Multichannel electrical stimulator with improved channel isolation

An electrical stimulator having a plurality of channels adapted to stimulate bodily tissue with an electrical current on each of the plurality of channels. First and second electrodes are coupled to each of the plurality of channels and adapted to be coupled to the bodily tissue for passing electrical current through the bodily tissue. A first current source and a second current source is supplied for each of the plurality of channels. The first current source being coupled to the first electrode and the second current source being coupled to the second electrode. First and second current sources operate in concert. The value of the current supplied being equal in magnitude and being oppositely oriented for each of the first and second current sources for each of the plurality of channels at any given instant of time. In one embodiment, one of the current sources for one of the electrodes in one of the plurality of channels may be eliminated.

BACKGROUND OF THE INVENTION 
The present invention relates to an electrical stimulator adapted to 
stimulate bodily tissue and more particularly to an electrical stimulator 
having a plurality of stimulation channels adapted to stimulate bodily 
tissue. 
Electrical stimulators adapted to stimulate bodily tissue are well known. 
Examples of such electrical stimulators include cochlear implants and 
transcutaneous electrical nerve stimulators. A cochlear implant supplies 
an electrical current to electrically stimulate the auditory nerve in 
order to simulate hearing in an otherwise deaf individual. Transcutaneous 
electrical nerve stimulators (TENS) are utilized for pain control or for 
controlled muscle activation. In both the cochlear implant and the TENS 
stimulators, a pair of electrodes are attached to the bodily tissue to be 
stimulated. Electrical current is then supplied to this electrode pair to 
provide a stimulation current between the electrodes which passes through 
the bodily tissue to be stimulated. This electrical current in the bodily 
tissue stimulates the appropriate nerves, i.e., the auditory nerve for the 
cochlear implant and pain bearing nerves for the TENS, to achieve the 
desired function, i.e., simulated hearing or alleviation of pain, 
respectively. 
In certain situations, it is desirable to have an electrical stimulator 
which has a plurality of channels. The plurality of channels may be 
designed to provide more than one type of information to the bodily tissue 
to be stimulated. With a cochlear implant, a plurality of channels may 
supply different types of information. As an example, one channel may 
provide information about a specific frequency range and a second channel 
may provide information about a different frequency range. This type of 
cochlear implant is designed to take advantage of the frequency place 
value relative to a position within the cochlea. As an example, a TENS 
stimulator may control different nerves and hence different muscles with 
different channels. 
Usually the theory of operation of such multichannel electrical stimulators 
is that each channel of the stimulator is completely independent of the 
others. In practice this may not be the case. The effectiveness of 
multichannel operation of electrical stimulators is impeded by diversion 
of current intended to pass between electrodes of one electrode pair to 
the electrode or electrodes of another electrode pair. Electrical current 
which is intended to pass between one electrode pair may be diverted to 
another electrode pair by the conduction of the bodily tissue. In general, 
attempts to control the interaction between the electrode pairs is 
accomplished by the physical spacing of the electrode pairs. However, 
spacing between electrode pairs cannot always be controlled. With a 
cochlear implant, for example, pairs of electrodes must be rather closely 
spaced to enable placement of more than one electrode pair within the 
cochlea. 
In essence, the effect is that a plurality of channels of stimulation in an 
electrical stimulator are not completely isolated from each other. That 
is, the stimulation of one electrode pair has an effect upon the 
stimulation of another electrode pair. Thus, less than ideal multichannel 
operation is achieved. The theoretical result of multichannel stimulation 
is significantly compromised. 
Another mechanism which has been used be achieve the isolation of multiple 
channels in a multichannel stimulator is to electrically isolate the 
output stages of the stimulator. This, however, requires rather complex 
circuitry, with attendant increased cost and decreased reliability, and, 
in the case where the current is being inductively coupled from external 
transmitter to an implanted electrode pair as in a typical cochlear 
implant, a plurality of receiving coils. 
SUMMARY OF THE INVENTION 
The present invention significantly reduces the detrimental interaction of 
one channel of the electrical stimulator by another channel of the 
electrical stimulator. 
In a conventional electrical stimulator, each channel has a current source 
which drives current between one electrode and electrical ground, which be 
common to electrodes of multiple channels. However, a single current 
source for each channel requires only that the current in the loop 
containing the electrode and electrical ground be equal to the value of 
the current source and not necessarily that the current passing to ground 
pass through any particular one of electrodes coupled to electrical 
ground. If the electrical power supplies of the channels of the electrical 
stimulator are not electrically isolated, i.e., if two or more electrodes 
are connected to the same electrical ground, a cross current from one 
electrode pair to another electrode pair could occur while still 
maintaining the loop current requirements of the single current source 
(for each channel). 
