Abstract:
A method and system of providing therapy to a patient using electrodes implanted adjacent tissue. The method comprises regulating a first voltage at an anode of the electrodes relative to the tissue, regulating a second voltage at a cathode of the electrodes relative to the tissue, and conveying electrical stimulation energy between the anode at the first voltage and the cathode at the second voltage, thereby stimulating the neural tissue. The system comprises a grounding electrode configured for being placed in contact with the tissue, electrical terminals configured for being respectively coupled to the electrodes, a first regulator configured for being electrically coupled between an anode of the electrodes and the grounding electrode, a second regulator configured for being electrically coupled between an anode of the electrodes and the grounding electrode, and control circuitry configured for controlling the regulators to convey electrical stimulation energy between the anode and cathode.

Description:
RELATED APPLICATION DATA 
     The present application is a continuation of U.S. patent application Ser. No. 12/821,043, filed Jun. 22, 2010, now U.S. Pat. No. 8,694,122, which claims the benefit under 35 U.S.C. §119 to U.S. Provisional Patent Application Ser. No. 61/220,131, filed Jun. 24, 2009, which applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to tissue stimulation systems. 
     BACKGROUND OF THE INVENTION 
     Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients. 
     Each of these implantable neurostimulation systems typically includes an electrode lead implanted at the desired stimulation site and an implantable pulse generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the electrode lead or indirectly to the electrode lead via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. A typical stimulation parameter set may include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation current at any given time, as well as the amplitude, duration, rate, and burst rate of the stimulation pulses. 
     The neurostimulation system may further comprise a handheld remote control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician&#39;s Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon. 
     Electrical stimulation energy may be delivered from the neurostimulator to the electrodes using one or more current-controlled sources for providing stimulation pulses of a specified and known current (i.e., current regulated output pulses), or one or more voltage-controlled sources for providing stimulation pulses of a specified and known voltage (i.e., voltage regulated output pulses). The circuitry of the neurostimulator may also include voltage converters, power regulators, output coupling capacitors, and other elements as needed to produce constant voltage or constant current stimulus pulses. Conventional battery-operated neurostimulators typically apply stimulation pulses to the tissue that are referenced to an internal circuit voltage in the neurostimulator, with a relatively low impedance connection being located between one or more stimulation electrodes and internal circuitry. This relatively low impedance effectively clamps the voltage on these stimulation electrodes to the internal circuit voltage. 
     For example, a voltage source can be coupled between the internal circuitry and an anode to create a cathode clamped voltage regulated circuit ( FIG. 1   a ), a current source can be coupled between the internal circuitry and an anode to create a cathode clamped current regulated circuit ( FIG. 1   b ), a voltage source can be coupled between the internal circuitry and a cathode to create an anode clamped voltage regulated circuit ( FIG. 1   c ), and a current source can be coupled between the internal circuitry and a cathode to create an anode clamped current regulated circuit ( FIG. 1   d ). It can be appreciated that the reference voltage will be at the cathodes for the topologies illustrated in  FIGS. 1   a  and  1   b  and will be at the anodes for the topologies illustrated in  FIGS. 1   c  and  1   d.    
     Because the voltage at the unregulated side of the electrode will be clamped to the voltage of the internal circuitry, and because the stimulation output circuitry may be unbalanced in that some components in the circuitry (coupling capacitors, protection circuits, etc.) may be present on the cathode side of the circuit but not the anode side of the circuit, or vice versa, the output stimulation circuitry between the cathode and the anode will be asymmetrical, such that the cathode and the anode will be asymmetrically referenced to the internal circuit. For example, a shift in voltage in the output stimulation circuit results in asymmetrical voltage shifts between the anodes and cathodes. 
     In particular, the voltage of the common mode signal (i.e., the average of the anode voltage shift and cathode voltage shift relative to the reference voltage) will be equal to or greater than the differential voltage between the cathode and anode. For example, as shown in  FIG. 2   a , when the cathode voltage is at the internal reference voltage, the common mode signal is equal to one-half the differential voltage between the cathode and anode. As shown in  FIG. 2   b , when the cathode voltage is above the internal reference voltage, the voltage of the common mode signal is greater than one-half the differential voltage between the cathode and anode. As shown in  FIG. 2   c , when the cathode voltage is below the internal reference voltage, the voltage of the common mode signal is likewise greater than one-half the differential voltage between the cathode and anode. The asymmetry between anodes and cathodes in the output stimulation circuitry may be associated with undesired side effects during stimulation that lead to reduced patient comfort. In particular, parasitic coupling of the common mode signal to the implantable device can give rise to an additional stimulation signal that is superimposed on the differential stimulation signal. Even if the common mode signal is subthreshold by itself, it can modulate the differential stimulation signal, causing unwanted activation of neural tissue. 
