Patent Publication Number: US-2004044371-A1

Title: Defibrillator with H-bridge output circuit referenced to common ground

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates generally to apparatus for generating defibrillation waveforms, and more particularly to a circuit for generating a defibrillation waveform in an external defibrillator.  
       BACKGROUND OF THE INVENTION  
       [0002] One of the most common and life-threatening medical conditions is ventricular fibrillation, a condition where the human heart is unable to pump the volume of blood required by the human body. The generally accepted technique for restoring a normal rhythm to a heart experiencing ventricular fibrillation is to apply a strong electric pulse to the heart using an external cardiac defibrillator. External cardiac defibrillators have been successfully used for many years in hospitals by doctors and nurses, and in the field by emergency treatment personnel, e.g., paramedics.  
       [0003] Conventional external cardiac defibrillators first accumulate a high-energy electric charge on an energy storage capacitor. When a switching mechanism is closed, the stored energy is transferred to a patient in the form of a large current pulse. The current pulse is applied to the patient via a pair of electrodes positioned on the patient&#39;s chest. The switching mechanism used in most contemporary external defibrillators is a high-energy transfer relay. A discharge control signal causes the relay to complete an electrical circuit between the storage capacitor and a wave shaping circuit whose output is connected to the electrodes attached to the patient.  
       [0004] certain studies indicate that there may be advantages to applying a biphasic rather than a monophasic waveform to the patient. For example, certain research indicates that a biphasic waveform may limit the resulting heart trauma associated with the defibrillation pulse. An H-bridge output circuit may be used for applying a biphasic defibrillation pulse.  
       [0005] The American Heart Association has recommended a range of energy levels for the first three defibrillation pulses applied by an external defibrillator. The recommended energy levels are: 200 joules for a first defibrillation pulse; 200 or 300 joules for a second defibrillation pulse; and 360 joules for a third defibrillation pulse, all within a recommended variance range of no more than plus or minus 15 percent according to standards promulgated by the Association for the Advancement of Medical Instrumentation (AAMI). These high-energy defibrillation pulses are required to ensure that a sufficient amount of the defibrillation pulse energy reaches the heart of the patient and is not dissipated in the chest wall of the patient.  
       [0006] Some implantable defibrillators, such as those shown in U.S. Pat. Nos. 5,083,562 and 4,880,357, use a bridge circuit with multiple silicon-controlled rectifiers (SCRs) to generate a biphasic waveform. Because implantable defibrillators only apply a low energy defibrillation pulse having a maximum energy of approximately 35 joules, however, the output circuit in implantable defibrillators is not adaptable for use in the external defibrillator. A 200-joule energy pulse applied to an implantable defibrillator bridge circuit may overload the bridge circuit components and cause the circuit to fail.  
       [0007] In addition, conventional external defibrillator circuits have typically been complex and expensive, with separate isolated circuitry required for the low voltage control circuitry and the high voltage defibrillation circuitry, due in part to the components required to conduct the large energy pulses that are generated in external defibrillators. It would be desirable to reduce the complexity and expense of such external defibrillator circuits, and to improve their efficiency.  
       [0008] The present invention is directed to providing apparatus that overcome the foregoing and other disadvantages. More specifically, the present invention is directed to an output circuit for an external defibrillator that is capable of applying a high-energy biaphasic defibrillation pulse to a patient, and which has reduced complexity and improved efficiency over prior external defibrillators.  
       SUMMARY OF THE INVENTION  
       [0009] An external defibrillator having an output circuit that allows a defibrillation pulse to be discharged to a patient from an energy storage device, preferably an energy storage capacitor, is disclosed. In accordance with one aspect of the invention, the output circuit is referenced to a common ground in the defibrillator. In the defibrillator, the preamplifier, impedance measurement circuit, charging circuit, battery, energy storage device and measurement and control circuits are all referenced to a common ground without requiring the commonly used isolation stages and circuits. For example, certain prior art defibrillators have utilized isolation circuits for circuits such as the preamplifier or output circuits. It will be appreciated that the utilization of a common ground for the high and low voltage circuitry is advantageous in that the resulting circuit design is simpler and less expensive than prior art designs.  
       [0010] In accordance with another aspect of the invention, the output circuit includes four legs arrayed in the form of an “H” (hereinafter the “H-bridge output circuit”). Each leg of the output circuit contains a solid-state switch. By selectively switching on pairs of switches in the H-bridge output circuit, a biphasic defibrillation pulse may be applied to the patient.  
       [0011] In accordance with another aspect of the invention, the switches in three of the legs of the H-bridge output circuit are silicon controlled rectifiers (SCRs). Preferably, only a single SCR is used in each leg. The switch in a fourth leg is an insulated gate bipolar transistor (IGBT). In one embodiment, only a single IGBT is used in the fourth leg. The use of single SCR and IGBT switches in each leg simplifies the circuit as compared to the use of semiconductor modules that are large and expensive or as compared to the use of lower voltage parts, which must be stacked. The use of three SCR legs further reduces the size, weight, and cost of the H-bridge output circuit in comparison with an implementation using two SCR and two IGBT legs. The use of a single IGBT in a leg of the H-bridge (as opposed to two or more IGBTs in series) also greatly simplifies the drive circuitry required to turn on and off the IGBT.  
       [0012] In accordance with another aspect of the invention, two of the SCR legs of the H-bridge output circuit are the two lower H-bridge legs, and a DC gate drive signal may be utilized to drive one or both of the SCR switches. Prior art defibrillators have typically isolated the H-bridge from the control circuit ground potential. This configuration has required a transformer to couple a drive signal to the SCR gates, and because the transformers are unable to pass the DC signals, the gate has been driven with AC signals. The utilization in the present invention of a common ground for both the high-voltage and low-voltage circuitry allows the gates of one or both of the SCR switches to be driven directly from field effect transistors (FETs) with a DC signal.  
       [0013] In accordance with another aspect of the invention, the IGBT leg is made to be the northwest leg of the H-bridge. Certain prior art defibrillators have placed the IGBT in the southeast leg. Making the northwest leg the IGBT leg helps avoid a design issue that occurs when attempting to modify certain prior art defibrillator configurations to meet the present design requirements. More specifically, in the present configuration, a current path can exist from the midpoint of the H-bridge through the preamp protection resistors (in one embodiment 12 kohms) to ground. The amount of current flowing through this path (in one embodiment 170 mA) is negligible compared with the current delivered to the patient (in one embodiment greater than 10 amps), but is sufficient to create a complication in the operation of the SCRs. More specifically, utilizing a prior art configuration where the IGBT is in the southeast leg, a current through this path (as noted above in one embodiment 170 mA), would flow through the SCRs at the top of the H-bridge. Once one of these SCRs was turned on, it could not be turned off again until the capacitor was mostly discharged because there is no mechanism for shutting off the current through the preamp path. The utilization of the IGBT in the northwest leg of the H-bridge allows the current through the preamp path to be shut off (along with the current through the patient) at the end of the first phase of the defibrillation pulse.  
       [0014] In accordance with still another aspect of the invention, a single power switch is utilized in each of the legs of the H-bridge output circuit, and is included in a single integrated surface mountable module. The use of single semiconductor switches in a single package simplifies the assembly and manufacturing of the defibrillator device.  
       [0015] In accordance with another aspect of the invention, the H-bridge output circuit is capable of conducting a biphasic waveform of 200 or more joules from the energy storage capacitor to the patient. Preferably, the H-bridge output circuit is capable of conducting a biphasic waveform equal to 360 joules, the industry standard for monophasic waveforms and the recommended level for a third defibrillation pulse by the American Heart Association. To store sufficient energy for such a biphasic defibrillation pulse, the size of the energy storage capacitor may in one embodiment fall within a range from 150 uF to 200 uF.  
       [0016] Moreover, in addition to being able to conduct a high energy defibrillation pulse of 200 to 360 joules, the H-bridge output circuit is also capable of conducting a low energy defibrillation pulse. In one embodiment, a lower energy defibrillation pulse of 150 joules may be delivered, while in other embodiments the defibrillator of the invention could also be used at a general lower energy range such as 1 to 50 joules. Some types of low energy defibrillation pulses are required when, for example, internal paddles are coupled to the defibrillator for use in surgery to directly defibrillate the heart, or for pediatric defibrillation, or for cardioversion of some arrhythmias in both pediatrics and adults.  
       [0017] In accordance with another aspect of the invention, a gate drive circuit biases on the IGBT in the first leg with a sufficient voltage over a short interval to allow the leg to conduct a high current without being damaged. In one embodiment, the leg can conduct a current of at least approximately 200 amps. Biasing the IGBT in this manner allows the IGBT to withstand a high-energy discharge such as occurs when a low resistance load is placed at the output of the circuit.  
       [0018] In accordance with still another aspect of the invention, all of the output circuit switches are selected to have sufficient current conducting capability to allow the switches in two of the legs on the same side of the H-bridge to provide a shorted path for the discharge of unwanted energy from the energy storage capacitor. The use of two legs on one side of the H-bridge to discharge the capacitor eliminates the need for an additional discharge circuit to perform this internal energy dump function. In addition, the H-bridge circuit is able to perform the internal energy dump quickly and accurately using advantageous component values that would not be practical to implement in a separate discharge circuit. For example, the H-bridge circuit is able to perform an internal dump in less than one second through the use of a resistive component with a value of less than 100 ohms. Also, because the H-bridge circuit is used for both the internal dump and defibrillation pulse operations, the resistive component of the H-bridge circuit serves to both absorb energy during the internal dump and also to limit current during the defibrillation pulse. The resistive value is selected to be small enough to allow sufficient current to provide both an effective defibrillation pulse and a fast internal energy dump, while also being large enough to limit the current so as to protect the switches of the H-bridge circuit. The resistive component is also selected to have a high thermal capacity so that it can withstand the heat produced by the high currents that result during the H-bridge internal dump and defibrillation pulse operations.  