The present invention provides improved channel isolation without requiring 
electrically isolated power supplies. Each channel of the electrical 
stimulator has a pair of electrodes. Each channel of the electrical 
stimulator has a pair of current sources (in one embodiment one channel 
may have a single current source) which work in concert with each other. 
One current source is coupled to each electrode of the electrode pair. The 
current sources operate harmoniously, i.e., when one current source is 
sourcing (or sinking) a certain current (at a given instant of time) its 
complementary current source is sinking (or sourcing) a substantially 
identical current in magnitude to certain current. This being the case, 
the current passing between the electrode pair is substantially equal to 
the desired amount of current. Current leak between electrode pairs is 
minimized and channel isolation is significantly improved. 
In one embodiment, one electrode may be left without a separate current 
source. If all other electrodes sink/source the proper amount of current, 
the only current left for the final electrode is the proper amount, thus, 
one current source may be saved. In this embodiment, for a two channel 
electrical stimulator, three current sources for the four electrodes (two 
electrode pairs) would be required. Similarly, for a three channel 
electrical stimulator, five current sources for six electrodes (three 
electrode pairs) would be required. 
In summary, the present invention provides an electrical stimulator having 
a plurality of channels having an electrical common. The stimulator is 
adapted to stimulate bodily tissue with an electrical current on each of 
the plurality of channels. A first electrode and a second electrode for 
each of the plurality of channels are provided. The first electrode and 
the second electrode are adapted to be coupled to the bodily tissue for 
passing the electrical current through the tissue between the first 
electrode and the second electrode. A first current source and a second 
current source for each of the plurality of channels are provided. The 
first current source is coupled between electrical common and the first 
electrode. The second current source is coupled between the second 
electrode and the electrical common. The first current source is 
substantially equal in magnitude to the second current source and is 
oppositely oriented for each of the plurality of channels at a given 
instant of time. Constructed in this manner the first electrode and the 
second electrode source and/or sink substantially identical currents, 
thus, achieving significantly improved isolation of stimuli between the 
plurality of channels. 
In one embodiment of the present invention, an electrical stimulator is 
provided having a plurality of channels having an electrical common. The 
stimulator is adapted to stimulate tissue with an electrical current on 
each of the plurality of channels. A first electrode and a second 
electrode for each of the plurality of channels are provided. The first 
electrode and the second electrode are adapted to be coupled to the tissue 
for passing the electrical current through the tissue between the first 
electrode and the second electrode. A first current source and a second 
current source for all but one of the plurality of channels are provided. 
The first current source is coupled between electrical common and the 
first electrode and the second current source are coupled between the 
second electrode and the electrical common. At a given instant of time and 
for each of all but one of the plurality of channels, the first current 
source is substantially equal in magnitude to the second current source 
and is oppositely oriented. The electrical stimulator also provides a 
current source for the one remaining of the plurality of channels being 
coupled between the electrical common and the first electrode and the 
second electrode being coupled directly to electrical common. Constructed 
in this manner the first electrode and the second electrode sink and/or 
source substantially identical currents achieving significantly improved 
isolation of stimulus between the plurality of channels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is illustrative of a prior art electrical stimulator 10. This 
electrical stimulator 10 is shown with two separate stimulation channels 
12 and 14. Stimulation channel 12 has a current source 16 coupled to 
electrodes 18 and 20. Electrodes 18 and 20 serve as an electrode pair for 
stimulation channel 12. Electrode pair (18, 20) is adapted to be coupled 
to bodily tissue 22 which is to be stimulated by the electrical stimulator 
10. Current source 16 supplies the desired amount of stimulating current 
for the stimulation channel 12 which is to be applied to the bodily tissue 
22 via electrode pair 18, 20. Similarly, stimulation channel 14 has a 
current source 24 coupled to electrodes 26 and 28 which serve as the 
electrode pair for stimulation channel 14. Electrode pair 26, 28 is also 
adapted to be coupled to bodily tissue 22. Current source 16 and current 
source 24 have a common electrical ground. Current source 24 supplies the 
desired amount of stimulation current for stimulation channel 14 which is 
to be applied via electrode pair 26, 28 to bodily tissue 22. Shown for 
convenience in the diagram of FIG. 1, current source 16 contains an arrow 
indicating that the momentary current supplied by current source 16 is in 
the upward direction. Similarly, current source 24 also contains an upward 
arrow indicating the same instantaneous direction of current flowing in 
stimulation channel 14. It is to be recognized and understood, however, 
that the arrow in current sources 16 and 24 are for convenience and that 
current sources 16 and 24 may provide steady state alternating or other 
instantaneous current supply waveforms and may or may not be equal at any 
given instant of time. That is, current source 16 is completely 
independent from current source 24. Current source 16 supplies current to 
electrode 18. That current then is designed to theoretically pass through 
bodily tissue 22 and be returned to current source 16 from electrode 20. 
