Abstract:
A method and apparatus for providing galvanic isolation for signal communication between two electrical circuits via a capacitive coupler that is constructed from conductive and non-conductive layers of a printed circuit board. The invention can provide at low cost and with substantial galvanic isolation, the communication of data at rates of greater than 1 megabit per second. The galvanic isolation provided by the invention protects against common mode fields as well as limited differential mode fields. The invention makes use of pre-existing layers of a circuit board and does not require many other or expensive electrical components.

Description:
CROSS-REFERENCE TO APPLICATIONS INCLUDING RELATED SUBJECT MATTER  
       [0001]     This application includes subject matter that is related to subject matter included within U.S. design patent application Ser. No. 29/217,149 filed Nov. 12, 2004 (attorney docket number  281   — 443). 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to providing galvanic isolation for communication of a signal between two electrical circuits, and in particular to providing galvanic isolation for communication of a signal within a medical device, such as for communication of an ECG signal between two electrical circuits via a capacitive coupler that is constructed from at least some of the conductive and non-conductive layers of a printed circuit board.  
       BACKGROUND OF THE INVENTION  
       [0003]     Medical devices are typically operated inside of a health care environment in close proximity to patients, other electrical devices and other objects made of conductive material. As a result, there is a risk of unwanted transfer of electrical energy and signal interference between such devices and such objects while providing health care to a patient.  
         [0004]     For example, an electrocardiogram (ECG) monitoring apparatus receives and processes electrocardiogram (ECG) signals generated by a circulatory system of a person. The apparatus typically includes a plurality of ECG (patient contact) electrodes that are each electrically connected to a lead wire and that are each configured to make physical contact with the person being monitored. The ECG electrodes and lead wires are also configured to receive and relay ECG signals generated by the person to components of the ECG monitoring apparatus that process the ECG signals.  
         [0005]     In some circumstances, the person may be experiencing some sort of cardiovascular instability, such as ventricular fibrillation. Ventricular fibrillation is a disturbance of electrical activity within a ventricular muscle of the heart. In order to arrest ventricular fibrillation, the patient may be administered a defibrillation shock via defibrillating device. In some circumstances, the patient may be administered the defibrillation shock while the patient is being monitored by an ECG monitoring apparatus. The defibrillation shock can create a voltage surge that can unintentionally conduct (travel) through one or more of the ECG contact electrodes and/or lead wires and cause interference with the communication of data between the ECG electrode and components of the ECG monitoring apparatus that process the ECG signals. Further, the defibrillation shock not only interferes with the communication of data, but if not galvanically isolated, the defibrillation shock could also travel through the device and harm the user.  
       SUMMARY OF THE INVENTION  
       [0006]     The invention provides galvanic isolation for signal communication between two electrical circuits via a capacitive coupler that is constructed from conductive and non-conductive layers of a printed circuit board. The invention can provide at low cost and with substantial galvanic isolation, the communication of data at rates of greater than 1 megabit per second. The galvanic isolation provided by the invention protects against common mode fields as well as limited differential mode fields. The invention makes use of pre-existing layers of a circuit board and does not require many other or expensive electrical components.  
         [0007]     In one type of embodiment, the invention provides for galvanic isolation of signal communication within a medical device, such as within an electrocardiogram (ECG) monitoring apparatus. For example, the invention provides for an electrocardiogram (ECG) monitoring apparatus and method with improved galvanic isolation between signal receiving low voltage electronics that process ECG signals and a front end portion of the apparatus that receives and relays the ECG signals to the signal receiving electronics.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The objects and features of the invention can be better understood with reference to the claims and drawings described below. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views. Differences between like parts may cause those parts to be indicated by different numerals. Unlike parts are indicated by different numerals.  
         [0009]      FIG. 1A  is a top view of an embodiment of an ECG monitoring apparatus including ten patient contact lead wires and electrodes that are configured to attach to a patient.  
         [0010]      FIG. 2  is an illustration of an embodiment of electronics that provide galvanic isolation for the communication of ECG signals.  
         [0011]      FIG. 3A  is an illustration of a side cross-sectional view of the capacitive coupler of  FIG. 3A  that is constructed from the conductive and non-conductive layers of a printed circuit board.  