       [0019] It will be appreciated that the disclosed defibrillator with a unique H-bridge output circuit that is referenced to a common ground with the preamplifier and charging circuits is advantageous in that it is simpler, less expensive, and operates more effectively than prior art defibrillators. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0020] The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
     [0021]FIG. 1 is a block diagram of an external defibrillator where the output circuit, charging circuit, and preamp circuit are referenced to a common ground in accordance with the present invention;  
     [0022]FIG. 2 is a more detailed block diagram of the external defibrillator of FIG. 1;  
     [0023]FIG. 3 is a schematic diagram of a preferred embodiment of the output circuit and transfer relay of the defibrillator of FIG. 2;  
     [0024]FIG. 4 is a block diagram of the charging circuit of the defibrillator of FIG. 2;  
     [0025]FIG. 5 is a schematic diagram of a preferred embodiment of the charging circuit of FIG. 4;  
     [0026]FIG. 6 is a block diagram of the preamp ECG and impedance drive and measurement circuit of FIG. 2; and  
     [0027] FIGS.  7 A- 7 C are schematic diagrams of a preferred embodiment of the preamp ECG and impedance drive and measurement circuitry of FIG. 6. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0028]FIG. 1 is a block diagram of an external defibrillator  8  that is connected to a patient  16 . The defibrillator includes a measurement and control circuit  10  that is connected to an energy storage capacitor and protective component  12  via a charging circuit  18 . During the operation of the defibrillator, the measurement and control circuit  10  controls the charging circuit  18  via a control line  25  to charge the energy storage capacitor to a desired voltage level. Feedback on the voltage level of the energy storage capacitor is provided to the measurement and control circuit  10  on a pair of lines  28  and  30 .  
     [0029] After charging to a desired level, the energy stored in the energy storage capacitor may be delivered to the patient  16  in the form of a defibrillation pulse. The energy storage capacitor and protective component  12  is connected by lines  26  and  28  to an output circuit  14 . Output circuit  14  includes defibrillation circuitry. The measurement and control circuit  10  is connected to the output circuit  14  by a control bus  42  and to an isolation circuit  35  by a control line  36 . Application of appropriate control signals over the control bus  42  and control line  36  causes the output circuit  14  to conduct energy from the energy storage capacitor. The energy is delivered to the patient  16  attached to the defibrillator  8  over a set of patient apex and sternum lines (aka electrodes)  15   a  and  15   b.  The apex line  15   a  is attached to an apex line  17  in output circuit  14  through the isolation circuit  35 . The sternum line  15   b  is attached to a sternum line  19  in output circuit  14  through the isolation circuit  35 .  
     [0030] The measurement and control circuit  10  also controls and receives measurements through a bus line  38  from a preamplifier ECG and impedance drive and measurement circuit  37 . The preamplifier ECG and impedance drive and measurement circuit  37  is coupled to the apex and sternum lines  15   a  and  15   b.  The preamplifier ECG and impedance drive and measurement circuit  37  provides measurements of the ECG and impedance of the patient  16 .  
     [0031] A battery and power supplies circuit  39  includes a battery or similar power source (e.g., a charge pack) for powering the defibrillator  8 . The battery and power supplies circuit  39  provides power to the other circuit components, including the measurement and control circuit  10 , the output circuit  14 , the charging circuit  18 , the isolation circuit  35 , and the preamplifier ECG and impedance drive and measurement circuit  37 . The battery and power supplies circuit  39  is referenced to the common ground  28 .  
     [0032] As illustrated in FIG. 1, the measurement and control circuit  10 , energy storage capacitor  12 , output circuit  14 , charging circuit  18 , preamplifier ECG and impedance drive and measurement circuit  37 , and the battery and power supplies circuit  39  are all referenced to the common ground  28 . This is in contrast to certain prior art defibrillators which utilize isolation circuits for various circuits such as the output circuit or preamplifier. It will be appreciated that the utilization of a common ground rather than requiring isolation circuitry between the high and low voltage circuitry is advantageous in that it results in simpler and less expensive circuitry.  
     [0033]FIG. 2 is a more detailed block diagram of the external defibrillator  8  of FIG. 1, according to one embodiment of the present invention. The defibrillator  8  is connected to a patient  16  and includes a microprocessor  20  that is connected to an energy storage capacitor  24  via a charging circuit  18 . It will be appreciated by those skilled in the art that energy storage capacitor  24  may be implemented with a multi-capacitor network (i.e., with capacitors connected in series and/or parallel). During the operation of the defibrillator  8 , microprocessor  20  controls charging circuit  18  using a signal on a control line  25  to charge energy storage capacitor  24  to a desired voltage level. To monitor the charging process, microprocessor  20  is connected to a scaling circuit  22  by a measurement line  47 , and by a control line  49 . It will be understood that while single measurement and control lines are shown, multiple lines may be used. Scaling circuit  22  is connected to energy storage capacitor  24  by a bridge line  28 , which connects to the negative lead of energy storage capacitor  24 , and by a line  30 , which connects to the positive lead of the capacitor. As will be described in more detail below, the bridge line  28  serves as a common ground for the defibrillator  8 . A clock  21  is also connected to microprocessor  20 .  
     [0034] After charging to a desired level, the energy stored in energy storage capacitor  24  may be delivered to patient  16  in the form of a defibrillation pulse. The H-bridge output circuit  14  is provided to allow the controlled transfer of energy from energy storage capacitor  24  to patient  16 . H-bridge  14  is an output circuit that includes four switches  31 ,  32 ,  33 , and  34 , which are driven by four driver circuits  51 ,  52 ,  53 , and  54 , respectively. Driver circuits  53  and  54  are further driven by a driver circuit  53   a,  as will be described in more detail below. Each of the switches  31 ,  32 ,  33  and  34  is connected in a leg of the output circuit that is arrayed in the form of an “H”. Switches  31  and  33  are coupled through a protective component  27  to the positive lead of the energy storage capacitor  24  by a bridge line  26 . Protective component  27  limits the current and voltage changes from energy storage capacitor  24 , and has both inductive and resistive properties. Switches  32  and  34  are coupled to energy storage capacitor  24  by the bridge line  28 .  
     [0035] Patient  16  is connected to the left side of H-bridge  14  by an apex line  17 , and to the right side of H-bridge  14  by a sternum line  19 . As depicted in FIG. 2, apex line  17  and sternum line  19  are connected to the patent apex and sternum electrode lines  15   a  and  15   b,  respectively, by a transfer relay circuit  35 . Microprocessor  20  is connected to driver circuits  51  and  52  by control lines  42   a  and  42   b,  respectively. Microprocessor  20  is coupled by a control line  42   cd  to driver circuit  53   a,  which is coupled by a control line  42   cd  to driver circuits  53  and  54 . Microprocessor  20  is also coupled to transfer relay circuit  35  by control line  36 .  
     [0036] As will be described in more detail below, application of appropriate control signals by microprocessor  20  over the control lines causes switches  31 - 34  to be appropriately opened and closed, thereby allowing H-bridge  14  to conduct energy from energy storage capacitor  24  to patient  16  in the form of a defibrillation pulse. The operation and components H-bridge output circuit  14  are described in more detail in U.S. patent application No. 10/186,218, filed Jun. 26, 2002, and U.S. patent application No. 10/141,687, filed May 7, 2002, each of which are commonly assigned and each of which are hereby incorporated by reference in their entireties.  
     [0037] The defibrillator  8  also includes preamplifier ECG and impedance drive and measurement circuitry  37 . The preamplifier ECG and impedance drive and measurement circuitry  37  is coupled to the patient apex and sternum lines  15   a  and  15   b.  The preamplifier ECG and impedance drive and measurement circuitry  37  is controlled by a control line  43  from the microprocessor  20  and provides measurements on a measurement line  44  to the microprocessor  20 . As shown in FIG. 2, the measurement and control circuit  10 , energy storage capacitor  12 , output circuit  14 , charging circuit  18 , and preamplifier circuit  37  are all referenced to the common ground  28 .  
     [0038] In order to simplify the block diagram of FIG. 2, the battery and power supplies circuit  39  of FIG. 1 has not been shown therein, although it will be understood that the battery and power supplies circuit  39  provides power to the circuit components as illustrated in FIG. 1. Furthermore, as described in more detail below, various power outputs from the battery and power supplies circuit  39  are coupled to circuit components as shown in the schematic diagrams of FIGS. 3, 5 and  7 A- 7 C. For example, some of the various power outputs from the battery and power supplies circuit  39  include the voltage line SW-VBATT, the voltage line PAD+, and the voltage line PAD−.  
     [0039] A schematic diagram of a preferred construction of H-bridge  14  and transfer relay  35  is shown in FIG. 3. H-bridge  14  uses four output switches SW 1 -SW 4  to conduct energy from energy storage capacitor  24  to patient  16 . Switches SW 1 -SW 4  correspond to switches  31 - 34  of FIG. 2, respectively. Switches SW 2 , SW 3  and SW 4  are semiconductor switches, preferably silicon controlled rectifiers (SCRs). Switch SW 1  is an insulated gate bipolar transistor (IGBT). Switches SW 1 -SW 4  can be switched from an off (nonconducting) to an on (conducting) condition.  