Similarly, for stimulation channel 14 current source 24 supplies current 
to electrode 26. In theory of operation, that current should pass through 
bodily tissue 22 and be returned to current source 24 from electrode 28. 
In practice, it has been found that with electrical stimulator 10, that a 
portion of the current from current source 16 of channel 12 of electrical 
stimulator 10 passes from electrode 18 to electrode 20 as theoretically 
designed. This current is indicated by arrow 30. However, a portion of the 
current passing from electrode 18 may be diverted to electrode 28, since 
electrode 20 and electrode 28 are coupled to the same electrical ground. 
This current is indicated by arrow 31. For channel 14 of the electrical 
stimulator 10, current from current source 24 passes from electrode 26 
into bodily tissue 22. Now, however, there are two potential paths for 
that current to flow. A portion of the current will flow, as designed, 
directly to electrode 28 to be returned to current source 24. This current 
is illustrated in the diagram by arrow 32. However, in practice, electrode 
20 competes for the current being supplied by electrode 26. Thus, some of 
the current supplied by electrode 26 does not pass to electrode 28 but 
rather is diverted, as i11ustrated by arrow 34, to electrode 20 instead. 
The result is a lack of isolation between channel 12 and channel 14 of 
electrica1 stimulator 10. Electrode 20 will receive more electrical 
current from bodily tissue 22 than desired while electrode 28 will receive 
less. The current illustrated by arrow 30 will be greater than designed 
while the current illustrated by arrow 32 will be less than designed. 
Thus, channel 14 of electrical stimulator 10 has an effect upon channel 12 
of electrical stimulator 10. This results in application of electrical 
currents to bodily tissue 22 which are not as theoretically designed and, 
thus, the results achieved will be less than theoretially designed and the 
results achieved may be significantly impaired. 
FIG. 2 illustrates one embodiment of the electrical stimulator 36 of the 
present invention. Electrical stimulator 36 also has two stimulation 
channels 38 and 40. Stimulation channel 38 has current source 42 and 
current source 44 coupled on either side of electrical common 46. Current 
source 42 is also coupled to electrode 48 while current source 44 is 
coupled to electrode 50. Electrode 48 and electrode 50 serve as the 
electrode pair for stimulation channel 38. Electrodes 48 and 50 are 
adapted to be coupled to bodily tissue 22 which is to be stimulated by the 
electrical stimulator 36. Similarly, stimulation channel 40 has current 
source 52 and current source 54 coupled on either side of electrical 
common 56. Current source 52 is also coupled to electrode 58 while current 
source 54 is also coupled to electrode 60. Electrodes 58 and 60 function 
as the electrode pair for stimulation channel 40. Electrodes 58 and 60 are 
adapted to be coupled to bodily tissue 22 which is to be stimulated by 
stimulation channel 40 of electrical stimulator 36. 
Current source 42 and current source 44 are constructed to operate in 
concert with each other. At any given instant of time for stimulation 
channel 38, the current flowing through current source 42 should be 
substantially equal to the current flowing through current source 44. 
Similarly, for stimulation channel 40 current source 52 and current source 
54 operate in concert. At any given instant in time for stimulation 
channel 40, the current flowing through current source 52 will be 
substantially equal to the current flowing through current source 54. That 
is, if current source 52 is sourcing a certain amount of current to 
electrode 58 then current source 54 will be arranged to sink that same 
certain amount of current from electrode 60. Current source 42 and current 
source 44 are arranged to operate on substantially identical currents. 
Similarly, current source 52 and current source 54 are arranged to operate 
on substantially identical currents. It is to be recognized and 
understood, however, that exact identicality between electronic circuits 
is extremely unlikely. Therefore, substantially identical currents refers 
to currents which are designed to be equal and are equal within the realm 
of reasonable circuit design constraints and practicality in component 
value variations. 
With a current source associated with each electrode, the electrical 
stimulator 36 of FIG. 2 achieves a much greater channel isolation in its 
operation in conjunction with bodily tissue 22. Current source 42 sources 
(or sinks) a given amount of current to electrode 48. That same amount of 
current is sinked (or sourced) into electrode 50 by current source 44. 