         [0012]      FIG. 3B  is an illustration of a top view of the capacitive coupler of  FIG. 3A  that is constructed from the conductive and non-conductive layers of a printed circuit board. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 1A  is a top conceptual view of an embodiment of an ECG monitoring apparatus  120  including (10) ECG patient contact lead wires  122   a - 122   j  that include patient contact electrodes  210   a - 210   j  and that are configured to be attached to a person, also referred to as a patient. When the ECG lead wires  122   a - 122   j  are attached to the patient, the ECG signals generated by the patient (not shown) are received by the patient contact lead wires  122   a - 122   j  and processed by the ECG monitoring apparatus  120 . The (8) ECG lead wires  122   a - 122   h  are configured to make contact with the upper body (chest and arms) of the patient. The (2) lead wires  122   i - 122   j  are configured to make contact with the lower body (legs) of the patient.  
         [0014]     In some circumstances, the patient may be administered a defibrillation shock (voltage surge), of typically about 2000 volts (200 joules) while being monitored by the ECG monitoring apparatus  120 . A defibrillation shock can arrest instabilities of cardiac activity occurring within the patient.  
         [0015]     The defibrillation shock can create a voltage surge that can unintentionally conduct (travel) through one or more of the ECG contact electrodes  210   a - 210   j  and/or lead wires  122   a - 122   j  and cause interference with the communication of data between the ECG electrodes  210   a - 210   j  and components of the ECG monitoring apparatus that process the ECG signals. Further, voltage surge can cause damage to the components of the ECG monitoring apparatus that process the ECG signals.  
         [0016]     Accordingly, embodiments of the invention include an apparatus for providing galvanic isolation to vulnerable components (electronics), including the components of the ECG monitoring apparatus that communicate and process the ECG signals, residing within the ECG monitoring apparatus  120 .  
         [0017]      FIG. 2  is an illustration of an embodiment of electronics  200  that provide galvanic isolation for the communication of ECG signals  202 . The ECG signals  202  are received from an analog front end portion  204  to a digital back end portion  206  of the electronics  200 .  
         [0018]     As shown, an analog ECG signal  202  is received from the analog front end section  204  of the circuitry that includes the patient contact electrodes of  FIG. 1 . The analog ECG signal  202  communicates ECG information received from the patient of  FIG. 1 . An analog to digital (A/D) converter  210  inputs an analog ECG signal  202  and outputs a digital signal  212 . The digital signal  212  also communicates the ECG information that is communicated by the analog signal  202 .  
         [0019]     A complex programmable logic device (CPLD)  214  inputs and converts the digital signal  212  into a Manchester encoded digital signal  216 . The Manchester encoded digital signal is  216  is output from the CPLD  214  and communicated into a differential driver  222 . In a preferred embodiment, the Manchester encoded signal  216  ranges from 0 to 3.3 volts.  
         [0020]     A Manchester encoding is a self clocking means of encoding arbitrary binary sequences. Each bit ( 1  or  0 ) is transmitted over a pre-defined time period. Each bit ( 1  or  0 ) is signified by at least one transition. Hence, each pre-defined time period has a transition which can also be used as a clock synchronization signal. Manchester encoding is considered to be a special case of binary phase shift keying.  
         [0021]     The differential driver  222  divides the incoming Manchester encoded digital signal  216  into two separate (differential) signals  218   a  and  218   b . Each of the two separate signals  218   a ,  218   b  are each respectively directed to a separate portion  222   a ,  222   b  of the differential driver  222 . The portion  222   a  of the differential driver  222  inputs the digital signal  218   a  and outputs a digital signal  219   a  without modification. Differential signals are less sensitive to noise and crosstalk.  
         [0022]     As a result, the digital signal  219   a  is equivalent to and has the same voltage characteristics over time, as the digital signal  218   a . However, the other portion  222   b  of the differential driver  222  inputs and inverts the digital signal  218   b  and outputs digital signal  219   b . As a result, the digital signal  219   b  is an inversion of signal  218   b  and has different (opposite) voltage characteristics as compared to the digital signal  218   b  over time.  
         [0023]     For example, with respect to the preferred type of embodiment, when voltage characteristic of digital signal  216  is equal to 0 volts, the voltage characteristic of digital signals  218   a ,  218   b  and  219   a  are also equal to 0 volts and the voltage characteristic of digital signal  219   b  is equal to 3.3 volts. However, when the voltage characteristic of digital signal  216  is equal to 3.3 volts, the voltage characteristic of digital signals  218   a ,  218   b  and  219   a  are also equal to 3.3 volts and the voltage characteristic of digital signal  219   b  is equal to 0 volts.  