     [0040] Each of the switches SW 1 -SW 4  is implemented as a single power switch device. Switches SW 1 -SW 4  are packaged in a single surface-mountable package  100  for ease in manufacturing. This circuit package achieves a substantial reduction in overall parts count over previous external defibrillator H-bridges which required multiple switches in each leg, e.g., two or more IGBTs in a leg, and which were not designed to be provided in a single package. The reduction in overall parts count and ease of manufacturing of the single-surface mountable package improves the reliability and manufacturability of the external defibrillator  8 . In addition, the use of the single IGBT device for a power switch in one of the legs of the H-bridge circuit simplifies the drive circuit requirements for the IGBT over previous H-bridge designs, which utilized multiple IGBT devices.  
     [0041] In the defibrillation mode, defibrillator  8  generates a biphasic defibrillation pulse for application to the patient  16 . Initially, switches SW 1 -SW 4  and the transfer relay  35  are opened. Charging of energy storage capacitor  24  is started, and monitored by microprocessor  20  (FIG. 2). When energy storage capacitor  24  is charged to a selected energy level and the transfer relay  35  is closed, switches SW 1  and SW 2  are switched on so as to connect energy storage capacitor  24  with apex line  17  and sternum line  19  for the application of a first phase of a defibrillation pulse to patient  16 . The first phase of the biphasic pulse is therefore a positive pulse from the apex to the sternum of patient  16 .  
     [0042] Before energy storage capacitor  24  is completely discharged, switch SW 1  is biased off to prepare for the application of the second phase of the biphasic pulse. Once switch SW 1  is biased off, switch SW 2  will also become nonconducting as the current though the SCR switch SW 2  drops below the holding current for the SCR.  
     [0043] After the end of the first phase of the biphasic defibrillation pulse, switches SW 3  and SW 4  are switched on to start the second phase of the biphasic pulse. Switches SW 3  and SW 4  provide a current path to apply a negative defibrillation pulse to patient  16 . The polarity of the second phase of the defibrillation pulse is therefore opposite in polarity to the first phase of the biphasic pulse. The end of the second phase of the biphasic pulse is truncated by switching on switch SW 1  and switch SW 2  to provide a shorted path for the remainder of the capacitor energy through switches SW 1  and SW 4  and also through switches SW 2  and SW 3 . After energy storage capacitor  24  is discharged, switches SW 1 -SW 4  go to a nonconducting state. Patient isolation relay  35  is then opened. Energy storage capacitor  24  may then be recharged to prepare defibrillator  8  to apply another defibrillation pulse.  
     [0044] As described above, the four output switches SW 1 -SW 4  can be switched from an off (nonconducting) state to an on (conducting) state by application of appropriate control signals on control lines  42   a,    42   b,  and  42   cd.  In order to allow the SCRs and IGBT to conduct a range of high and low currents required for various applications, special switch driving circuits  51 - 54  are coupled to switches SW 1 -SW 4 , respectively. Control lines  42   a  and  42   b,  are connected to switch driving circuits  51  and  52 , and control line  42   cd  is connected to switch driving circuit  53   a,  which is in turn connected by control line  42   cd′  to driving circuits  53  and  54 , so as to allow microprocessor  20  to control the state of the switches.  
     [0045] As noted above, IGBT switch SW 1  is driven by switch driving circuit  51 . Switch driving circuit  51  amplifies the control signal  42   a  and provides it to the gate of the IGBT switch SW 1 . It is desirable to drive the IGBT switch SW 1  with a high voltage at its gate so that the switch will be able to conduct high currents, as will be described in more detail below. As will also be described in more detail below, it is also desirable to control a turn-on and turn-off time of the IGBT switch SW 1  so as to ensure proper operation of the other switches within the H-bridge  14 .  
     [0046] Switch driving circuit  51  includes resistors R 11 -R 19 , capacitors C 11 -C 17 , switches SW 11 -SW 13 , a component U 11 , a transformer T 11 , components CR 11  and CR 12 , and a zener diode ZD 11 . On the primary side of the transformer T 11 , the control signal  42   a  determines the current through the transformer. Capacitor C 11  and resistor R 11  are coupled in parallel between the control signal line  42   a  and ground. The control signal line  42   a  is coupled to the input of component U 11 . The negative power supply input of component U 11  is coupled to ground while the positive power supply input is coupled to the battery voltage line SW-VBATT. Capacitor C 12  is coupled between the positive power supply input of component U 11  and ground. The output of component U 11  is coupled to the gate of switch SW 11 . The source of switch SW 11  is coupled to ground, while the drain is coupled through the parallel resistors R 12   a  and R 12   b  to the non-dotted end of the primary winding of transformer T 11 . The dotted end of the primary winding of transformer T 11  is coupled to the battery voltage line SW-VBATT. When an oscillating control signal is provided on control line  42   a,  switch SW 11  is caused to be turned off and on, thus creating an oscillating current through the primary winding of the transformer T 11 , which results in a current being generated in the secondary windings of the transformer T 11 , as will be described in more detail below.  
     [0047] The secondary windings of the transformer T 11  include two windings coupled in parallel. Capacitor C 14  is also coupled in parallel with the secondary windings. Component CR 11  is shown as being schematically represented by two diodes connected at their anodes, with three pins  1 ,  2 , and  3 , with pins  1  and  2  being connected to the cathodes of the respective two diodes and pin  3  being connected at the junction of the anodes of the two diodes. Pin  3  of the component CR 11  is coupled to the dotted ends of the secondary windings of transformer T 11 . Capacitor C 15  is coupled between pin  2  of component CR 11  and the non-dotted ends of the secondary windings of the transformers T 11 . A resistor R 13  is coupled in parallel with the capacitor C 15 . Component CR 12  is similar to component CR 11 , and also includes three pins  1 ,  2 , and  3 . Pin  1  of component CR 12  is coupled to pin  2  of component CR 11 . Pin  2  of component CR 12  is coupled to pin  3  of component CR 12 . The base of switch SW 12  is coupled to pin  3  of component CR 12 . Resistor R 16  is coupled between the collector of switch SWl 2  and the non-dotted ends of the secondary windings of transformer T 11 . Resistor R 15  is coupled between the emitter of switch SW 12  and pin  3  of component CR 12 . Resistor R 14  is coupled between the emitter of switch SWl 2  and pin  2  of component CR 11 . Resistor R 17  and capacitor C 16  are coupled in parallel between the emitter of switch SWl 2  and the non-dotted end of the secondary windings of transformer T 11 . The collector of switch SW 12  is coupled to the gate of switch SW 13 . The drain of switch SW 13  is coupled to pin  1  of component CR 11 , and the source of switch SW 13  is coupled to the non-dotted end of the secondary windings of the transformer T 11 . Resistor R 18  and zener diode ZD 11  are coupled in parallel between pin  1  of component CR 11  and the non-dotted ends of the secondary windings of transformer T 11 . Resistor R 19  and capacitor C 17  are coupled in series between pin  1  of component CR 11  and the non-dotted ends of the secondary windings of transformer T 11 . The circuit node between the resistor R 19  and capacitor C 17  is coupled to the gate of the IGBT switch SW 1 . As noted above, IGBT switch SW 1  is coupled between the bridge line  26  and the apex line  17 .  
     [0048] As noted above, control signal  42   a  determines the current through the primary winding of transformer T 11 . The resulting current generated in the secondary windings of transformer T 11  travels through the above-described components to apply a voltage to the gate of IGBT switch SW 1 . The turn-on and turn-off time of IGBT switch SW 1  is thus controlled, at least in part, by the above-described components which control the voltage applied to the gate of IGBT switch SW 1 .  
     [0049] It will be appreciated that transformer T 11  provides isolation of the high voltage circuitry including IGBT switch SW 1 , from the low voltage control circuitry including control signal  42   a.  It will also be appreciated that the switch driving circuit  51  amplifies the control signal  42   a  for use in driving the gate of the IGBT switch SW 1 . In one embodiment, the gate of the IGBT switch SW 2  may be driven with up to 30 volts.  
     [0050] High currents may sometimes occur in H-bridge  14 . One way that high currents may be created is when low resistance is placed between the shock paddles. When this happens, a high current flows between apex line  17  and sternum line  19 . In this embodiment, to accommodate high currents without damaging IGBT switch SW 1 , IGBT switch SW 1  may be biased by a high gate voltage (e.g., 30 volts) such that the IGBT can safely conduct upwards of 200 amperes of current. When very low patient impedances are detected, the control circuitry of the defibrillator  8  limits the charge voltage so as to attempt to ensure that the defibrillator does not deliver a current of more than 200 amps to the patient.  
     [0051] In one embodiment, the drive circuit  51  is designed so that IGBT switch SW 1  is turned on relatively slowly when compared to the fast turn-on of SCR switches SW 2 , SW 3 , and SW 4 . A slow turn-on for IGBT switch SW 1  is desirable because the IGBT switch is on the same side of H-bridge  14  as SCR switch SW 4 . SCR switch SW 4  is controlled by the control signal on control line  42   cd,  but due to the nature of SCR switches, the SCR switch may be accidentally turned on regardless of the signal on control line  42   cd  if a rapid voltage change occurs across SCR switch SW 4 . If IGBT switch SW 1  was therefore turned on too quickly, the resulting rate of change of the voltage across SCR switch SW 1  might cause it to turn on accidentally. In contrast to the slow turn-on of IGBT switch SW 1 , the turn-off of the IGBT switch may be performed relatively quickly. The IGBT switch can be quickly turned off because at turn-off there is no concern that the sensitive SCR switches will accidentally turn on.  
     [0052] It will be appreciated that driving circuit  51  allows the IGBT to be used in external defibrillator  8  where extremely high voltages must be switched in the presence of SCRs. The driving circuit and the use of the single IGBT switch minimizes the number of components required to switch a defibrillation pulse of 200 or more joules. In addition to conducting high currents associated with high-energy defibrillation pulses, the IGBT is also able to conduct very low currents that are associated with low energy defibrillation pulses.  