     In addition to the problem of asymmetry in the output stimulation circuit, referencing the voltage at the cathodes and anodes to an internal circuit may require excessive voltage levels at the cathodes and anodes in order to maintain the desired voltage potential therebetween. For example, if the desired voltage potential between a cathode and an anode is 5V, and if the internal voltage is 20V, the voltage at the anode would have to be 25V and the voltage at the cathode would have to be 20V. The increased voltage at the electrodes will increase the voltage relative to the tissue, which may cause problems such as unwanted stimulation and even electro-chemical reactions resulting in corrosion of the electrodes. 
     There, thus, remains a need for an improved method and system for conveying stimulation to tissue in a controlled manner. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present inventions, a method of providing therapy to a patient using an array of electrodes implanted adjacent neural tissue (e.g., spinal cord tissue) of the patient is provided. The method comprises regulating a first voltage at an anode of the electrodes relative to the neural tissue, regulating a second voltage at a cathode of the electrodes relative to the tissue, and conveying electrical stimulation energy between the anode at the first voltage and the cathode at the second voltage, thereby stimulating the neural tissue. In one method, the voltages on the anode and cathode are regulated in a balanced fashion, such that an average shift in voltage on the anode and cathode relative to the neural tissue is equal to or less than one half a differential voltage between the anode and cathode. Optimally, the voltage shifts at the anode and cathode relative to the neural tissue may be equal in magnitude, but opposite in polarity (i.e., anode voltage shifts up and cathode voltage shifts down by the amount). An optional method comprises regulating a first current flowing through the anode, and regulating a second current flowing through the cathode. Furthermore, the values for the first current and the second current necessary to achieve the first and second voltages may be computed. 
     In accordance with a second aspect of the present inventions, a neurostimulation system is provided. The neurostimulation system comprises a grounding electrode configured for being placed in contact with neural tissue, and a plurality of electrical terminals configured for being respectively coupled to an array of electrodes. The neurostimulation system further comprises a first regulator configured for being electrically coupled between an anode of the electrodes and the grounding electrode, a second regulator configured for being electrically coupled between an anode of the electrodes and the grounding electrode, and control circuitry configured for controlling the first and second regulators to convey electrical stimulation energy between the anode and the cathode. 
     In one embodiment, each of the first and second regulators comprises a voltage source. In another embodiment, each of the first and second regulators comprises a current source. In the latter case, the control circuitry is configured for controlling the current sources to output the same current value and/or the control circuitry may be further configured for determining values for the first current and the second current necessary to achieve the first and second voltages. In another embodiment, the control circuitry may be configured for controlling the regulators such that an average shift in voltage on the anode and cathode relative to the neural tissue is equal to or less than one half a differential voltage between the anode and cathode. Optimally, the voltage shifts at the anode and cathode relative to the neural tissue may be equal in magnitude, but opposite in polarity (i.e., anode voltage shifts up and cathode voltage shifts down by the amount). The neurostimulation system may comprise a housing containing the plurality of electrical terminals, first and second voltage regulators, and control circuitry. 
     Other and further aspects and features of the invention will be evident from reading the following detailed description of the preferred embodiments, which are intended to illustrate, not limit, the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate the design and utility of preferred embodiments of the present invention, in which similar elements are referred to by common reference numerals. In order to better appreciate how the above-recited and other advantages and objects of the present inventions are obtained, a more particular description of the present inventions briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIGS. 1   a - 1   d  are prior art circuit diagrams of different prior art tissue stimulation topologies; 
         FIGS. 2   a - 2   c  are prior art diagrams of a cathode voltage and an anode voltage generated by the tissue stimulation topologies of  FIGS. 1   a - 1   d;    
         FIG. 3  is plan view of one embodiment of a spinal cord stimulation (SCS) system arranged in accordance with the present inventions; 
         FIG. 4  is a plan view of the SCS system of  FIG. 3  in use with a patient; 
         FIG. 5  is a profile view of an implantable pulse generator (IPG) used in the SCS system of  FIG. 3 ; 
         FIG. 6  is a block diagram of the internal components of the IPG of  FIG. 5 ; 
         FIGS. 7   a  and  7   b  are circuit diagrams of two tissue stimulation topologies used by the SCS system of  FIG. 3 ; 
         FIGS. 8   a  and  8   b  are circuit diagrams of two alternative tissue stimulation topologies used by the SCS system of  FIG. 3 ; 
         FIGS. 9   a  and  9   b  are circuit diagrams of two alternative tissue stimulation topologies used by the SCS system of  FIG. 3 ; and 
         FIG. 10  is a diagram of a cathode voltage and an anode voltage generated by the tissue stimulation topologies of  FIGS. 1   a - 1   d.    
