eleeLr.ode area of 1. cm, the line 7 passes approximately through a capacitive value of about 40 picoFarads. For capacitors with an electrode area of around 25 em' and capacitive valuen below 200 picoFarad,3, region 6 approximately corre.5ponds Lo the zoric, with d = 0.1 ram or loss; while for such values region 8 approximately corresponds to the values above d. = 0 - 1 mm.

   An important _Lrnplication of 'igur(- 5 is that sensors lthin regime 6 are very sensitive to 20 with capacitance values wi sinall additiona.l changes in the separation distance (delta-d) in contrast, sensors with capacitance values corresponding to region 8 are relatively insensitive to such changes. This i-, illustrated more succinctly in Figure 6. 25 In Figure 6, the percentage change in capacitance corresponding to a del-ta-d - 0.1 iTun is graphed as a function of the nominal separation distance dFigure 6 is dirnOTISionle:5- s along the C axis afid applies to all capaciLive sensors whi.ch obey or approximately 3 JO obey the relation C=kAM. A.ccording Lo the invention the.. capacitive value of the Pickup electrod.--, and other capacitive sensors when employed, are designed la operate- in region 81 of F'igure 6, as opposed to regiorl 6' frorn which it is sepazated by boundary I.ine 71.

   Tn this latter regime the capacitance, cind hence the output signal is sufficiently insensitive to spatial and temporal body surface variations so as to provide the advantages of signal stability inherent in the invention.

   Fi-gures 5 and 6 pre.miso that operation in regions 6 and 6' can be effected hy iciieving low capacitance coupling between the body and the pickup, electrode. Figures 10 and 17.

   apply to an alternate case whetein Lhe capacitive coupling between the pickup electrode arid the body is high, but the results of achieving system opuration in preferred regions 6,61 is still obtained. This is achieved by insertion of a series llin.i.ting capacitor C. in the input to the first stage 1.5 amplifier of the sensor- Thi3 series limiting capa(jibor may have a preterred value that is greater than the iriput Capacitance of the first stage amplifi.er, and less than the offective value of the capacitarice coupling between the, pickup elecLrode and the body whose electrical field is being sensed, e.g. between 5 and 40 picoFdrads.

   Tn Figures 1A and 1E the pj,e--k.up capacitor C, is shown as being directly coupled to the operational amplifier 1 CIA. In Figures 1C and ID a series, capacitor. C, is shown added between the pickup capacitor C,, and lhe amplifier input (at which V,, is detected). The effect of L-his limitinq capacitor C, is to place a maximum value on the capacitance extending between the body 3 and Lkic signal serising mearis 1C1A. The pickup elecLiode's capacitance C, is in serles with the I.imiting capacitor C,.- Collectively, they behave as a single capacitor having a total net value CT -1 1 C1 + 1 1 0 4 Figures 10 and 11 plot the behaviour of C., as a function of the separation distance present for the pickup capacitor Cp.

   This net value capacitor Cr provides a more stable, 5 separationinsensitive- circuit performance that occurs in its absence. This is particularly true when C, is smaller than CP, A convenient formula. for esLablishing a value for C, is that CL should be less tan 5 (picoFarads/c:m2) times the area of the pickup electrode (in cm').

   The consequence is that a similax region W' of insensitivity to displacement of the pickup electrode exists i.ii rigures 10 and 11, parallelling regions 8 and 81 in Figures 5 and 6. A similar preferred criterion for. performance of the invention can also be established for the circul t arrangement of Figure IC, 1D, namely, a 0.1 mni displacement. of the pickup, electrode cause, a 5OS- or small change in the net capacitance Cri- Preferably the change is less than 20%_ Thus, the same effect of desensitizing the signal pickup and <-nuriling capacitance from motli.on art.ifacts can be achieved through the presence of a limiting capacitor Cy, in the input link between the pickup capacitor C,., and the signal sensing nteans 1C1A.

   PY For the present j.ilver.ti..on, the input resistance -4,5 present at the input to the high impedance amplifier can be provided from two sources:

   1) the inherent input. resiS-,.aTICe of the amplifier, typically 101 -101, ohins; 2) the input resistance of an added, external, input resistor, R,.

   21 A preferred value for this resistance R. may be determined by considering the pickup electrode and input reSistance as an PC high freqiiency passing fiLter.

   Assuming an efFecLive pickup elect-rode capacitive value of 60 picoFarads and a low froquence CUL-Off of 0-05 Hz established by the RC input value of the first stage amplifier, a preferyed value of 4 x 1012 ohms may be provided for t-he input stage input rc-s.i.sLarice R, .