     [0053] It will be appreciated that the above-described circuit configuration in which the IGBT switch SW 1  is placed in the northwest leg of the H-bridge is advantageous over previous prior art designs, in which the IGBT switch was placed in the southeast leg of the H-bridge  14 . The placement of the IGBT switch SW 1  in the northwest leg is particularly advantageous in the present circuit design due to a current path that exists from the midpoint of the H-bridge  14  through the preamp protection resistors (which in one embodiment may be 12 kohms) to ground. The amount of current flowing through this path (in one embodiment 170 mA) is negligible compared with the current delivered to the patient (in one embodiment greater than 10 amps), but is sufficient to disturb the operation of the H-bridge. More specifically, using a design in which the IGBT switch is placed in the southeast leg, the current through the preamp protection resistors (in one embodiment 170 mA) would flow through the SCRs at the top of the H-bridge  14 . Once one of these SCRs was turned on, it could not be turned off until the capacitor was essentially discharged because there is no mechanism for shutting off the current through the preamp path, and as is well known in the art, once current begins flowing through an SCR, it generally cannot be turned off until the current drops below a specified level. The placement of the IGBT switch SW 1  in the northwest leg allows the current through the preamp path to be shut off (along with the current through the patient) at the end of the first phase of a multiphasic defibrillation pulse.  
     [0054] SCR switch SW 2  is driven by drive circuit  52 , while SCR switch SW 4  is driven by the combination of drive circuits  53   a  and  54 . The components of drive circuit  52  and the combination of drive circuits  53   a  and  54  are similar. For purposes of this description, therefore, only the construction and operation of the combination of switch driving circuits  53   a  and  54  will be described. Those skilled in the art will recognize that the combination of switch driving circuits  53   a  and  54  operate in a similar manner to switch driving circuit  52 . The combination of switch driving circuits  53   a  and  54 , and the switch driving circuit  52 , are designed to drive the SCR switches SW 4  and SW 2 , respectively, so that they are both able to conduct the high-energy defibrillation pulses of 200 or more joules, as well as remaining conducting during low-energy defibrillation pulses.  
     [0055] Switch driving circuit  53   a  receives control signal  42   cd  and outputs control signal  42   cd′ . Switch driving circuit  53   a  includes capacitors C 43  and C 44 , resistor R 44 , and component U 41 . Capacitor C 43  and resistor R 44  are coupled in parallel between the control signal line  42   cd  and ground. The negative power supply input of component U 41  is coupled to ground, while the positive power supply input is coupled to the battery voltage line SW-VBATT. Capacitor C 44  is coupled between the positive power supply input of component U 41  and ground. The input of component U 41  is coupled to the control signal line  42   cd,  while the output is coupled to the control signal line  42   cd′.    
     [0056] Driver circuit  54  receives control signal line  42   cd′ , and drives SCR switch SW 4 . Driver circuit  54  includes capacitors C 41  and C 42 , resistors R 41  and R 42 , and switch SW 41 . Control signal line  42   cd′  is coupled to the gate of switch SW 41 . Resistor R 41  is coupled between the gate of switch SW 41  and the source of switch SW 41 . Capacitor C 41  is coupled between the source of switch SW 41  and ground. The source of switch SW 41  is also coupled to the battery voltage line SW-VBATT. Resistor R 42  is coupled between the drain of switch SW 41  and the gate of SCR switch SW 4 . Capacitor C 42  is coupled between the gate of SCR switch SW 4  and ground.  
     [0057] It will be appreciated that the combination of the above-described driver circuits  53   a  and  54  are generally driven by the control signal lines  42   cd  and  42   cd′ , which as will be described in more detail below may in one embodiment carry an oscillating drive signal. It will be understood that in other embodiments, the SCR switch SW 4  may be driven by a DC gate drive signal, similar to the one used on control line  42   b  for SCR switch SW 2 . It will be appreciated that the driver circuit  52  may be advantageously used in combination with a DC gate drive signal that is applied to the lower SCR switch SW 2 . The benefits of this design can be illustrated by comparison with certain prior art defibrillators, which isolated the high-voltage H-bridge from the low-voltage control circuit ground potential. This type of prior art configuration required a transformer to couple a drive signal to the SCR gate, which consequently required the gate to be driven with AC signals, in that transformers are unable to pass DC signals. The utilization of a common ground for the high-voltage and low-voltage circuits in the present invention allows the gate of the SCR switch SW 2  to be driven directly from FET switch SW 21  with a DC signal.  
     [0058] SCR switch SW 3  is driven by drive circuit  53 . Drive circuit  53  includes a transformer T 31  for isolating the high-voltage SCR switch SW 3  from the low-voltage control circuitry. The isolation of the gate drive allows for the use of small, low voltage parts, in contrast to the relatively high-voltage gate drive components that would be required if the gate drive was not isolated. Switch driving circuit  53  is designed to drive the SCR switch SW 3  so that it is able to both conduct the high-energy defibrillation pulses of 200 or more joules as well as remaining conducting during lower energy defibrillation pulses.  
     [0059] Switch driving circuit  53  receives the control signal line  42   cd′  from switch driving circuit  53   a,  and drives SCR switch SW 3 . Switch driving circuit  53  includes switch SW 31 , resistors R 31   a,  R 31   b,  R 32 , transformer T 31 , diode D 31 , capacitor C 32 , and component ZD 31 . The gate of switch SW 31  receives the control signal line  42   cd′ . Resistor R 32  is coupled between the source of switch SW 31  and the gate of switch SW 31 . The source of switch SW 31  receives the battery voltage line SW-VBATT. The drain of switch SW 31  is coupled to the dotted end of the primary winding of transformer T 31 . Resistors R 31   a  and R 31   b  are coupled in parallel between the non-dotted end of the primary winding of transformer T 31  and ground. Component ZD 31  is coupled in parallel with the secondary winding of transformer T 31 . The anode of diode D 31  is coupled to the dotted end of the secondary winding of transformer T 31 , while the cathode is coupled to the gate of SCR switch SW 3 . Capacitor C 32  is coupled between the gate of SCR switch SW 3  and sternum line  19 . The non-dotted end of the secondary winding of transformer T 31  is coupled to sternum line  19 .  
     [0060] On the secondary winding side of transformer T 31 , the anode of diode D 31  is connected to the dotted end of the secondary winding of transformer T 31 , and the cathode of diode D 31  is coupled to the gate of SCR switch SW 3 . Capacitor C 32  is coupled between the cathode of diode D 31  and sternum line  19 . Sternum line  19  is coupled to the non-dotted end of the secondary winding of transformer T 31 . Component ZD 31  is coupled between the dotted and non-dotted ends of the secondary winding of transformer T 31 . As noted above, the anode of SCR switch SW 3  is coupled to the bridge line  26 , while the cathode is coupled to sternum line  19 .  
     [0061] To turn on switch SW 3 , an oscillating control signal is provided on control line  42   cd.  In this embodiment, the oscillating control signal may be a pulse train. In one embodiment the pulse train control signal on control line  42   cd′  is provided as a series of 10 pulses, the pulses being 1 microsecond wide and being provided every 2.5 microseconds. The pulse train control signal repeatedly turns control switch SW 31  on and off, producing a changing voltage across the primary winding of transformer T 31 . The voltage is stepped down by transformer T 31  and rectified by diode D 31  before being applied to the gate of SCR switch SW 3 . In one embodiment, a 10% duty cycle pulse train on the control line  42   cd  has been found to be adequate to maintain SCR switch SW 3  in a conducting state. As long as the control signal is applied to the switch driving circuit  53 , the switch SW 3  will generally remain in the conducting state. The switch SW 3  remains in the conducting state even when conducting relatively low defibrillation currents. As is well known, once triggered or latched on, an SCR generally remains in the conducting state until the current through the SCR drops below a minimum level (e.g., 90 mA), even if the gate voltage of the SCR is grounded.  
     [0062] Protection for the switches SW 1 -SW 4  is provided in part by protective component  27 , which has both inductive and resistive properties. Protective component  27  is coupled between bridge lines  26  and  30 . In one embodiment, protective component  27  is implemented with a coil of resistance wire that provides an inductive resistance. Protective component  27  limits the rate of change of the voltage across, and current flow to, SCR switches SW 2 , SW 3 , and SW 4 . Too high of a rate of change of the voltage across an SCR switch is undesirable because it can cause the SCR switch to inadvertently turn on. For example, since SCR switches SW 2  and SW 3  are on the same side of H-bridge  14 , any time SCR switch SW 3  is abruptly turned on, a rapid voltage change may also result across SCR switch SW 2 . To prevent rapid voltage changes, protective component  27  reduces the rate of change of the voltage across SCR switch SW 2  when SCR switch SW 3  is turned on. Also, too high of a current flow can damage the switches SW 2 , SW 3 , and SW 4 , and protective component  27  limits the current flow in H-bridge  14 . The use of protective component  27  therefore reduces the need for additional protective components that would otherwise need to be coupled to switches SW 2 , SW 3 , and SW 4 .  
     [0063] The H-bridge  14  also includes resistors R 1   a  and R 1   b . Resistor R 1   a  is coupled between apex line  17  and ground. Resistor R 1   b  is coupled between sternum line  19  and ground.  
     [0064] It will be appreciated that one advantage of H-bridge  14  described above is that it allows external defibrillator  8  to generate and apply a high-energy biphasic waveform to a patient. For prior defibrillators providing a monophasic waveform, the standard energy level in the industry for the discharge has been equal to or greater than 200 joules. The above described circuit allows the same amount of energy (approximately equal to or greater than 200 joules) to be delivered to the patient in a biphasic waveform, thereby resulting in a greater certainty of defibrillation effectiveness for a broader range of patients. At the same time, the circuit incorporates special driving circuitry to allow even very low energy biphasic waveforms to be delivered to the patient.  