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The description that follows relates to a spinal cord stimulation (SCS) system. However, it is to be understood that the while the invention lends itself well to applications in SCS, the invention, in its broadest aspects, may not be so limited. Rather, the invention may be used with any type of implantable electrical circuitry used to stimulate tissue. For example, the present invention may be used as part of a pacemaker, a defibrillator, a cochlear stimulator, a retinal stimulator, a stimulator configured to produce coordinated limb movement, a cortical stimulator, a deep brain stimulator, peripheral nerve stimulator, microstimulator, or in any other neural stimulator configured to treat urinary incontinence, sleep apnea, shoulder sublaxation, headache, etc. 
     Turning first to  FIG. 3 , an exemplary spinal cord stimulation (SCS) system  10  generally includes one or more (in this case, two) implantable stimulation leads  12 , a pulse generating device in the form of an implantable pulse generator (IPG)  14 , an external control device in the form of a remote controller RC  16 , a clinician&#39;s programmer (CP)  18 , an external trial stimulator (ETS)  20 , and an external charger  22 . 
     The IPG  14  is physically connected via one or more percutaneous lead extensions  24  to the stimulation leads  12 , which carry a plurality of electrodes  26  arranged in an array. In the illustrated embodiment, the stimulation leads  12  are percutaneous leads, and to this end, the electrodes  26  are arranged in-line along the stimulation leads  12 . In alternative embodiments, the electrodes  26  may be arranged in a two-dimensional pattern on a single paddle lead. As will be described in further detail below, the IPG  14  includes pulse generation circuitry that delivers electrical stimulation energy in the form of a pulsed electrical waveform (i.e., a temporal series of electrical pulses) to the electrode array  26  in accordance with a set of stimulation parameters. 
     The ETS  20  may also be physically connected via the percutaneous lead extensions  28  and external cable  30  to the stimulation leads  12 . The ETS  20 , which has similar pulse generation circuitry as that of the IPG  14 , also delivers electrical stimulation energy to the electrode array  26  in accordance with a set of stimulation parameters. The major difference between the ETS  20  and the IPG  14  is that the ETS  20  is a non-implantable device that is used on a trial basis after the stimulation leads  12  have been implanted and prior to implantation of the IPG  14 , to test the responsiveness of the stimulation that is to be provided. Further details of an exemplary ETS are described in U.S. Pat. No. 6,895,280, which is expressly incorporated herein by reference. 
     The RC  16  may be used to telemetrically control the ETS  20  via a bi-directional RF communications link  32 . Once the IPG  14  and stimulation leads  12  are implanted, the RC  16  may be used to telemetrically control the IPG  14  via a bi-directional RF communications link  34 . Such control allows the IPG  14  to be turned on or off and to be programmed with different stimulation parameter sets. The IPG  14  may also be operated to modify the programmed stimulation parameters to actively control the characteristics of the electrical stimulation energy output by the IPG  14 . 
     The CP  18  provides clinician detailed stimulation parameters for programming the IPG  14  and ETS  20  in the operating room and in follow-up sessions. The CP  18  may perform this function by indirectly communicating with the IPG  14  or ETS  20 , through the RC  16 , via an IR communications link  36 . Alternatively, the CP  18  may directly communicate with the IPG  14  or ETS  20  via an RF communications link (not shown). The clinician detailed stimulation parameters provided by the CP  18  are also used to program the RC  16 , so that the stimulation parameters can be subsequently modified by operation of the RC  16  in a stand-alone mode (i.e., without the assistance of the CP  18 ). The external charger  22  is a portable device used to transcutaneously charge the IPG  14  via an inductive link  38 . Once the IPG  14  has been programmed, and its power source has been charged by the external charger  22  or otherwise replenished, the IPG  14  may function as programmed without the RC  16  or CP  18  being present. 