         [0024]     A capacitive coupler  220   a  includes first  224   a  and second  224   b  signal transmitting capacitor plates  224   a ,  224   b  respectively, and first  226   a  and a second  226   b  signal receiving capacitor plates  226   a ,  226   b  respectively. The signal transmitting capacitor plates  224   a ,  224   b  are configured to function as a capacitor within the transmitting circuit  206 .  
         [0025]     The signal receiving capacitor plates  226   a ,  226   b  are configured to function as a receiver (antenna) within a receiving circuit  208 . The signal receiving capacitor plates  226   a ,  226   b  are located substantially parallel to and between the transmitting capacitor plates  224   a ,  224   b  and are configured to receive a signal communicated between the transmitting capacitor plates  224   a ,  224   b . The signal transmitting capacitor plates  224   a ,  224   b  effectively shield the signal receiving capacitor plates  226   a ,  226   b.    
         [0026]     The capacitive coupler  220   a  includes a data signal transmitting circuit  206  and a data signal receiving circuit  208 . The data signal transmitting circuit  206  includes the signal transmitting capacitor plates  224   a ,  226   b . The data signal receiving circuit  208  includes the signal receiving plates  226   a ,  224   b . The capacitive coupler  220   a  provides galvanic isolation between the data signal transmitting circuit  206  and the signal receiving circuit  208 . The data signal receiving circuit  208  is also referred to as the primary circuit and the data signal transmitting circuit  206  is also referred to as the secondary circuit, of the ECG monitoring device  120 .  
         [0027]     The digital signal received by the receiving plates  226   a ,  226   b  is communicated as a digital signal  228   a  and  228   b  respectively, which are each input into a comparator  230 . The comparator  230  outputs a digital signal  232  based upon the voltage value of its input signals  228   a ,  228   b . Preferably and typically, the comparator  230  performs amplification of its input signals  228   a ,  228   b  in order to output the digital signal  232 . The digital signal  232  is input into a digital signal processor (DSP)  240 .  
         [0028]     The DSP  240  inputs and processes the digital signal  232  output from the comparator  230 . The digital signal  232  is processed and stored as digital data (not shown) into a memory  242 . The digital data represents information communicated by the analog ECG signal  202 . Other hardware (not shown) further processes the digital data stored into memory  242 . Preferably, the memory  242  is byte addressable and can be implemented as FLASH or random access memory (RAM). Preferably, the DSP  240  is implemented as a Texas Instruments 5502 digital signal processor.  
         [0029]     The DSP  240  also generates a clock signal  234  residing within the primary (signal receiving) circuit  208  which is communicated through and output by a second capacitive coupler  220   b  as clock signal  236  residing within the secondary (signal transmitting) circuit  206 . The second capacitive coupler  220   b  functions like the first capacitive coupler  220   a  as previously described. In other embodiments, the role of the DSP  240  described above is instead implemented as a microprocessor.  
         [0030]     The CPLD  214  inputs and processes the clock signal  236  in order to generate the Manchester encoded signal  216  previously described. The DSP  240  also employs an oscillator (not shown) that generates the clock signal  234  that is used to time the processing of the signal  232  input by the DSP  240  from the comparator  230 .  
         [0031]      FIG. 3A  is an illustration of a side cross-sectional view of the capacitive coupler  220   a ,  220   b  of  FIG. 2  that is constructed from a plurality of conductive and non-conductive layers of a printed circuit board (PCB)  310 . As shown, the PCB  310  includes layers of conductive and non-conductive material  310   a - 310   i.    
         [0032]     The (4) PCB layers  310   b ,  310   d ,  310   f  and  310   h  include conductive material and are preferably and approximately 0.65 mil (thousandths of one inch) in thickness and made from copper. The (5) layers  310   a ,  310   c ,  310   e ,  310   g  and  310   i  include non-conductive (dielectric) material. The outer layers  310   a  and  310  can be bounded with a respectively an upper outer and lower outer 0.65 mil copper plates. The upper outer and lower outer copper plates can be further bounded with outer soldermask layers.  
         [0033]     Preferably, the non-conductive layers  310   a ,  310   c ,  310   e ,  310   g  and  310   i  are made of PREPEG or FR406 circuit board isolation material. The non-conductive layers  310   c  and  310   g are preferably and approximately 15.15 mil in thickness and made of PREPEG board isolation material. The non-conductive layers  310   a ,  310   e  and  310   i  are preferably and approximately 10 mil in thickness and made from FR406 circuit board isolation material. The non-conductive layers constructed from FR-4 and/or PREPEG insulating (dielectric) material have high isolating properties. In some embodiments, one or more non-conductive (isolating) layers is constructed from (3) thinner isolating layers instead of (1) thicker isolating layer.  