     [0065]FIG. 3 also shows transfer relay  35 , which includes a relay  35   a  which is driven by drive signals RELAY 0  and RELAY 1 . As illustrated in FIG. 3, relay  35   a  has pins  1 ,  3 , and  4  on its right side, and pins  5 ,  6 , and  8  on its left side. Pin  4  is coupled to sternum line  19 , which when the relay is closed is connected to pin  3 , which is coupled to patient sternum line  19 ′, which is coupled to patient sternum electrode line  15   b.  Pin  5  is coupled to apex line  17 , which when the relay is closed is connected to pin  6 , which is coupled to patient apex line  17 ′, which is coupled to patient apex electrode line  15   a.  Pin  1  is coupled to the battery voltage line SW-VBATT. It will be appreciated that in some embodiments, the patient apex line  17 ′ and the patient apex electrode line  15   a  may actually be the same line, as may also be the case with the patient sternum line  19 ′ and the patient sternum electrode line  15   b.    
     [0066] Relay  35  also includes driving circuitry for driving relay  35   a,  which includes resistors R 51 -R 55 , switches SW 51  and SW 52 , and diodes D 51  and D 54 . The anode of diode D 54  is coupled to pin  8  of relay  35   a,  while the cathode of diode D 54  is coupled to pin  1  of relay  35   a.  The drain of switch SW 51  is coupled to pin  8  of relay  35   a.  The gate of switch SW 51  is coupled to the drive signal RELAY 0 . Resistor R 51  is coupled between the gate of switch SW 51  and ground. The source of switch SW 51  is coupled to the drain of switch SW 52 . The gate of switch SW 52  is coupled to the drive signal RELAY 1 . Resistor R 52  is coupled between the gate of switch SW 52  and ground. The source of SW 52  is coupled to ground. Resistor R 53  is coupled between the drain of switch SW 51  and the source of SW 51 . Resistor R 54  is coupled between the drain of switch SW 52  and ground. The anode of diode D 51  is coupled to the drain of switch SW 52 , and the cathode of diode D 51  is coupled to the voltage line +3.3V-A. Resistor R 55  is coupled between the drain of switch SW 52  and the control signal line RELAY-FETS.  
     [0067] Transfer relay  35  is operated such that when the defibrillator  8  is to apply a defibrillation pulse to a patient  16 , the relay  35   a  is closed. When the relay  35   a  is open, it isolates the patient  16  from the rest of the defibrillator  8  circuitry. As described above, the transfer relay  35  includes drive circuitry for driving the relay  35   a.  The drive circuitry is controlled by the control signal lines RELAY 0 , RELAY 1 , and RELAY-FETS.  
     [0068]FIG. 4 is a block diagram of the charging circuit  18  that is used to charge the energy storage capacitor  24 . As described above, the charging circuit  18  is referenced to the same common ground  28  as the output circuit  14  and the preamplifier circuit  37 . As described above, the measurement and control circuit  10  controls the charging circuit  18  to charge the energy storage capacitor to a desired voltage level. After charging to a desired level, the energy stored in the energy storage capacitor may be delivered to the patient  16  in the form of a defibrillation pulse.  
     [0069] As shown in FIG. 4, the charging circuit  18  includes a transformer circuit  110 , a capacitor voltage measurement circuit  120 , a flyback sense circuit  130 , an overvoltage comparator circuit  140 , and a charger control circuit  150 . Transformer circuit  110  receives the battery voltage line SW-VBATT and provides a stepped-up voltage on the voltage line CAP+. This stepped up voltage on the voltage line CAP+ is used to charge the energy storage capacitor  24  (FIG. 2), and is also provided as an input to the capacitor voltage measurement circuit  120 . The capacitor voltage measurement circuit  120  provides two output signal lines VCAP-HV 1  and VCAP-HV 2 , which represent the voltage on the energy storage capacitor  24 , and provides the measurements to the control circuit for the defibrillator  8 . The signal line VCAP-HV 2  is also coupled as an input to the overvoltage comparator circuit  140 . Overvoltage comparator circuit  140  also receives a control signal from the flyback-sense circuit  130 .  
     [0070] Flyback-sense circuit  130  receives a control signal line CHARGE 0 , and also a control signal line FLYBACK-SENSE from the charger control circuit  150 . Flyback sense circuit  130  inhibits the signal on control signal line FLYBACK-SENSE for a specified time period (e.g., 40 milliseconds) after the signal on the control signal line CHARGE 0  goes high. The overvoltage comparator circuit  140  receives the measurement signal line VCAP-HV 2 , as well as the control signal from the flyback sense circuit  130 . The overvoltage comparator circuit  140  provides an output to the charger control circuit  150 . Charger control circuit  150  receives control signal lines CHARGE 1  and CHARGE-RATE-DAC, and outputs a control signal to control the charging of the transformer  110 .  
     [0071]FIG. 5 is a schematic diagram of a preferred embodiment of the charging circuit  18  of FIG. 4. As illustrated in FIG. 5, the transformer circuit  110  includes a transformer T 111 , a diode D 111 , and capacitors C 111 A and C 111 B. In one embodiment, the transformer T 111  is able to step, up the voltage on the battery voltage line SW-VBATT to 2300 volts. The dotted end of the primary winding of the transformer T 111  is connected to the battery voltage line SW-VBATT. Capacitors C 111 A and C 111 B are coupled in parallel between the battery voltage line SW-VBATT and ground. The dotted end of the secondary winding of transformer T 111  is coupled to ground, while the non-dotted end of the secondary winding of transformer T 111  is coupled to the anode of diode D 111 . The cathode of diode D 111  is coupled to the positive terminal of the energy storage capacitor through the charging line CAP+/ 30  (referenced as line  30  in FIG. 2).  
     [0072] The charging line CAP+/ 30  is coupled to the capacitor voltage measurement circuit  120 . The capacitor voltage measurement circuit  120  includes resistors R 121  to R 126 , capacitors C 121  and C 122 , and components U 121  and U 122 . The resistor/capacitor structure for the components U 121  and U 122  are similar, therefore only the structure for component U 121  will be described herein. The negative input of component U 121  is coupled to the output of the component U 121 . The positive input of the component U 121  is coupled through resistor R 123  to ground. One side of resistor R 122  is coupled to the positive input of the component U 121 , while the other side of resistor R 122  is coupled to a circuit node between resistor R 121  and capacitor C 121 . Capacitor C 121  is coupled between the circuit node and ground, while resistor R 121  is coupled between the circuit node and the capacitor charging line CAP+/ 30 .  
     [0073] In one embodiment, the sizes of resistors R 121 -R 123  and capacitor C 121  are selected so that the gain of component U 121  is 1/735, while the sizes of the components R 124 -R 126  and C 122  are selected so that the gain of the component U 122  is 1/592. The output of the component U 121  is provided on the measurement line VCAP-HV 1 , while the output of the component U 122  is provided on the measurement line VCAP-HV 2 . The measurement line VCAP-HV 2  is also coupled to the overvoltage comparator circuit  140 . As described above, overvoltage comparator circuit  140  also receives a control signal from flyback sense circuit  130 .  
     [0074] Flyback sense circuit  130  includes resistors R 131 -R 141 , capacitors C 131 -C 135 , switches SW 131  and SW 132 , and components U 131  and U 132 . The signal line CHARGE 0  is coupled to the gate of switch SW 131 . Resistor R 131  is coupled between the gate of switch SW 131  and ground. The drain of switch SW 131  is coupled to the overvoltage comparator circuit  140 , while the source of switch SW 131  is coupled through resistor R 141  to ground. The gate of switch SW 131  is coupled through resistor R 132  to the negative input of component U 131 . The negative input of component U 131  is also coupled through capacitor C 131  to ground. The negative power supply input of component U 131  is coupled to ground, while the positive power supply input of component U 131  is coupled to the voltage line PAD+. Capacitor C 132  is coupled between the positive power supply input of component U 131  and ground. Resistor R 133  is coupled between a voltage line 2.5-VREF and the positive input of the component U 131 . Resistor R 134  is coupled between the positive input of component U 131  and the output of component U 131 . The output of component U 131  is coupled to the gate of switch SW 132 .  
     [0075] The source of switch SW 132  is coupled to ground, while the drain is coupled to the positive input of component U 132 . Capacitor C 133  and resistor R 136  are coupled in parallel between the drain of switch SW 132  and ground. Resistor R 137  is coupled between the drain of switch SW 132  and the signal line FLYBACK-SENSE from control charger circuit  150 . Resistor R 135  is coupled between the positive input of component U 132  and the output of component U 132 . The negative power supply input of component U 132  is coupled to ground, while the positive power supply input is coupled to the voltage line PAD+. Capacitor C 135  is coupled between the positive power supply input of the component U 132  and ground.  
     [0076] Capacitor C 134  and resistor R 138  are coupled in parallel between the negative input of component U 132  and ground. Resistor R 140  is coupled between the negative input of component U 132  and the voltage line 2.5-VREF. Resistor R 139  is coupled between the battery voltage line SW-VBATT, and the resistor R 138 . The output of component U 132  is coupled to the source of switch SW 131 . Resistor  141  is coupled between the source of switch SW 131  and ground. The drain of switch SW 131  is coupled to the overvoltage comparator circuit  140 .  