     For purposes of brevity, the details of the RC  16 , CP  18 , ETS  20 , and external charger  22  will not be described herein. Details of exemplary embodiments of these devices are disclosed in U.S. Pat. No. 6,895,280, which is expressly incorporated herein by reference. 
     As shown in  FIG. 4 , the electrode leads  12  are implanted within the spinal column  42  of a patient  40 . The preferred placement of the electrode leads  12  is adjacent, i.e., resting upon near, or upon the dura, adjacent to the spinal cord area to be stimulated. Due to the lack of space near the location where the electrode leads  12  exit the spinal column  42 , the IPG  14  is generally implanted in a surgically-made pocket either in the abdomen or above the buttocks. The IPG  14  may, of course, also be implanted in other locations of the patient&#39;s body. The lead extension  24  facilitates locating the IPG  14  away from the exit point of the electrode leads  12 . As there shown, the CP  18  communicates with the IPG  14  via the RC  16 . 
     Referring now to  FIG. 5 , the external features of the stimulation leads  12  and the IPG  14  will be briefly described. One of the stimulation leads  12  has eight electrodes  26  (labeled E 1 -E 8 ), and the other stimulation lead  12  has eight electrodes  26  (labeled E 9 -E 16 ). The actual number and shape of leads and electrodes will, of course, vary according to the intended application. The IPG  14  comprises an outer case  50  for housing the electronic and other components (described in further detail below), and a connector  52  to which the proximal ends of the stimulation leads  12  mate in a manner that electrically couples the electrodes  26  to the internal electronics (described in further detail below) within the outer case  50 . The outer case  50  is composed of an electrically conductive, biocompatible material, such as titanium, and forms a hermetically sealed compartment wherein the internal electronics are protected from the body tissue and fluids. In some cases, the outer case  50  may serve as an electrode. 
     As briefly discussed above, the IPG  14  includes battery and pulse generation circuitry that delivers the electrical stimulation energy in the form of a pulsed electrical waveform to the electrode array  26  in accordance with a set of stimulation parameters programmed into the IPG  14 . Such stimulation parameters may comprise electrode combinations, which define the electrodes that are activated as anodes (positive), cathodes (negative), and turned off (zero), percentage of stimulation energy assigned to each electrode (fractionalized electrode configurations), and electrical pulse parameters, which define the pulse amplitude (measured in milliamps or volts depending on whether the IPG  14  supplies constant current or constant voltage to the electrode array  26 ), pulse width (measured in microseconds), and pulse rate (measured in pulses per second), pulse shape, and burst rate (measured as the stimulation on duration per unit time). 
     Electrical stimulation will occur between two (or more) activated electrodes, one of which may be the IPG case  50 . Simulation energy may be transmitted to the tissue in a monopolar or multipolar (e.g., bipolar, tripolar, etc.) fashion. Monopolar stimulation occurs when a selected one of the lead electrodes  26  is activated along with the case  50  of the IPG  14 , so that stimulation energy is transmitted between the selected electrode  26  and case  50 . Bipolar stimulation occurs when two of the lead electrodes  26  are activated as anode and cathode, so that stimulation energy is transmitted between the selected electrodes  26 . For example, electrode E 3  on the first lead  12  may be activated as an anode at the same time that electrode E 11  on the second lead  12  is activated as a cathode. Tripolar stimulation occurs when three of the lead electrodes  26  are activated, two as anodes and the remaining one as a cathode, or two as cathodes and the remaining one as an anode. For example, electrodes E 4  and E 5  on the first lead  12  may be activated as anodes at the same time that electrode E 12  on the second lead  12  is activated as a cathode. 
     The stimulation energy may be delivered between electrodes as monophasic electrical energy or multiphasic electrical energy. Monophasic electrical energy includes a series of pulses that are either all positive (anodic) or all negative (cathodic). Multiphasic electrical energy includes a series of pulses that alternate between positive and negative. For example, multiphasic electrical energy may include a series of biphasic pulses, with each biphasic pulse including a cathodic (negative) stimulation pulse and an anodic (positive) recharge pulse that is generated after the stimulation pulse to prevent direct current charge transfer through the tissue, thereby avoiding electrode degradation and cell trauma. That is, charge is conveyed through the electrode-tissue interface via current at an electrode during a stimulation period (the length of the stimulation pulse), and then pulled back off the electrode-tissue interface via an oppositely polarized current at the same electrode during a recharge period (the length of the recharge pulse). 
