U.S. patent number 3,744,482 [Application Number 05/158,040] was granted by the patent office on 1973-07-10 for dry contact electrode with amplifier for physiological signals.
This patent grant is currently assigned to Hittman Associates, Inc.. Invention is credited to William M. Kaufman, Donald P. Powell.
United States Patent |
3,744,482 |
Kaufman , et al. |
July 10, 1973 |
DRY CONTACT ELECTRODE WITH AMPLIFIER FOR PHYSIOLOGICAL SIGNALS
Abstract
An electrode and amplification circuitry connected thereto
mounted within a housing for capacitively coupling physiologically
produced potential along the skin surface of the body to the
amplification circuitry without undesirable direct current shifts
in potential and with rapid recovery from saturation due to a
transient by means of a Zener diode or leak-resistor circuit
arrangement in the amplification circuitry.
Inventors: |
Kaufman; William M. (Chevy
Chase, MD), Powell; Donald P. (Baltimore, MD) |
Assignee: |
Hittman Associates, Inc.
(Columbia, MD)
|
Family
ID: |
22566462 |
Appl.
No.: |
05/158,040 |
Filed: |
June 29, 1971 |
Current U.S.
Class: |
600/372 |
Current CPC
Class: |
A61B
5/30 (20210101); A61B 5/302 (20210101) |
Current International
Class: |
A61B
5/04 (20060101); A61B 5/0402 (20060101); A61B
5/0428 (20060101); A61b 005/04 () |
Field of
Search: |
;128/2.6B,2.6E,2.6R,2.1E,DIG.4 ;330/109 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Claims
I claim:
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.
Description
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.
DETAILED DESCRIPTION
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 C.sub.c and cup 12, which is
preferably of stainless steel, serves as the other plate of C.sub.c
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 C.sub.c
and a capacitor C.sub.f in the feedback loop. The resistance
R.sub.f in the feedback loop sets the low frequency cut-off point
of the pass band F.sub.1 which is:
f.sub.L = 1/2.pi.R.sub.F C.sub.F.
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
10.sup.5, input impedance on the order of 10.sup.9 ohms or greater,
and output impedance on the order of 10.sup.2 or 10.sup.3 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:
E.sub.2 /E.sub.1 = j2.pi.f R.sub.F C.sub.F /1 + j2.pi.f R.sub.F
C.sub.F
using conventional electrical engineering notation and terminology.
The amplitude of the frequency response is:
.vertline.E.sub.2 /E.sub.1 .vertline.= 2.pi.f R.sub.F C.sub.F /1 +
(2 .pi.f R.sub.F C.sub.F).sup.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 R.sub.F is needed. A
coupling film of commercially available insulating material (e.g.,
0.00025 inch Mylar) has approximately 2.5 .times. 10.sup..sup.-3
.mu.f 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.sup..sup.-3 .mu.f capacitance. The corresponding value of
R.sub.f for f.sub.L = 0.1 Hz is R.sub.f = 1.6 .times. 10.sup.9
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 10.sup.5, then
.+-.90.mu.V at the summing point will saturate the amplifier. Use
of Zener diodes as back-to-back diodes Dz making up R.sub.F 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 R.sub.1 to the circuit of FIG. 7. In this
circuit R.sub.1 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
R.sub.1 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.
* * * * *