     [0077] Overvoltage comparator circuit  140  includes resistors R 142 -R 146 , capacitors C 141 , component U 141 , and switch SW 141 . The source of switch SW 141  is coupled to the drain of switch SW 131  of the flyback sense circuit  130 . The drain of switch SW 141  is coupled to the charger control circuit  150 . The gate of switch SW 141  is coupled to the output of component U 141 . Resistor R 142  is coupled between the measurement line VCAP-HV 2  and the negative input of the component U 141 . Resistor R 143  and capacitor C 141  are coupled in parallel between the negative input of the component U 141  and ground. Resistor R 144  is coupled between voltage line 2.5-VREF and the positive input of component U 141 . Resistor R 145  is coupled between the positive input of component U 141  and the output of component U 141 . Resistor R 146  is coupled between the output of component U 141  and the voltage line PAD+. In one embodiment, the overvoltage comparator circuit  140  operates to cut off the charging voltage for the capacitor at 2300 volts.  
     [0078] Charger control circuit  150  receives a signal from the drain of switch SW 141  in the overvoltage comparator circuit  140 . Charger control circuit  150  includes resistors R 151 -R 164 , capacitors C 151 -C 158 , switches SW 151 -SW 153 , diodes D 151  and D 152 , components U 151 -U 153 , heat sink H 151 , and zener diode ZD 151 . The input signal line CHARGE 1  is coupled to the gate of switch SW 151 . Resistor R 151  is coupled between the gate of switch SW 151  and ground. The source of switch SW 151  is coupled to ground, while the drain of switch SW 151  is coupled to the gate of switch SW 152 . Resistor R 152  is coupled between the gate of switch SW 152  and the source of switch SW 152 . The source of switch SW 152  is coupled to the battery voltage line SW-VBATT. The drain of switch SW 152  is coupled to the VCC pin  14  of component U 151 .  
     [0079] Component U 151  has  14  pins, including a DRIVE+ pin  1 , a DRIVE− pin  2 , a RAMP pin  3 , an INHIBIT pin  4 , an RT/CT pin  5 , a 2.5 V-REF pin  6 , a GND pin  7 , a 1.25 V-REF pin  8 , an ERR+ pin  9 , an ERR− pin  10 , an FB pin  11 , an SFT-STRT pin  12 , a RUN/STRT pin  13 , and a VCC pin  14 . DRIVE+ pin  1  is coupled through resistor R 162  to the gate of switch SW 153 . DRIVE− pin  2  is coupled to ground. RAMP pin  3  is coupled to the output of component U 152 . INHIBIT pin  4  is coupled through resistor R 153  to VCC pin  14 . INHIBIT pin  4  is also coupled to the drain of switch SW 151  from the overvoltage comparator circuit  140 . RT/CT pin  5  is coupled through capacitor C 154  to ground, and is also coupled through resistor R 156  to the reference voltage line 2.5-VREF. Capacitor C 153  is coupled between the reference voltage line 2.5-VREF and ground. 2.5 V-REF pin  6  is coupled to the reference voltage line 2.5-VREF. GND pin  7  is coupled to ground. 1.25 V-REF pin  8  is coupled to the reference voltage line 1.25-VREF. Capacitor C 152  is coupled between the reference voltage line 1.25-VREF and ground. ERR+ pin  9  is coupled to the reference voltage line 2.5-VREF and is also coupled through a resistor R 154  to FB pin  11 . FB pin  11  is coupled through resistor R 155  to ground. ERR−+pin  10  is coupled to pin  8 . SFT-STRT pin  12  is coupled through capacitor C 155  to ground.  
     [0080] As noted above, DRIVE+ pin  1  of component U 151  is coupled by resistor R 162  to the gate of switch SW 153 . Resistor R 163  is coupled between the gate of switch SW 153  and ground. The source of switch SW 153  is coupled to ground, while the drain is coupled to the dotted end of the primary winding of transformer T 151 . Heat sink H 151  is coupled between the dotted end of the primary winding of transformer T 151  and the cathode of zener diode ZD 151 . The anode of zener diode ZD 151  is coupled to ground. The non-dotted end of the primary winding of transformer T 151  is coupled to the non-dotted end of the primary winding of transformer T 111  of transformer circuit  110 .  
     [0081] The dotted end of the secondary winding of the transformer T 151  is coupled to ground. The anode of diode D 152  is coupled to ground, while the cathode is coupled through resistor R 164  to the non-dotted end of the secondary winding of transformer T 151 . The anode of diode D 151  is coupled to the non-dotted end of the secondary winding of the transformer T 151 . The cathode of diode D 151  is coupled through resistor R 157  to the positive input of component U 152 . The cathode of diode D 151  is also coupled through resistor R 158  to ground. The positive input of the component U 152  is also coupled through capacitor C 156  to ground.  
     [0082] As described above, the output of the component U 152  is coupled to RAMP pin  3  of component U 151 . Capacitor C 157  and resistor R 159  are coupled in parallel between the negative input of component U 152  and ground. Resistor R 160  is coupled between the negative input of component U 152  and the output of component U 153 . The negative input of component U 153  is coupled to the output of component U 153 . Resistor R 161  and capacitor C 158  are coupled in parallel between the positive input of the component U 153  and ground. The positive input of component U 153  is also coupled to the control signal line CHRG-RATE-DAC.  
     [0083]FIG. 6 is a block diagram of the preamplifier ECG and impedance drive and measurement circuit  37  of FIG. 2. As shown in FIG. 6, the preamplifier ECG and impedance drive and measurement circuit  37  includes an ECG preamp circuit  37   a,  an impedance demodulator circuit  37   b,  and an ECG A-to-D converter circuit  37   c.  A preferred embodiment of the ECG preamplifier circuit  37   a  will be described in more detail below with reference to the schematic diagram shown in FIG. 7A, while a preferred embodiment of the impedance demodulator circuit  37   b  will be described in more detail below with reference to the schematic diagram shown in FIG. 7B, and a preferred embodiment of the ECG A-to-D converter circuit  37   c  will be described in more detail below with reference to the schematic diagram shown in FIG. 7C.  
     [0084] As shown in FIG. 6, the ECG preamplifier circuit  37   a  includes a preamp instrumentation amplifier  210 , a self test switch  225 , an ECG gain and filter circuit  230 , and a self test switches and impedance drive circuit  245 . The preamp instrumentation amplifier circuit  210  measures the apex and sternum voltages over the lines APEX/ 15   a  and STERNUM/l  5   b.  Preamp instrumentation amplifier circuit  210  outputs a signal on the signal line PREAMP. Self test switch circuit  225  passes the signal on the signal line PREAMP depending on the control signal from the self test switches and impedance drive circuit  245  and the control signal line SELFTEST-IA. Self test switches and impedance drive circuit  245  receives control signals on signal lines SELFTEST-STEP, SELFTEST-Z, and DRIVE-IMP-SW. The ECG gain and filter circuit  230  receives the signal line PREAMP and provides an output on the signal line ECG. The ECG gain and filter circuit  230  includes low frequency filter and gain stages for processing the information on the signal line PREAMP for ECG signals.  
     [0085] The impedance demodulator circuit  37   b  includes the impedance gain circuit  250 , the impedance demodulator drive circuit  260 , the impedance demodulator circuit  270 , the impedance reactive filter circuit  280 , the impedance resistive filter circuit  285 , and the impedance motion filter circuit  290 . In general, the impedance demodulator circuit  37   b  utilizes high frequency filter and gain stages to process the information received on the signal line PREAMP to determine relevant impedance information. As shown in FIG. 6, the impedance gain circuit  250  receives the signal line PREAMP and outputs a signal to the impedance demodulator circuit  270 . Impedance demodulator circuit  270  is controlled in part by impedance demodulator drive circuit  260 , which receives a clock signal on the signal line Z-CLOCK. Impedance demodulator circuit  270  outputs a first signal to the impedance reactive filter circuit  280 , and a second signal to the impedance resistive filter circuit  285 . The impedance reactive filter circuit  280  outputs a signal line Z-REACTIVE, while the impedance resistive filter circuit  285  outputs a signal line Z-RESISTIVE. The impedance motion filter circuit  290  receives the signal line Z-RESISTIVE as an input, and outputs a signal line Z-MOTION. The ECG A-to-D converter circuit  37   c  receives the signal lines Z-REACTIVE, Z-RESISTIVE, Z-MOTION, and ECG, and outputs a signal line AD-DOUT.  
     [0086]FIG. 7A is a schematic diagram of a preferred embodiment of the ECG preamp circuit  37   a  of FIG. 6. As shown in FIG. 7A, the preamp instrumentation amplifer circuit  210  includes resistors R 211 -R 226 , capacitors C 211 -C 219 , and components U 211 -U 213 . Resistor R 211  is coupled between the patient apex signal line  15   a /APEX and a circuit node with the capacitor C 211 , which is also coupled through resistor R 212  to ground. The other side of capacitor C 211  is coupled through resistor R 213  to the output of component U 211 . The output of component U 211  is coupled through resistor R 214  to the negative input of component U 211 . The negative input of component U 211  is also coupled through resistor R 215  to a voltage line ZDP. The positive input of component U 211  is coupled to ground.  
     [0087] Resistor R 216  is coupled between the patient sternum line  15   b /STERNUM and a circuit node with the capacitor C 212 , which is also coupled through a resistor R 217  to ground. The other side of capacitor C 212  is coupled through resistor R 218  to the output of component U 211 . Capacitor C 213  and resistor R 219  are coupled in parallel between the output of component U 211  and the negative input of component U 211 . The negative input of component U 211  is also coupled through resistor R 220  to the self test switches and impedance drive circuit  245 . The positive input of component U 212  is coupled to ground.  
     [0088] Resistor R 221  is coupled between the patient apex line  15   a /APEX and a circuit node with capacitor C 214 , capacitor C 216 , and resistor R 222 . The other side of capacitor C 214  is coupled to ground, while the other side of capacitor C 216  is coupled to a circuit node with resistor R 223 , capacitor C 215 , and resistor R 224 , and the other side of resistor R 222  is coupled to the positive input of component U 213 . Resistor R 223  is coupled between the patient sternum line  15   b /STERNUM and the circuit node with capacitor C 216 . The other side of capacitor C 215  is coupled to ground while the other side of resistor  224  is coupled to the negative input of component U 213 . Capacitor C 217  and resistor R 225  are coupled in series between the positive input of the component U 213  and the negative input of the component U 213 .  