   Occasionally, the near-DC signals delivered to the pickup electrode will be so substantial as to drive the signal at the input amplifier to the 1 Lmit of its range of' respon-se. When ovcrdriven, the recovery period before a normal input leve 1 can be re-esLablished by the input resistor is increased. To shorten the recovery period in such cases it is convenient to provide the input stage- i,7i,th a non-linear input resstance. This- can be achieved by grounding the input through pairs of Schotkey diodes, L),, D, in Figure 3, connected in parallel- The forward resistance of Schetkey dLQdes before 2- 0 br.eakdown cc<-u.rs can be on Lhe order o 10" ohms. by diodes with a forward bre-adown volLage that is above the level of the signal of interest, the ".respt" function ol: the input resistance of the high impedance amplifier c,;-in be improved. If the breakdown voltage of the Schotkey diode is chosen Le bu at the voltage level for saturation of the input amplifier, then the " short ing" effect occurring after breakdown will not distort the signal of interest as long as the amplifier is operating within or inside its;aturat.(:.)n cut-off limits.

   As the forward resistance of the Schotkey diodes prior to breakdown may be higher than the appropri, ate value to provide an input resistance suitod to the given low frequency 22 cut-off for the RC filter, such diodes D,, D, may have to be accompanied by a paraMel input resistor R. that establishes the appropriate net value for input resistance for small).evel signals- In Figure 3 two pick-ups similar to that of Fiqure 1A (except for the substitution of diodes D:,, D; For the input.. R,) are used to d.rivc a differential amplifier IC3A. The securid additional pickup electrode]A is piaced at a location 10, separated from the first arid second locations 2 and 5. Within the body the signal. source V.. may be treated as distribut-ing its potential o-,7er the resistors Re., R'r, W,,.

   BY use of this differential signal detection circuit, common mode noise present in the two pick-up circuits will be minim-'9.zed.

   Figure 4 shows the circuit of E'igure 3 extended by an optical isolator 1501 driven by an operational amplifier IC4A which is, in turn, driveri by the output from the differenLial amplifier 1C3A. By mciinting these circuits as cLo as possible to the pick-up electrode 1 1A, interference from ambent 60 Hz electromagnetic signals can be minimized.

   In. Figure 4, a shielding conductive layer 11 is depicted a.s overlying the ey.ternally-dirp(-t-.(:.(i side the ci rcuitry. This layer/structure 11 is pref erab ly connecLed o Llic circuit common point but need rict nec(-,-ssaril-y be so connected. In some configurations thin- shield may be f loat -ng" Its role is Lo exclude effects arisingfrom int.ruding.electro-magnetic e.q. 60Hz, ociginating in thet, environment. 1n non-earthed applications the shield distributes ambient, intruding signals equally to both pickups, contributing to common inode ricise rejection. It is 23 highly desirable that such a shield be employed in one or other of such configurations. In Figure 7 a belt 12 in depicted that carries the cirau!U of Figure 4.

   The hatched areas are decorative. The pick-up electrodes 1, 1A are mounted on a MYLAR"" or KAPTIQN(lm' film 13 that serves both as a spacer and as an insulatinq dielectric of approximately 0-13 wt thickness- The pick-up electrodes I, 1A have been measured against a copper plate as providing a capacitance value of 20 picoFarad5.

   The belt 12 of Figure 7 has its own on-board power supply in the form of batteries 1_ Thc case 15 of the. batteries 14 is connecWd to circuit common point and serves as an electrode to provide the reference capacitor C,,. A measured value for its capacitance, when placed against a copper plate, of 160 piceFarads has been obsorved wj_Lh the case 35 coupled to the entiie circuit. The subsLrate tor the belt 12 is made of MPTON" having a thickness of 5 thousandLhs of an inch. This forms the principal dielectric elemont. for both of the napacitors C. and C.. The naturc of the dielectric material has little effect on the invenLion whcri the pickup electrodes are located at a sufficient"stand-off" gap from the bodyThe shield 11 (not shown) in the belt 12 of Figurc 7 is in the form of a flexible conductive layer, with an 2b insulated undersurface that overlics the circuitry on the outer side portion of the belt 12. This shielding layer must be close enouqh to the pickup electrodeR T to evenly distribute ambient noise s.iqrials, and sufficienGly spaced froin the pickup electrode/body interface so as Lo not detract from signal pickup by the pickup electrodes.