     Turning next to  FIG. 6 , the main internal components of the IPG  14  will now be described. The IPG  14  includes analog output circuitry  60  configured for generating electrical stimulation energy in accordance with a defined pulsed waveform having a specified pulse amplitude, pulse rate, pulse width, pulse shape, and burst rate under control of control logic  62  over data bus  64 . Control of the pulse rate and pulse width of the electrical waveform is facilitated by timer logic circuitry  66 , which may have a suitable resolution, e.g., 10 μs. The electrical stimulation energy generated by the output analog circuitry  60  is output via capacitors C 1 -C 16  to electrical terminals  68  corresponding to the electrodes  26 . 
     The analog output circuitry  60  may either comprise independently controlled current sources for providing electrical stimulation energy of a specified and known amperage to or from the electrical terminals  68 , or independently controlled voltage sources for providing electrical stimulation energy of a specified and known voltage at the electrical terminals  68  or to multiplexed current or voltage sources that are then connected to the electrical terminals  68 . The operation of the analog output circuitry  60 , including alternative embodiments of suitable output circuitry for performing the same function of generating stimulation pulses of a prescribed amplitude and width, is described more fully in U.S. Pat. Nos. 6,516,227 and 6,993,384, which are expressly incorporated herein by reference. 
     Significantly, as will be described in further detail below, the analog output circuitry  60  presents symmetrical outputs to both the anodes and cathodes that will not be subject to the differential voltage shifts in the circuitry discussed in the background. Furthermore, the analog output circuitry  60  references the voltages at the anodes and cathodes to the tissue rather than a voltage internal to the IPG  14 . 
     The IPG  14  further comprises monitoring circuitry  70  for monitoring the status of various nodes or other points  72  throughout the IPG  14 , e.g., power supply voltages, temperature, battery voltage, and the like. The monitoring circuitry  70  is also configured for measuring electrical parameter data (e.g., electrode impedance and/or electrode field potential). The IPG  14  further comprises processing circuitry in the form of a microcontroller (μC)  74  that controls the control logic  62  over data bus  76 , and obtains status data from the monitoring circuitry  70  via data bus  78 . The IPG  14  further comprises memory  80  and oscillator and clock circuit  82  coupled to the microcontroller  74 . The microcontroller  74 , in combination with the memory  80  and oscillator and clock circuit  82 , thus comprise a microprocessor system that carries out a program function in accordance with a suitable program stored in the memory  80 . Alternatively, for some applications, the function provided by the microprocessor system may be carried out by a suitable state machine. 
     Thus, the microcontroller  74  generates the necessary control and status signals, which allow the microcontroller  74  to control the operation of the IPG  14  in accordance with a selected operating program and stimulation parameters. In controlling the operation of the IPG  14 , the microcontroller  74  is able to individually generate stimulus pulses at the electrical terminals  68  using the analog output circuitry  60 , in combination with the control logic  62  and timer logic circuitry  66 , thereby allowing each electrical terminal  68  (and thus, each electrode  26 ) to be paired or grouped with other electrical terminals  68  (and thus, other electrodes  26 ), including the monopolar case electrode, to control the polarity, amplitude, rate, pulse width, pulse shape, burst rate, and channel through which the current stimulus pulses are provided. The microcontroller  74  facilitates the storage of electrical parameter data measured by the monitoring circuitry  70  within memory  80 . 
     The IPG  14  further comprises a receiving coil  84  for receiving programming data (e.g., the operating program and/or stimulation parameters) from the external programmer (i.e., the RC  16  or CP  18 ) in an appropriate modulated carrier signal, and charging, and circuitry  86  for demodulating the carrier signal it receives through the receiving coil  84  to recover the programming data, which programming data is then stored within the memory  80 , or within other memory elements (not shown) distributed throughout the IPG  14 . 