     [0089] Resistor R 226  is coupled between a first input RG of the component U 213  and a second input RG of the component U 213 . The positive power supply of component U 213  is coupled to the voltage line PAD+, which is also coupled through capacitor C 218  to ground. The negative power voltage line of the component U 213  is coupled to the voltage line PAD−, which is also coupled through capacitor C 219  to ground. The reference input REF of component U 213  is coupled to ground. The output of component U 213  is coupled to the self test switch circuit  225 .  
     [0090] Self test switch circuit  225  includes capacitors C 226 -C 228  and a component U 225 . Component U 225  has eight pins, including a D pin  1 , a S 1  pin  2 , a GND pin  3 , a VDD pin  4 , an LV pin  5 , an IN pin  6 , a VSS pin  7 , and an S 2  pin  8 . D pin  1  provides the signals on signal line PREAMP and is coupled to both the ECG gain and filter circuit  230  and to the impedance gain circuit  250  (FIG. 7B). S 1  pin  2  is coupled to the output of component U 213  of the preamp instrumentation amplifier circuit  210 . GND pin  3  is coupled to ground, and is also coupled through capacitor C 228  to VSS pin  7 . VSS pin  7  is also coupled to the voltage line PAD−. VDD pin  4  is coupled to the voltage line PAD+. Capacitor C 226  and C 227  are coupled in parallel between VDD pin  4  and ground. LV pin  5  is coupled to the voltage line PAD+. IN pin  6  is coupled to the control signal line SELFTEST-IA. S 2  pin  8  is coupled to the self test switches and impedance drive circuit  245 .  
     [0091] The ECG gain and filter circuit  230  includes resistors R 231 -R 245 , capacitors C 231 -C 247 , and components U 231 -U 233 . Resistor R 231  is coupled between D pin  1  of component U 225  of self test switch  225  and a circuit node with resistor R 236 , resistor R 233 , and capacitor C 231 . The other side of capacitor C 231  is coupled to ground, while the other side of resistor R 233  is coupled to the negative input of component U 231 , and the other side of resistor R 236  is coupled to a circuit node with capacitor C 235 , resistor R 235 , resistor R 237 , the cathode of diode D 31 , the anode of diode D 32 , and resistor R 242 . The other side of capacitor C 235  is coupled to the negative input of component U 231 , while the other side of resistor R 235  is coupled to the output of component U 231 , and the other side of resistor R 237  is coupled to a circuit node with resistor R 238  and resistor R 239 , and the other side of resistor R 242  is coupled to a circuit node with resistor R 243 , resistor R 245 , and capacitor C 243 . The anode of diode D 231  is coupled to the cathode of diode D 232 . Capacitor C 239  is coupled between the anode of diode D 231  and the voltage line PAD−, while the cathode of diode D 232  is coupled through resistor R 241  to the voltage line BIAS.  
     [0092] From the circuit node between the resistor R 242 , the resistor R 243 , the resistor R 245 , and the capacitor U 243 , the other side of the resistor R 243  is coupled to the negative input of component R 233 , while the other side of resistor R 245  is coupled to the output of component U 233 , and the other side of capacitor C 243  is coupled to ground. Resistor R 244  and capacitor C 246  are coupled in series between the negative input of component U 233  and the positive input of component U 233 . The positive input of component U 233  is also coupled to the voltage line BIAS. Capacitor C 247  is coupled between the negative input of component U 233  and the output of component U 233 . The positive input of component U 233  is coupled through capacitor C 245  to ground. The negative input of component U 233  is coupled through capacitor C 244  to ground.  
     [0093] The positive input of component U 232  is coupled to the voltage line BIAS. Capacitor C 241  is coupled between the positive input of component U 232  and ground, while capacitor C 242  is coupled between the negative input of component U 232  and ground. Resistor R 240  and capacitor C 240  are coupled in series between the positive input of component U 232  and the negative input of component U 232 . With regard to the circuit node between resistors R 237 , R 238 , and R 239 , the other side of resistor R 238  is coupled to the negative input of component U 232 , while the other side of resistor R 239  is coupled to the voltage line BIAS. The negative input of component U 232  is coupled through capacitor C 238  to the output of component U 232 . The output of component U 232  is coupled through resistor R 232  to ground.  
     [0094] The positive power supply input of component U 232  is coupled to the voltage line PAD+, while the negative power supply input of component U 232  is coupled to the voltage line PAD−. Capacitor C 236  is coupled between the positive power supply input for component U 232  and ground, while capacitor C 237  is coupled between the negative power supply input for component U 232  and ground. Capacitor C 234  and resistor R 234  are coupled in series between the negative input of component U 231  and the positive input of component U 231 . Capacitor C 232  is coupled between the positive input of component U 231  and ground, while capacitor C 233  is coupled between the negative input of component U 231  and ground.  
     [0095] Self test switches and impedance drive circuit  245  includes resistors R 246 -R 252 , capacitors C 248 -C 251 , and components U 245 -U 249 . Component U 245  has five connected pins, including a D pin  1 , a S pin  2 , a GND pin  3 , an IN pin  4 , and a VDD pin  6 . S pin  2  is coupled through resistor R 246  to ground, and is also coupled through resistor R 247  to the voltage line VDN. IN pin  4  is coupled to the control signal line SELFTEST-STEP. GND pin  3  is coupled to ground. VDD pin  6  is coupled to the voltage line PAD+. D pin  1  is coupled to the positive input of component U 246 .  
     [0096] Resistor R 248  is coupled between the positive input of component U 246  and the voltage line PAD+, while resistor R 249  is coupled between the positive input of component U 246  and ground, and resistor R 250  is coupled between the positive input of component U 246  and the voltage ZDN. The negative input of component U 246  is coupled to the output of component U 246 . The positive power supply input of component U 246  is coupled to the voltage line PAD+, while the negative power supply input of component U 246  is coupled to the voltage line PAD−. Capacitor C 248  is coupled between the positive voltage line input of component U 246  and ground, while capacitor C 249  is coupled between the negative power supply input of component U 246  and ground. The output of component U 246  is coupled to the S 2  pin  8  of component U 225  of self test switch  225 .  
     [0097] Component U 247  has six connected pins, including an N pin  1 , a VDD pin  2 , a GND pin  3 , a S 1  pin  4 , a D pin  5 , and a S 2  pin  6 . N pin  1  is coupled to the control signal line DRIVE-IMP-SW. VDD pin  2  is coupled to the voltage line PAD+. GND pin  3  is coupled to ground. S 1  pin  4  is coupled to the voltage line BIAS. D pin  5  is coupled through resistor R 251  to the positive input of component U 249 . S 2  pin  6  is coupled to ground.  
     [0098] Component U 248  has five connected pins, including a D pin  1 , an S pin  2 , a GND pin  3 , an N pin  4 , and a VDD pin  6 . N pin  4  is coupled to the control signal line SELFTEST-Z. S pin  2  is coupled through resistor R 252  to ground. GND pin  3  is coupled to ground. VDD pin  6  is coupled to the voltage line PAD+. D pin  1  is coupled to the positive input of component U 249 . The positive input of component U 249  is coupled through capacitor C 250  to ground. The negative input of component U 249  is coupled to the output of component U 249 . Capacitor C 251  is coupled between the output of component U 249  and a circuit node with resistor R 220  of the preamp instrumentation amplifier circuit  210 . As described above, the other side of resistor R 220  is coupled to the negative input of component U 212 .  
     [0099] With regard to the above-described components in the schematic diagram of FIG. 7A, it will be understood that certain of the components are included primarily for self test purposes. For example, capacitors C 226 -C 228 , components U 245 -U 246 , capacitors C 248 -C 249 , resistors R 246 -R 250 , component U 248 , and resistor R 252  are included only for self test purposes. Component U 225  is also included only for self test purposes, although its removal would require adding a line between pin  6  of component U 213  and resistor R 231 .  
     [0100]FIG. 7B is a schematic diagram of the impedance demodulator circuit  37   b  of FIG. 6. As shown in FIG. 7B, the impedance demodulator drive circuit  260  includes resistors R 261 -R 269 , components U 261 -U 266 , and a capacitor C 261 . Signal line Z-CLOCK is coupled to the input of component U 261 . The input of component U 261  is also coupled through resistor R 261  ground. The output of component U 261  is coupled to an input pin  11  of component U 262 .  
     [0101] Component U 262  has six connected pins including a Q pin  8 , a Q pin  9 , a PR pin  10 , an input pin  11 , a D pin  12 , and a CL pin  13 . Q pin  8  is coupled to D pin  12 , and is also coupled to pin  3  of component U 265 . Q pin  9  is coupled to input pin  3  of component U 263 . PR pin  10  is coupled by resistor R 262  to the voltage line PAD+. CL pin  13  is coupled by resistor R 263  to the voltage line PAD+.  
     [0102] Component U 263  has six connected pins including a CL pin  1 , a D pin  2 , an input pin  3 , a PR pin  4 , a Q pin  5 , and a Q pin  6 . CL pin  1  is coupled by resistor  265  to the voltage line PAD+. D pin  2  is coupled to Q pin  6 , and is also coupled to pin  2  of component U 265 . Input pin  3  is coupled to pin  9  of component U 262 . PR pin  4  is coupled by resistor R 264  to the voltage line PAD+. Q pin  5  is coupled by signal line ZDM 1  to pin  1  of component  270 .  