   The pick-up electrodes 1, 1A in Fiqure 4 are held hy the substrate 13 of the heTU 12, at. a fixed, intervening 24 interval. This interval is dimensioned to permit the electrodes 1 to respectively overlie electrical nodes (not shown) on the body 3 oil a wearer 16 as shown in Figure 8- The belt 12 is held in place by Lefision developed by connectors, e.g. nook-and-loop fastening means, once positioned on the body 3. While a narrow belt 12 is depicted in Figure 8, a wider belL or vest 1.5 could carry three, four or more electrodes 1 as shown in Figure 9.

   An advantaqe of the invention is that nAulLiple pick- 1 -ctrodes can be assembled in a preformated, fixed array 0 up e1L that can be fitted to the body collectively, as a unitar-y assembly, much as in the manner of donniiig an article of clothing. 'rhis permiLs a wearc-r to be "fitted-up" for electrical field measurement in a -very -short period of time- L)ata acquisition can readily be suspended and resumed by the simple act of removing and then re-donning the pre-assembled array. No components are consumed in this process.

   The electrodes 1 of such a pi.eco of app-arel as shown in Figure 9 may feed signals Uo a radio transmil't.er 19 carried by the wearer 16. In this manner an especially converiient form oE Lcle- monitoring can be achieved.

   Apart from providing Lhe pickup electrode with an insulative layer that inherently suits its operation in the pre.ferred, separation-insensitive vonc, the. actual rcedom from havinq Lo place the pickup e1ectrode in intimate contact with the body whose field is to be sensed, has considerable advantages. These include:

   the pickup electrode need riot be tightly fixed at a specific location on Lhe skin. Small. lateral displacements are permissible. Adhesives are avoided; 2) the pickup electrode need not be appliedunder excessive pressure against the skin. Discomfort is avoided; 3) the skin need not be prepared to receive the electrode, as by shaving or rubbing; and 4) an i.nsulaLive layer, such as a pad or layer of clothing may be present between the electrode and the skin. This car) be useful Lo increase comfort and absorb sweat. With the e.lecLrode at a removed location, the appearance of sweat on the skin does not substantially affect the 6CTree of capacitive coupling.

   These are substantial conveniences for patients who must submit to ECG examinations. This is particularly true in respect to extended-period ECG munitoring pr-ocediires.

   C014CLUSION The foregoing has constituted a description ef specific embodiments showing how the invention may be:ippiied J_ and put into useThese embudiments are only exemplary. The 20 invention in it.s broadest, and more specifi-c aspects, is turLher described and defined in Lhe claims which now follow.

   These claims, and the languaqo used therein, are br.) be understood in terms of the variants of the invention whi-ch have been described- They are not to be restricted to su:li var.iants, but are to be read as covering the tull scope of the invention as is implicit within the invention and the disclosu--e that has been provided herein.

   26 THE EMBODIMENTS OF TEE INVENTION IN WRICH ANEXCLUSIVE PROPERTY ARE CLAIMED AS F0TiLOWS:

   1- An electric f.ield sensor tor. detecting an eloctric field present over a surface comprising:

   b (1) a voltage divider network inc-luding at one end a pick-up electrode with a face b-urface having arl insulating dielectric layer positioned adjacent to said face surface for placement- next to a source surface whose electrical field is to be sensed through capacitive coupling, the voltage divider network including at another end an electrica] coupling for connection to another portion of the source surface over which an electrical potential di.ff(--rr,rice exists; and is (2) signal sen,iiig niean having an input capacitance that forms a portion of the voltage divider network, the signal- sensing Tneans beinq connected for measuring t-.he.,, voltage appearing across thaL porLion of the voltage divider network provided by the input capacitance of the signal sensing means, wherein the thickness of the insulating dielectric layer is sufficient so that, when the pickup electrode is placed adjacent the source surface whose field is to b(--,.iiLeasured, the change in the capacitive coupling between the signal sensing means and the source Surface arising from a change in the separation distance between the pickup electrnde. and said surface varies insensitively with displacement of the eleeLrode towards or away from the surface- A sensor as in claim 1 wherein the overall, effective capacitance that may be 'Lormed betwccri said source 27 surface and L-he signal sonsing means through the pick-up electrode has a value in the region of a pl(,)t of capacitance value versus separation distancp wherein the pr-rceriLdge change in capacitance is les.5 than 50 percent when E-:xibj ected to a 0 - 1 mm change in the separation d.istance occurring between the pick-up electrode and the confronted surface.

   3. A sensor as in cldim 2 whe.rei.n the percentage change in capacitance is less than 20% when a 0-1. mrri change in the separation distance occurs.

   4. A sensor sis in claim 1 wherein said insulative layer is of such dimensions as to preclude Lhe electrode from providinq a capacitance value of over 40 tnicc)Farads/cm-'.