     The IPG  14  further comprises back telemetry circuitry  88  and a transmission coil  90  for sending informational data to the external programmer. The back telemetry features of the IPG  14  also allow its status to be checked. For example, when the CP  18  initiates a programming session with the IPG  14 , the capacity of the battery is telemetered, so that the CP  18  can calculate the estimated time to recharge. Any changes made to the current stimulus parameters are confirmed through back telemetry, thereby assuring that such changes have been correctly received and implemented within the implant system. Moreover, upon interrogation by the CP  18 , all programmable settings stored within the IPG  14  may be uploaded to the CP  18 . 
     The IPG  14  further comprises a rechargeable power source  92  and power circuits  94  for providing the operating power to the IPG  14 . The rechargeable power source  92  may, e.g., comprise a lithium-ion or lithium-ion polymer battery or other form of rechargeable power. The rechargeable source  92  provides an unregulated voltage to the power circuits  94 . The power circuits  94 , in turn, generate the various voltages  96 , some of which are regulated and some of which are not, as needed by the various circuits located within the IPG  14 . The rechargeable power source  92  is recharged using rectified AC power (or DC power converted from AC power through other means, e.g., efficient AC-to-DC converter circuits, also known as “inverter circuits”) received by the receiving coil  84 . 
     To recharge the power source  92 , the external charger  22  (shown in  FIG. 3 ), which generates the AC magnetic field, is placed against, or otherwise adjacent, to the patient&#39;s skin over the implanted IPG  14 . The AC magnetic field emitted by the external charger induces AC currents in the receiving coil  84 . The charging and forward telemetry circuitry  86  rectifies the AC current to produce DC current, which is used to charge the power source  92 . While the receiving coil  84  is described as being used for both wirelessly receiving communications (e.g., programming and control data) and charging energy from the external device, it should be appreciated that the receiving coil  84  can be arranged as a dedicated charging coil, while another coil, such as the coil  90 , can be used for bi-directional telemetry. 
     Additional details concerning the above-described and other IPGs may be found in U.S. Pat. No. 6,516,227, U.S. Patent Publication No. 2003/0139781, and U.S. patent application Ser. No. 11/138,632, entitled “Low Power Loss Current Digital-to-Analog Converter Used in an Implantable Pulse Generator,” which are expressly incorporated herein by reference. 
     It should be noted that rather than an IPG, the SCS system  10  may alternatively utilize an implantable receiver-stimulator (not shown) connected to the stimulation leads  12 . In this case, the power source, e.g., a battery, for powering the implanted receiver, as well as control circuitry to command the receiver-stimulator, will be contained in an external controller inductively coupled to the receiver-stimulator via an electromagnetic link. Data/power signals are transcutaneously coupled from a cable-connected transmission coil placed over the implanted receiver-stimulator. The implanted receiver-stimulator receives the signal and generates the stimulation energy in accordance with the control signals. 
     As briefly discussed above, the analog output circuitry  60  presents symmetrical outputs to both the anodes and cathodes. For example, with reference to  FIG. 7   a , a first voltage source  102   a  is coupled to an anode  100   a , and a second voltage source  102   b  is coupled to a cathode  100   b . This is in contrast to the single-ended voltage regulated circuit illustrated in  FIG. 1   a . Thus, because there is a voltage source at both the anode  100   a  and the cathode  100   b , voltage shifts within the analog output circuitry  60  will not be conducted to the anode  100   a  and cathode  100   b  differentially. Alternatively, with reference to  FIG. 7   b , a first current source  104   a  is coupled to the anode  100   a , and a second current source  104   b  is coupled to the cathode  100   b . This is in contrast to the single-ended current regulated circuit illustrated in  FIG. 1   b . In this case, the current sources present a high impedance to the respective anode  100   a  and cathode  100   b , thereby isolating the anode  100   a  and cathode  100   b  from voltage shifts within the analog output circuitry  60 . 
     As also briefly discussed above, the analog output circuitry  60  references the voltages at the anodes and cathodes to the tissue rather than a voltage internal to the IPG  14 . To this end, the IPG  14  is provided with a grounding electrode  106  configured for being placed in contract with tissue. For example, the grounding electrode  106  may be located on the case  50  or may be the case  50  itself. In the illustrated embodiment, the analog output circuitry  60  regulates the voltages at the anodes and cathodes, such that the common mode signal (i.e., the average of the anode voltage shift and cathode voltage shift relative to the reference voltage (in this case, the grounding electrode  106 )) will be equal to or less than the differential voltage between the cathodes and anodes, as illustrated in  FIG. 10 . 