     [0103] Component U 264  has five connected pins including a Q pin  8 , a PR pin  10 , an input pin  11 , a D pin  12 , and a CL pin  13 . Q pin  8  is coupled to the signal line DRIVE−IMP−SW. PR pin  10  is coupled by resistor R 266  to the voltage line PAD+. Input pin  11  is coupled to the output of component U 266 . CL pin  13  is coupled by resistor R 267  to the voltage line PAD+.  
     [0104] Component U 265  has five connected pins, including a CL pin  1 , a D pin  2 , an input pin  3 , a PR pin  4 , and a Q pin  5 . CL pin  1  is coupled by resistor R 269  to the voltage line PAD+. D pin  2  is coupled to the circuit node between pin  12  of component U 264 , pin  6  of component U 263 , and pin  2  of component U 263 . Input pin  3  is coupled to the circuit node between pin  8  of component U 262  and pin  12  of component U 262 . PR pin  4  is coupled by resistor R 268  to the voltage line PAD+. Q pin  5  is coupled by signal line ZDM 2  to pin  5  of component  270 .  
     [0105] The input of component  266  is coupled to the output of component U 261 . The positive power supply input of component U 266  is coupled to the voltage line PAD+. The positive power supply input of component U 266  is also coupled by capacitor C 261  to ground. The negative power supply input of component U 266  is coupled to ground. The output of component U 266  is coupled to pin  11  of component U 264 .  
     [0106] Impedance gain circuit  250  includes resistors R 254 -R 259 , capacitors C 252 -C 255 , and components U 251 -U 253 . Resistor R 254  is coupled between the signal line PREAMP from the self test switch  225  (FIG. 7A) and a circuit node with the capacitor C 252 . The other side of the capacitor C 252  is coupled to the negative input of component U 251 . Capacitor C 253  and resistor R 255  are coupled in parallel between the negative input of component U 251  and the output of component of U 251 . The positive input of component U 251  is coupled to ground. Resistor R 256  is coupled between the output of component U 251  and a circuit node with capacitor  254 . The other side of capacitor  254  is coupled to the negative input of component U 252 .  
     [0107] Capacitor C 255  and resistor R 257  are coupled in parallel between the negative input of component U 252  and the output of component U 252 . The positive input of component U 252  is coupled to the voltage line BIAS. The output of component U 252  is coupled to pins  2  and  4  of component  270 . The output of component U 252  is also coupled by resistor R 258  to the negative input of component U 253 . The negative input of component U 253  is coupled by resistor R 259  to the output of component U 253 . The positive input of component U 253  is coupled to the voltage line BIAS. The output of component U 253  is coupled to pins  9  and  7  of impedance demodulator circuit  270 .  
     [0108] Impedance demodulator circuit  270  has ten connected pins, including an IN 1  pin  1 , an S 1 A pin  2 , a GND pin  3 , an S 2 A pin  4 , an IN 2  pin  5 , a D 2  pin  6 , an S 2 B pin  7 , a VDD pin  8 , an S 1 B pin  9 , and a D 1  pin  10 . IN 1  pin  1  is coupled to the signal line ZDM 1 . IN 2  pin  5  is coupled to the signal line ZDM 2 . S 1 A pin  2  is coupled to S 2 A pin  4 , which is also coupled to the output of component U 252  of the impedance gain circuit  250 . S 1 B pin  9  is coupled to S 2 B pin  7 , which is also coupled to the output of component U 253  of the impedance gain circuit  250 . GND pin  3  is coupled to ground, while VDD pin  8  is coupled to the voltage line PAD+. VDD pin  8  is also coupled by capacitor C 270  to ground D 1  pin  10  is coupled to the impedance reactive filter circuit  280 . D 2  pin  6  is coupled to the impedance resistive filter circuit  285 .  
     [0109] Impedance reactive filter circuit  280  and impedance resistive filter circuit  285  are of similar construction, and thus only the construction of impedance reactive filter circuit  280  will be described herein. Impedance reactive filter circuit  280  includes resistors R 280 -R 284 , capacitors C 280 -C 281  and a component U 280 . Resistor R 280  is coupled between D 1  pin  10  of impedance demodulator  270  and a circuit node between capacitor C 280  and resistor R 281 . The other side of capacitor C 280  is coupled to ground, while the other side of resistor R 281  is coupled to a circuit node with capacitor C 282  and resistor R 282 . The other side of capacitor C 282  is coupled to the output of component U 280 , while the other side of resistor R 2822  is coupled to the positive input of component U 280 .  
     [0110] The positive input of component U 280  is coupled by capacitor C 281  to ground. The negative input of component U 280  is coupled by resistor R 283  to the output of component U 280 . The negative input of component U 280  is also coupled by resistor R 284  to the voltage line BIAS. The output of component U 280  is coupled to the signal line Z-REACTIVE. As noted above, the impedance resistive filter circuit  285  is of similar construction to the impedance reactive filter circuit  280  and will not be described further herein, other than to note that within the impedance resistive filter circuit  285  the resistor R 285  is coupled to the D 2  pin  6  of the impedance demodulator  270 , and the output of component U 285  is coupled to the signal line Z-RESISTIVE.  
     [0111] Impedance motion filter  290  includes resistors R 290 -R 295 , capacitors C 290 -C 293  and components U 290  and U 291 . Capacitor C 290  is coupled between the signal line Z-RESISTIVE and the positive input of component U 290 . The positive input of component U 290  is also coupled by resistor R 290  to the voltage line BIAS. The negative input of component U 290  is coupled by resistor R 291  to the voltage line BIAS. Capacitor C 291  and resistor R 292  are coupled in parallel between the negative input of component U 290  and the output of component U 290 . Capacitor C 292  is coupled between the output of component U 290  and the positive input of component U 291 . The positive input of component U 291  is coupled by resistor R 293  to the voltage line BIAS. The negative input of component U 291  is coupled by resistor R 294  to the voltage line BIAS. Capacitor C 293  and resistor R 295  are coupled in parallel between the negative input of component U 291  and the output of component U 291 . The output of component U 291  is coupled to the signal line Z-MOTION.  
     [0112]FIG. 7C is a schematic diagram of the ECG A-to-D converter circuit  37   c  of FIG. 6. As shown in FIG. 7C, the ECG A-to-D converter circuit  37   c  includes resistors R 296 -R 299 , components U 296 -U 299 , and a capacitor C 296 . Signal line AD-DIN is coupled to the input of component U 297 . Resistor R 297  is coupled between the input of component U 297  and ground. A signal line AD-CLK is coupled to the input of component U 298 . Resistor R 298  is coupled between the input of component U 298  and ground. A signal line AD-CS is coupled to the input of component U 299 . Resistor R 299  is coupled between the input of component U 299  and ground. The output of component U 297  is coupled to pin  17  of component U 296 . The output of component U 298  is coupled to pin  19  of component U 296 . The output of component U 299  is coupled to pin  18  of component U 296 .  
     [0113] Component U 296  has nineteen connected pins, including a CH 0  pin  1 , a CH 1  pin  2 , a CH 2  pin  3 , a CH 3  pin  4 , a CH 4  pin  5 , a CH 5  pin  6 , a CH 6  pin  7 , a CH 7  pin  8 , a COM pin  9 , an SHDN pin  10 , a VREF pin  11 , a VCC pin  12 , a GND pin  13 , a GND pin  14 , a DOUT pin  15 , a DIN pin  17 , an SC pin  18 , an SLC pin  19 , and a VCC pin  20 . CH 0  pin  1  is coupled to the signal line ECG. CH 1  pin  2  is coupled to the signal line Z-RESISTIVE. CH 2  pin  3  is coupled to the signal line Z-REACTIVE. CH 3  pin  4  is coupled to the signal line Z-MOTION. CH 4  pin  5  is coupled to the voltage line 3.3V-A. CH 5  pin  6  is coupled to the signal line TEMPERATURE. CH 7  pin  8  is coupled to the signal line VCAP-HV 2 . CH 7  pin  8  is coupled by resistor  296  to ground. COM pin  9  is coupled to GND pin  13  and to GND pin  14 , each of which are coupled to ground. SHDN pin  10  is coupled to REF pin  11 , which is coupled to the voltage line PAD+. VCC pin  12  is coupled to VCC pin  20 , which is coupled to the voltage line PAD+. VCC pin  12  and VCC pin  20  are also coupled by capacitor C 296  to ground. The OUT pin  15  is coupled to the signal line AD-DOUT. DIN pin  17  is coupled to the output of component U 297 . CS pin  18  is coupled to the output of component U 299 . CLK pin  19  is coupled to the output of component U 298 .  
     [0114] As described above, the preamplifier ECG and impedance drive and measurement circuit  37  is referenced to the same common ground as the output circuit  14  and charging circuit  18 . In one embodiment, it will be understood that it is the preamplifier power supply that is referenced to the common ground, as opposed to the preamplifier input. In other words, the preamplifier power supply is not isolated from the output circuit  14 . Thus, the power supplies are not electrically isolated from one another.  
     [0115] It will be appreciated that the defibrillator  8  described above with reference to FIGS.  1 - 7 C provides a number of advantages over prior art defibrillators. The utilization of a common ground for the defibrillator results in a simpler circuit design than was required in prior art defibrillators which utilized isolation circuits for the high and low voltage circuitry. The utilization of the common ground also allows one or both of the SCRs in the two lower legs of the H-bridge to be driven with DC gate drive signals, thus reducing the complexity of the drive circuits. In addition, the placement of the IGBT in the northwest leg of the H-bridge is an improved design over prior art defibrillators which placed the IGBT in the southeast leg. These aspects of the design result in a defibrillator that is simpler, less expensive, and operates more effectively than prior art defibrillators.  
     [0116] While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.