         [0034]     The PCB layer  310   a  includes non-conductive material located adjacent to the conductive layer  310   b  and may optionally include or abut from above conductive material not located adjacent to the conductive layer  310   b , such as a copper plate (upper outer plate) or foil (as described above). The PCB layer  310   i  also includes non-conductive material located adjacent to the conductive layer  310   h  and may optionally include or abut from below conductive material not located adjacent to layer  310   h , such as a copper plate (lower outer plate) or foil (as described above).  
         [0035]     The first and second signal transmitting capacitor plates  224   a  and  224   b  are constructed within PCB layers  310   b  and  310   h  respectively. The first and second signal receiving plates  226   a  and  226   b  are constructed within PCB layers  310   d  and  310   f  respectively. The signal  219   a  is communicated to the capacitor plate  224   a  and the signal  219   b  is communicated to the capacitor plate  224   b . The digital signal  228   a  is communicated from receiver plate  226   a  and the digital signal  228   b  is communicated from receiver plate  226   b  to the comparator  230  of  FIG. 2 .  
         [0036]     In the preferred embodiment, the capacitive coupler  220   a  is designed to transfer the digital data signal  234  at 1 megabit per second while having a break down voltage of 795 volts per mil. The capacitive coupler is also designed to withstand a defibrillation voltage impulse surge of 5 kilovolts for 20 milliseconds and a sustained voltage surge of 4 kilovolts (RMS) for one minute. For safety, the both capacitive couplers  220   a ,  220   b  are limited to a rating of 20 Pico farads.  
         [0037]      FIG. 3B  is an illustration of a top view of the capacitive coupler of  FIG. 3A  that is constructed from the conductive and non-conductive layers of a printed circuit board. As shown, a top view of the capacitor plate  224   a  receives the digital signal  219   a , that is preferably communicated via a copper conductor (not shown). The receiver plate  226   a  is obstructed from this view by the transmitting plate  224   a . A dotted line represents the perimeter of the receiver plate  226   a  located below the transmitting plate  224   a.    
         [0038]     In a preferred embodiment, the largest area dimension of the capacitor plate  224   a  is 6 millimeters by 6 millimeters and the largest area dimension of the receiver plate  226   a  is 5 millimeters by 5 millimeters. Preferably, the depth (See  FIG. 2, 3A ) of both the transmitter and receiver plates is 0.65 millimeters.  
         [0039]     Preferably, the capacitive coupler is configured to withstand a maximum voltage pulse of 5000 volts and 300 joules without break down. Preferably, clearance around the conductors  219   a ,  291   b  from other conductive material is at least 1.25 millimeters.  
         [0040]     The invention can be applied to various types of devices including signal receiving electronics and where outside electrical sources can interfere with the communication of such signals. This is particularly applicable to signal reception by low voltage electronics coupled to a conductive path that can make unwanted contact with outside sources of electrical energy.  
         [0041]     For example, medical devices that are configured to receive signals from wire connected pressure and/or thermal transducers, can be vulnerable from voltage surges from outside electrical sources. Also for example, other devices monitoring EKG signals (brainwaves), cardiac output, blood pressure or other physiological data from a patient can be vulnerable to unwanted contact and damage from outside electrical sources.  
         [0042]     Besides a defibrillator, there are many other electrical sources within proximity to a patient within a health care environment that can potentially create a contact and signal interference with the operation of devices that include signal receiving electronics. For example, electrical cutting tools used for surgery on a patient, or electrical thermal devices that apply heat to a patient, are likely sources of electrical signal interference.  
         [0043]     Operation of these types of tools may cause an unwanted transfer of electrical energy and signal interference to other devices that include signal receiving electronics and that are located in proximity to a patient. Devices that simply draw line voltage from a standard electrical outlet, such as a lamp, can possibly cause unwanted transfer of electrical energy and/or signal interference with other devices that include signal receiving electronics and that are located in proximity to a patient.  
         [0044]     While the present invention has been explained with reference to the structure disclosed herein, it is not confined to the details set forth and this invention is intended to cover any modifications and changes as may come within the scope and spirit of the following claims.