Kaufman, William M. (Chevy Chase, MD)
Powell, Donald P. (Baltimore, MD)
1. A dry contact electrode for receiving physiological signals from the surface of the skin, comprising a casing, conducting means secured to said casing and adapted to form one plate of an input capacitor, amplifying means housed within said conducting means, coupling means for insulating said conducting means from the surface of the skin and adapted to physically connect the surface of the skin to an input of said amplifying means through said conducting means so that the skin forms the other plate of said input capacitor, said coupling means comprising inert material which eliminates skin irritation and other medical complications arising from contact with the skin surface, wherein said amplifying means has feedback means comprising a feedback capacitor and a pair of diodes connected back-to-back in parallel with said feedback capacitor for providing high capacitive reactance and limiting leakage, and an output of said amplifier means from which amplified physiological signals can be extracted.
2. The electrode of claim 1, wherein said coupling means includes an insulating film covering the outside surface of said conducting means.
3. The electrode of claim 2, including retaining means adjacent said casing and physically engaging said insulating film for removably mounting said insulating film over the outside surface of said conducting means.
4. The electrode of claim 1, including resistive means connected across the input of said amplifying means for leaking off charge at the input of said amplifying means when going into saturation.
5. The electrode of claim 1, wherein said pair of diodes breakdown to a low resistance level at a predetermined voltage level hastening the decay of a transient voltage allowing rapid recovery from saturation.
6. The electrode of claim 1, wherein said diodes are Zener diodes.
7. A dry contact electrode for receiving physiological signals from the surface of the skin of a patient, the electrode comprising: a main casing; conducting means supported by said main casing for defining one plate of an input capacitor; insulating means associating with said conducting means for insulating said conducting means from the skin of a patient in such a manner that when said dry contact electrode is in contact with the skin of the patient, the skin defines the other plate of said input capacitor; amplifying means housed by said main casing; circuit means for connecting said input capacitor to said amplifying means; output means for transmitting the output signal developed by said amplifying means to a load; feedback capacitance connected between an input and an output of said amplifying means; and a pair of feedback diodes connected back-to-back in parallel across said feedback capacitance.
8. The electrode of claim 7, and further comprising resistor means connected across the input of said amplifying means.
9. The electrode of claim 7, wherein said diodes are Zener diodes.
10. The electrode of claim 7, wherein said insulating means takes the form of a film covering the outside surface of said conducting means.
11. The electrode of claim 10, and further comprising retaining means for removably mounting said insulating film over the outside surface of said conducting means.
12. A dry contact electrode for receiving physiological signals from the surface of the skin of a patient, the electrode comprising: a main casing; conducting means supported by said main casing for contacting the surface of the skin of the patient; amplifying means housed by said main casing; capacitor means connected between said conducting means and an input of said amplifying means for transmitting physiological signals to said amplifying means; output means for transmitting the output signals developed by said amplifying means to a load; feedback capacitance connected between the input and the output of said amplifying means; and a pair of feedback diodes connected back-to-back in parallel across said feedback capacitance.
BACKGROUND OF THE INVENTION
The measurement of physiologically generated electrical potentials on the surface of the body is common in medical practice and in research. Two examples are electrocardiography (ECG) and electroencephalography (EEG), both of which are frequently employed diagnostically. Typically, electrodes are placed at various locations on the surface of the body and the voltage between selected pairs of electrodes is measured (usually recorded) as a function of time.
If the electrodes in contact with the body are metallic conductors, various electrochemically induced electrical potentials can appear between the metal and the skin, for example, because of perspiration. This problem of contact noise with metallic electrodes necessitated the development of special conductive pastes in combination with specific metals that would minimize noise generation at the electrode contact. The most popular combination of this sort consists of a silver metal contact and a concentrated silver chloride aqueous solution in the form of a conductive paste.
Although the use of electrode paste minimizes the problem of contact noise, there are several problems associated with the application of paste electrodes for long-term monitoring. When paste contact electrodes are used for many hours or several days continuously, skin irritation is a common problem. The continuous contact of the concentrated salt solution on the skin is not fully acceptable and puffy irritated welts may arise. As the paste dries under the metal electrode, contact noise is created which can become so severe that the electrodes must be removed, cleaned and repositioned on the body in order to obtain an acceptable signal-to-noise ratio.