   1 A sensor as in claim 1 wherein said insulative layer is of such dimensions as to preclude the electrode front providing a capacitance value of over. 20 picoFarads /CM2 _ 6_ A serisor as in claim 1 whercin s,-.jid insulative layer is oC such dimens.ions as Le preclude the electrocie from providing a capacitance value of over 10 picoFarads/cm'.

   -7. Ari electric fiel.d sensor for dete(-,(--'Lfig an electric field presenL over a surface comprising:

   (1) a voltage divider network including at one end a pick-up electrode with a face surface having an i-n.RiLiting dielectri.c layer posi tioned adjac:..eiiL to 2h said face.5urface for placement next to a source surface whese electrical field is to be serised through capacItive couplinq, the voltage dj,,,ider network including at anoL-her end an electri-cal 28 coupling for connection to another portion of the source surface over which an Cloctrical potential difference exists; (2) signal sensing means having an input capac-i-Larice that forms a portion of the voltage divider network, the signal sensing mE:.,a. ns being corinec-ted for measuring the voltage appearing across that portion of the voltage divider network provided by the input capaciLance of the signal sensing means; and (3) a series capacitor, positioned within said voltagc dividcr nctwork bctween said pickup electrode and the signal sensing means,:3aid -series, capacitor having a value in picoFarads of less Own five times the area of the pickup ciectrode in cmz.

   S. A sensor as in claim "/ wherein said series capacitor has a value of at between 5 and 40 picoFarads.

   9. A sensor as in claim 4 comprising a leakage resisUor in parallel with the input capacitance of Lhe signal sensinq means of between 10" and 10" ohms.

   10. A sensor as in claim 1. cumprising a capacitive, coupling to the surface aU Ulie end of the voltage divider network opposite the pick-up elecLrodn.

   11. A sensor as in claim I comprisinq a resistive- contact coupling to the surface at. the end of the voltage divider network opposite the pick-up electrode, said resistive contact coupling having a resistance value of SOO k ohms, or less.

   29 12--- A sensor as in clairn 1 having a cen(JucL-ive elcmerit positioned over the externally-directed side of the serisor to exclude the effects oE ey.tc-rn<:lly generaled electromagnetic signals.

   3.3. A serlsor assembly system comprising two pick-up sensor.-, as in claim 1 applied at a spaced separation over the surface and connected to a differcntial amplifier to obtain the difference in 1-he output signals fropi two locations em the surface with common mode noise rc-,jectien.

   11. A sensor assembly comprising multiple sensors each as i..ri claim 1 assembled on a carrier to locate- the pick-up clectrodes of each sensor in a fixed, preformated array.

   15- A --ensor assembly as in claim 1,4 wherein the carrier.Ls a piece of c.lothing that can be readily donned or removed with minimal inconvenience.

   16. A sensor a.,tsembly as in claim 14 c-o.rnhined with telemorilLoring means.

   1-7. A method of,,,t:-,nsirig an electj.-ic Eield present over a surface comprising:

   (1) presenting a pickup, ejectrede to confront surface and to establish a capacitive coupling to said surface and receive a signal based upon th(-, electric field emanating therefrom; (2) applying the signal. so received to a voltage 2 5 divider network which includes at one end the pj.c:k up electrode and at another ond an electrical coupling means connected to another portion of the 31 Sur f a ce over which an electrical potential difference exists, there being a high impedance amplifier wiLh an input capacitance connected there between; (3) maintaining the pickup electrode at a spaced separation from the confronted, field-emanating surface so that the overall effective capacitance between said surface and said amplifier has a value in the region of a plot of capacitance value versus separation distance wherein the percentage change in capacitance is less than 50 percent when subjected to a 0.1 mm change in He separaticn distance occurring between the pick-up electrode and the confronted surface whereby a signal is provided to the amplifier and the capacYtive coupling between the field-emanating surface and the pickup electrode varies insensitively with displacement of tqe electrode towards or away from said surface.

   18- A method as in clai,m 17 wherein the percentage change irt the capacitance is less than 20 when a 0.1 mat change in the separation distance occurs.

   19. A method as in claim 17 wLerein the pickup electrode has a surface confronting face that is provided with an ins ulative dielectric layer having a thickness such as to preclude the electrode from providing a capacitance value of over 40 picoFarads per contimeter squartd.

   20. A method as in claim 17 wherein the voltage divider network includes a series limiting cap,_-.icdtor between the 31 pickup electrode and the inpift to the amplfier, the pic-up electrode having a value of betweon S and 40 picoFarads.

   32