Therefore, the current illustrated by arrow 62 represents the current 
sourced (or sinked) by current source 42 and the current sinked (or 
sourced) by current source 44. Similarly, for stimulation channel 40, 
electrode 58 sources (or sinks) the exact amount of current supplied by 
current source 52. Electrode 60 sinks (or sources) the exact amount of 
current determined by current source 54. Therefore, the current flowing 
between electrodes 58 and 60 represented in the diagram by arrow 64 is 
substantially that current determined by current source 52 and current 
source 54. The result is that electrical stimulator 36 has significantly 
greater isolation between stimulation channel 38 and stimulation channel 
40. There is significantly less contamination of the current flowing in 
bodily tissue 22 between electrode pairs 58 and 60 due to the physical 
proximity of electrode pairs 48 and 50. 
FIG. 3 illustrates an alternative embodiment of the electrical stimulator 
36. In this embodiment, stimulation channel 38 is identical to stimulation 
channel 38 of electrical stimulator 36 of FIG. 2. Again, current source 42 
and current source 44 connected on either side of electrical common 46. 
Current source 42 is coupled to electrode 48 while current source 44 is 
coupled to electrode 50. Electrode 48 and electrode 50 are adapted to be 
coupled to bodily tissue 22 and produce current in bodily tissue 22 
illustrated by arrow 62. Stimulation channel 40 in electrical stimulator 
36 of FIG. 3 is similar to stimulation channel 40 of the electrical 
stimulator 36 of FIG. 2. Again, current source 52 is coupled on one side 
of electrical common 56 and is coupled to electrode 58. Electrode 58 and 
electrode 60 are adapted to be coupled to bodily tissue 22 to supply a 
stimulation current to bodily tissue 22 which is represented by arrow 64. 
The difference between stimulation channel 40 of FIG. 3 and stimulation 
channel 40 of FIG. 2 is that current source 54 is omitted in the 
embodiment illustrated in FIG. 3. Since the current sourced (or sinked) by 
electrode 48 is determined by current source 42 and since the current 
sinked (or sourced) by electrode 50 is determined by current source 44 and 
the current sourced (or sinked) by electrode 58 is determined by current 
source 52. The only current left to be sinked (or sourced) by electrode 60 
is that remaining current. Since current sources 42 and 44 are balanced, 
the current sourced (or sinked) by electrode 48 equals the current sinked 
(or sourced) by electrode 50. The remaining current available to be sinked 
(or sourced) by electrode 60 is that current which is sourced (or sinked) 
by electrode 58 as determined by current source 52. Thus, one current 
source, namely current source 54, can be omitted from the diagram and the 
isolation between channel 38 and channel 40 may still be achieved. 
FIG. 4 represents still another alternative embodiment of the electrical 
stimulator 36 of the present invention. In the electrical stimulator 36 
illustrated in FIG. 4, both stimulation channel 38 and stimulation channel 
40 have their outputs capacitively coupled to their respective electrode 
pairs 48, 50 and 58, 60. For stimulation channel 38, shunt resistances 66 
and 68 serve to carry the difference in current between non-exactly 
matched current sources 42 and 44. Capacitors 70, coupled between current 
source 42 and electrode 48, and capacitor 72, coupled between current 
source 44 in electrode 50 serve to capacitively couple stimulation channel 
38 to electrode pair 48, 50. Resistor 66 is coupled across current source 
42 and resistor 68 is coupled across current source 44. Similarly, for 
stimulation channel 40, resistor 74 is coupled across current source 52 
and resistor 76 is coupled across current source 44. Again, resistor 74 
and 76 serve to take up the mismatch, if any, between current source 52 
and current source 54. Capacitor 78, coupled between current source 52 and 
electrode 58 and capacitor 80, coupled between current source 54 and 
electrode 60, serve to capacitively couple stimulation channel 40 to 
electrode pair 58 and 60. 