For these reasons, investigators have been pursuing the development of capacitively coupled electrodes. With typical capacitive coupling to the body, the skin is in contact with a stable insulating material, such as a metallic oxide, which is relatively chemically inert and non-irritating. Such an electrode does not depend upon electrical conduction; therefore, the conductivity of the horny layer of skin and the presence or absence of perspiration will not affect signal quality. Since the electrical impedance of a capacitive coupling increases with decreasing frequency and since the frequency band of interest for most biological signals is very low (from as low as fractions of one hertz), it is necessary to provide an amplifier circuit with a very high input impedance. Previous investigators have described very high input resistance dc amplifiers for this application and have mounted these amplifiers in close proximity to the coupling insulator to minimize pickup of electromagnetic interference.
Unfortunately, zero signal stability of dc amplifiers is a major problem area. Induction of a small charge on the control electrode of the initial amplifying circuit element due to an input transient or a leakage current can cause the dc amplifier to shift its quiescent operating point significantly. These effects make a dc amplifier somewhat undesirable for circuit applications that do not require dc response capability. The pass bands that are used for the recording of the EGC and the EEG include low frequency components but do not include dc. Therefore, an ac amplifier could be suitable for these applications if properly designed and constructed for compatibility therewith.
SUMMARY OF THE INVENTION
This invention relates to ac coupled electrodes for the acquisition of electrical signals, particularly those potentials generated physiologically which are normally measured at the surface of the body.
A primary object of the invention is the provision of dry contact means involving a relatively inert material touching the skin thereby preventing irritation and other medical complications at the skin surface.
Another object of this invention is the elimination of electrical conduction with the skin so that the conductivity of the horny layer of skin and the presence or absence of perspiration will not affect signal quality and so that direct current shifts in potential are avoided due to capacitive coupling.
Still another object of this invention is the provision of means facilitating replacement of the coupling film.
A still further object of the invention is the provision of non-linear circuit motion action and high capacitive reactance to respectively allow rapid recovery from saturation due to a transient and enhance patient safety by limiting 60 Hz leakage.
BRIEF DESCRIPTION OF THE DRAWING
The above and other objects of this invention will become apparent to those skilled in the art after a detailed description of preferred embodiments of this invention taken together with the accompanying drawing wherein:
FIG. 1 is a side elevational view of an insulated electrode;
FIG. 2 is a plan view thereof;
FIG. 3 is a side elevational view of an embodiment of the electrode;
FIG. 4 is a plan view of the embodiment;
FIG. 5 is schematic view of the capacitance coupling portion of the electrode shown in FIGS. 1 and 2;
FIG. 6 is a schematic view of the capacitance coupling portion of the electrode shown in FIGS. 3 and 4;
FIG. 7 is a schematic view of the electrode circuitry; and
FIG. 8 is a schematic view of an embodiment of the electrode circuitry.
Referring in detail to the drawing, there is shown in FIGS. 1 and 2 an electronic amplifier 10 housed within a cup 12 formed of conducting material and extending below the bottom surface of a molded casing 14 formed of insulating material. An insulating film 16 is removably positioned over the exposed cylindrical wall and bottom surface of cup 12 by means of a retainer ring 18. An electrical cable 20 containing power and signal leads connects amplifier 10 to a power supply and coupling filters (not shown) so that the electrode can be used to transmit the physiological signal to an instrument for visual display or recording. The molding compound used for making cable 20, casing 14, and cup 12 into a compact signal unit is an insulated material such as epoxy or acrylic resins.
As shown in FIG. 5, insulating film 16 along the bottom surface of cup 12 is placed in contact with the skin 30 of the subject being measured. The skin 30, being electrically conductive, serves as one of the plates of a coupling capacitor Cc and cup 12, which is preferably of stainless steel, serves as the other plate of Cc with film 16 being the dielectric medium.
A second method of capacitively coupling the physiological signal is to eliminate the use of insulating film 16. Cup 12 is housed directly within casing 14 in a manner so that the bottom surface of cup 12 is flush with the bottom surface of casing 14 for exposure to the skin as clearly illustrated in FIG. 3. As shown in FIG. 6, the skin 30 and the bottom of conducting cup 12 form a terminal point 32 which is connected to one terminal 34 of a conventional capacitor C which couples the signal to the remainder of the electrode circuit.