     With reference back to  FIG. 7   a , the first voltage source  102   a  is electrically coupled between the anode  100   a  and the grounding electrode  106 , and the second voltage source  102   b  is electrically coupled between the cathode  100   b  and the grounding electrode  106 . As a result, the voltages at the respective anode  100   a  and cathode  100   b  relative to the tissue may be controlled, so that large voltages are not applied to the tissue. The voltage values respectively output by the first and second voltage sources  102   a ,  102   b  can be set to be equal in order to minimize the maximum voltage seen by the tissue. For example, if the desired voltage potential between the anode  100   a  and the cathode  100   b  is 5V, the first voltage source  102   a  can be set to output a voltage of 2.5V relative to the grounding electrode  106  (and thus, the tissue), and the second voltage source  102   b  can be set to output a voltage of −2.5V relative to the grounding electrode  106  (and thus, the tissue). Essentially, in this case, the voltage of the common mode signal would be zero. Notably, the internal reference voltage of the analog output circuitry  60  is irrelevant, since the voltage sources  102   a ,  102   b  are not referenced to this internal voltage. 
     With reference to  FIG. 7   b , the first current source  104   a  is electrically coupled between the anode  100   a  and the grounding electrode  106 , and the second current source  104   b  is electrically coupled between the cathode  100   b  and the grounding electrode  106 . Thus, the electrical current flowing through each of the anode  100   a  and the cathode  100   b  can be controlled. In this case, where there the anode  100   a  and cathode  100   b  are the only active electrodes, the absolute value of the electrical current magnitude flowing through the anode  100   a  will be essentially equal to the electrical current magnitude flowing through the cathode  100   b ; however, the electrical currents flowing through the anode  100   a  and cathode  100   b  will be oppositely polarized. For example, the current output by the first current source  104   a  may be set at 2.5 mA, while the current output by the second current source  104   b  may be set at −2.5 mA. Essentially, in this case, the voltage of the common mode signal would be zero assuming that the tissue impedances on the cathodes and anodes are equal. 
     Although the current sources  104   a ,  104   b  regulate the current flowing through the anode  100   a  and cathode  100   b , the voltages at the respective anode  100   a  and cathode  100   b  relative to the tissue may still be controlled, so that large voltages are not applied to the tissue. In particular, the currents required to be output by the respective current sources  100   a ,  100   b  to achieve the voltage distribution desired at the respective anode  100   a  and cathode  100   b  relative to the tissue can be computed in a conventional manner. 
     Although each of the voltage sources  102   a ,  102   b  and current sources  104   a ,  104   b  in the topologies illustrated in  FIGS. 7   a  and  7   b  are coupled to only a single electrode, it should be appreciated that each of these sources can be coupled to multiple electrodes (either a group of anodes  100   a  or a group of cathodes  100   b ), as illustrated in  FIGS. 8   a  and  8   b . Furthermore, multiple sources of the same type can be respectively connected to multiple electrodes at the same time. For example, two voltage sources  102   a  or two current sources  104   a  can be respectively connected to two anodes  100   a  at the same time, or two voltage sources  102   b  or two current sources  104   b  can be respectively connected to two cathodes  100  at the same time, as illustrated in  FIGS. 9   a  and  9   b . Thus, this concept can be applied to a multiplicity of anodes and a multiplicity of cathodes where the positive shifts in voltage on the anode and negative shifts in voltage on the cathodes are such that the average shift is zero or at least less than one half of the maximum differential voltage between any anode and cathode pair during the stimulation pulse. 
     It can be appreciated from the foregoing that the voltage or voltages at the anode or anodes  100   a  relative to the tissue can be regulated, and the voltage or voltages at the cathode or cathodes  100   b  can be regulated, while the electrical stimulation energy is conveyed between the anode or anodes  100   a  and the cathode or cathodes  100   b . If any of the topologies illustrated in  FIG. 7   b ,  8   b , or  9   b  are used, the currents flowing through the anode or anodes  100   a  and the cathode or cathodes  100   b  can be regulated. 
     Although particular embodiments of the present inventions have been shown and described, it will be understood that it is not intended to limit the present inventions to the preferred embodiments, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present inventions. Thus, the present inventions are intended to cover alternatives, modifications, and equivalents, which may be included within the spirit and scope of the present inventions as defined by the claims.