FIG. 5 illustrates a detailed circuit diagram of a preferred embodiment of 
electrical stimulator 36. Again, electrical stimulator 36 consists of 
stimulation channel 38 and stimulation channel 40. Stimulation channel 38 
is adapted to be coupled to bodily tissue 22 with electrodes 48 and 50 
while stimulation channel 40 is adapted to be coupled to bodily tissue 22 
through electrodes 58 and electrode 60. Stimulation channel 38 is coupled 
to a voltage source at point 82. Operational amplifier 84 along with 
resistors 86, 88, 90 and 92 operate as current source 42. Resistor 86 is 
coupled between voltage source point 82 and the negative input to 
operational amplifier 84. Resistor 88 is coupled between the negative 
input to operational amplifier 84 and the output of operational amplifier 
84. Resistor 92 is coupled between the positive input to operational 
amplifier 84 and the output of operational amplifier 84. Resistor 90 is 
coupled between the positive input to operational amplifier 84 and to 
electrical common 46. Positive input to operational amplifier 84 is also 
coupled to electrode 48. The values for resistors 86, 88, 90 and 92 can be 
determined by making the value of resistor 92 divided by the value of 
resistor 90 equal to the value of resistor 86 divided by the value of 
resistor 88. The magnitude of the current supplied by this current source 
will be roughly equal to the value of the voltage source from voltage 
point 82 divided by the value of resistor 90. Current source 44 in this 
diagram is shown schematically consisting of operational amplifier 94, 
operational amplifier 96, resistors 98, resistor 100, resistor 102, 
resistor 104, resistor 106 and resistor 108. Resistor 106 is coupled 
between voltage source point 82 and the negative input to operational 
amplifier 96. Operational amplifier 96 serves to invert the voltage 
appearing at voltage point 82 to enable operational amplifier 94 to 
generate a current equal and opposite to the current generated by 
operational amplifier 84. The positive input to operational amplifier 96 
is coupled to electrical common 46. Resistor 108 is coupled between the 
negative input of operational amplifier 96 to the output of operational 
amplifier 96. Resistor 98 is coupled between the output of operational 
amplifier 96 and the negative input of operational amplifer 94. Resistor 
100 is coupled between the negative input to operational amplifier 94 in 
the output of operational amplifer 94. Resistor 104 is coupled between the 
positive input to operational amplifier 94 and the output of operational 
amplifier 94. Resistor 102 is coupled between the positive input to 
operational amplifier 94 into e1ectrical common. The positive input to 
operational amplifier 94 is also coupled to electrode 50. 
The value of resistor 104 divided by the value of resistor 102 should equal 
the value of resistor 98 divided by the value of resistor 100. The 
magnitude of the current supplied by this current source is roughly equal 
to the value supplied by voltage point 82 divided by the value of resistor 
102. The value of resistor 106 and the value of resistor 108 should also 
be equal. Operational amplifier 96 and resistors 106 and 108 serve to 
invert the voltage appearing at voltage point 82. The value of resistor 86 
and value of resistor 98 should match as should the value of resistor 88 
and the value of resistor 100, the value of resistor 90 and the value of 
resistor 102, and the value of resistor 92 and the value of resistor 104. 
Similarly, operational amplifier 84 and operational amplifier 94 should 
also be matched. In one preferred embodiment, operational amplifiers 84 
and 94 are Model No. 741 operational amplifiers obtained from suppliers 
such as Texas Instruments and National Semiconductor. In a preferred 
embodiment, the voltage appearing at voltage point 82 would vary 
between-10 volts and +10 volts and the value of all resistors 86, 88, 90, 
92, 98, 100, 102, 104, 106 and 108 would be 2 kilohms. 
The electrical schematic for stimulation channel 40 is identical to that 
previously described for stimulation channel 38. Voltage point 110 serves 
to supply stimulation channel 40. Current source consisting of operational 
amplifiers 112, resistors 114, 116, 118 and 120 are coupled to electrode 
58. Operational amplifier 122 resistors 124 and 126 serve to invert the 
voltage appearing at voltage point 110. The other current source 
consisting of operational amplifier 128 and resistors 130, 132, 134 and 
136 are coupled to electrode 60. The same constraints or the values of 
these components of stimulation channel forty are identical to those 
constraints for stimulation channel 38. It is to be recognized and 
understood, however, that the component values between electrical 
stimulation channel 38 and electrical stimulation channel 40 may not be 
equal. The component values just need to be consistent within one of the 
stimulation channels 38 or 40. It is also to be recognized and understood, 
of course, that even if the component values between electrical 
stimulation channel 38 and electrical stimulation channel 40 are equal 
that the stimulation currents supplied by electrode pairs 48 and 50 and 58 
and 60, respectively, at any given instant in time need not be identical. 
The stimulation current may be varied according to the voltage source 
appearing at voltage points 82 and 110, respectively. 
Thus, there has been shown and described a novel electrical stimulator. It 
is to be recognized and understood, however, that various changes, 
substitutions and modifications in the form and the details of the 
described invention may be made by those skilled in the art without 
departing from the scope of the following claims.