The basic electronic circuit for the electrode employs an operational amplifier A with an input coupling capacitor Cc and a capacitor Cf in the feedback loop. The resistance Rf in the feedback loop sets the low frequency cut-off point of the pass band F1 which is:
fL = 1/2πRF CF.
Operational amplifier A is a very high gain inverting amplifier with very high input impedance and very low output impedance. In the idealized limit, the amplifier gain and input impedance are infinite and output impedance is zero. Micro-circuit operational amplifiers are commercially available with gains on the order of 105, input impedance on the order of 109 ohms or greater, and output impedance on the order of 102 or 103 ohms. A good example of such a commercially available device is the Amelco 2741 operational amplifier. Using the idealization defined above, the circuit response of amplifier A is:
E2 /E1 = j2πf RF CF /1 + j2πf RF CF
using conventional electrical engineering notation and terminology. The amplitude of the frequency response is:
│E2 /E1 │= 2πf RF CF /1 + (2 πf RF CF)2
which is a "high pass" response. The upper cutoff frequency is not apparent from this equation because of the idealized assumption for the operational amplifier A. Since all realizable operational amplifiers have an upper frequency limit to their amplification, this will provide an upper limit to the pass band of the electrode circuit. This upper limit is far beyond the frequency spectrum of physiological signals for typical commercial operational amplifiers.
Clinical quality ECG amplifiers must have a lower cutoff frequency of 0.1 Hz or lower. Since practical coupling capacitors must be relatively small, a very high value for RF is needed. A coupling film of commercially available insulating material (e.g., 0.00025 inch Mylar) has approximately 2.5 × 10-3 μf capacitance per square inch. Conventional conductive electrodes are on the order of 0.25 to 1.00 square inch in area. Therefore, a comparably sized coupling film will have approximately 10-3 μf capacitance. The corresponding value of Rf for fL = 0.1 Hz is Rf = 1.6 × 109 ohms. This is a very high resistance value not easily obtained with linear resistance material. It has been found in solving this dilemma that it is possible to use the reverse characteristics of a semiconductor junction diode to obtain resistors with this high level of resistance. FIG. 7 shows two diodes D connected "back-to-back" so that reverse characteristics dominate for current in either direction.
An improvement on this design is desirable because of the very low frequency response of the preamplifier. A low frequency response capability implies that the preamplifier will be sensitive to transient dc shifts. The decay time for recovery from a transient step input is longer, the lower the value of the low end cutoff frequency. A transient large enough to saturate the preamplifier and block signal transmission may last many seconds in a typical ECG monitoring application.
Clinical requirements are conflicting in that low frequency response capability is required for ECG, but a loss of ECG signal for more than a few seconds (typically 9 to 10 seconds) will strike an alarm. This problem can be obviated by means of non-linear circuit action.
A non-linear circuit action is needed that will prevent the input terminal of operational amplifier A (the summing point) from drifing either positively or negatively in voltage beyond very narrowly defined limits. For example, if the amplifier power supply is ±9V and if the operational amplifier gain is 105, then ±90μV at the summing point will saturate the amplifier. Use of Zener diodes as back-to-back diodes Dz making up RF will provide saturation protection as shown in FIG. 8. If the Zener voltage is considerably less than the voltage of the power supply, then, as the amplifier output voltage increases in magnitude beyond the Zener voltage level, diodes Dz begin to pass current readily thereby reducing the feedback resistance and hastening the decay of the transient voltage. Once the output falls below the Zener voltage level, then amplifier A resumes linear operation.
Another means of obtaining this non-linear action is to add an input leak resistor R1 to the circuit of FIG. 7. In this circuit R1 is a resistor whose value has been specially selected to be large enough so as not to affect the low end frequency response and small enough to leak off the charge at the input once amplifier A goes into saturation. In the normal linear mode of operation, the effective value of any input leak resistance is multiplied by the operational amplifier gain. When the amplifier saturates, the normal high gain drops off and input resistor R1 permits a relatively rapid discharge of the potential at the summing point.
While preferred embodiments of this invention have been illustrated and described, it should be understood by those skilled in the art that many changes and modifications may be resorted to without departing from the spirit and scope of this invention.