Abstract:
An apparatus, method and system for collection of physiological electrical potential signals. In one embodiment, an apparatus for use in measuring electrical potentials in a subject (e.g. an animal or a human), having an amplifier being removably mountable to a ground electrode and electrically coupled to at least two signal electrodes, wherein the amplifier is configured to communicate with a signal processing device and indicate if one of the at least two signal electrodes is poorly affixed to, or detached from, said subject. In another embodiment, the at least two signal electrodes comprise a first signal electrode and a second signal electrode, and the amplifier is configured to detect differential electrical potential signals presented by the first signal electrode and the second signal electrode, amplify the differential electrical potential signals by a predetermined gain level to generate an amplified signal, and transmit the amplified signal to the signal processing device.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/690,630, filed Oct. 23, 2003, now U.S. Pat. No. 7,206,625, the contents of which are hereby incorporated herein by reference. 
    
    
     FIELD 
     This invention relates to physiological electrical potentials and, more particularly, to a method and device for the collection of these electrical potentials. 
     BACKGROUND 
     Living animals generate electrical potentials which, when collected, detected and analysed, can be used for a variety of purposes. For example, synchronous neural activity in a live animal or human brain produces electrical potentials that can be detected at the surface of the scalp with conductive electrodes. These detected potentials can then be used in a wide variety of clinical applications, particularly diagnostic applications. 
     It is known to collect these electrical potentials generated by living animals through the application of passive electrodes applied to the skin of the animal. These electrodes consist of a conductive surface or pad that is coupled or adhered to the skin of a subject. The operation of the conductive pad is often facilitated by the additional application of a conductive substance, such as gel, between the skin and the electrode. The conductive pad of the electrode is connected to a lead wire which, in turn, is electrically coupled to an amplifier. The length of the lead wire is typically in excess of 1 m (usually from approximately 1 m to 2.5 m) and electrically connects the amplifier (housed in a signal processing device) and the electrode. The amplifier amplifies the difference in electric potentials between a signal electrode and a reference electrode, both of which are affixed to the subject (human or animal). The amplifier is typically housed together with some signal processing device which, typically, is also adapted to record and analyse any detected electrical potentials which have been amplified by the amplifier. Unfortunately, this known arrangement of the electrode, lead wire and amplifier has significant shortcomings, particularly for the following reasons. 
     Unlike typically well known electrical potentials in common use in other industries and other areas of activity, the electrical potentials generated by living animals are often very small in amplitude—often in the millivolt, microvolt, or even nanovolt range. As a result, these electrical potentials are easily “drowned out” or lost due to noise from the electrical potentials generated by other items in the vicinity of the subject (e.g., lighting, the signal processing device, other equipment, etc.). That is, the differential electric potentials of interest in most applications (often smaller than 1 microvolt) are usually smaller than the electrical noise that is detected by the amplifier when no signal is present. 
     Significant sources of electrical noise which will often be detected by the amplifier are caused by the plurality of time-varying and time-invariant electromagnetic fields that are often present in a test environment where the electrode-lead wire-amplifier arrangement is employed. These time varying electromagnetic fields are inductively and capacitively coupled to the lead wire that carries the signal from the electrode to the amplifier. Consequently, these time varying electromagnetic fields introduce noise onto the lead wire that will be detected and amplified by the amplifier. A second significant source of noise is motion artefacts; i.e., the noise induced in the lead wire as it moves through a static (i.e., time-invariant) electromagnetic field. 
     To address these known shortcomings, efforts have been made to shorten the lead wire in an attempt to reduce noise. However, these efforts have had limited success. Amongst the problems with these efforts is that it is impractical in many applications to tether a subject (whether it is an animal or human) with a wire that is less than about 1 meter long to the amplifier. 
     Another measure to reduce noise that has had some success, albeit limited, is achieved with differential measurements since common mode noise, i.e. noise that is identically present in two wires, can be cancelled to a certain degree. Unfortunately, not all of the noise induced by the various electromagnetic fields is identical in both signal wires and, thus, some significant amount of noise will still be not cancelled and thus present in the recording system. 
     Additional efforts to reduce the effect of noise include conducting multiple tests and then averaging the results of these multiple tests. Unfortunately, conducting repeated tests in an attempt to eliminate or reduce any noise detected has the unwanted effect of significantly lengthening the testing process. Since it is often preferred that the subject remain still or, in some cases, unconscious, a lengthening of the testing process is quite undesirable especially when the test subject is a young child or animal. 
     Another shortcoming with the known electrode-based systems of measuring electrical potentials is the difficulty in determining whether the electrodes have been properly attached or affixed to the subject (e.g., animal or human), while proper attachment, as typically indicated by low electrical impedance between the electrodes, is crucial for the recorded signal-to-noise ratio. As a result, significant care must be taken by the clinician to properly attach these electrodes and then carefully monitor any potentials measured to assess whether the measurements are indicative of improper electrode attachment. If a clinician or other operator is of the opinion that at least one of the electrodes is improperly attached to the subject, a time consuming review of each electrode is necessary to determine which electrode is improperly attached to the subject. To overcome this time consuming process some clinical systems include impedance detection, i.e., a means for automatically detecting if an electrode is poorly connected with the skin of a subject. The accepted method of impedance detection (see for example U.S. Pat. No. 5,368,041) is to introduce a small-current signal to each electrode. The voltage from each electrode to ground is measured and is proportional to the impedance of the electrode. However, such an impedance-detection system requires additional circuitry and the introduction of another electrical current. This additional current and circuitry will be a further source of noise in any signal detected. Moreover, the additional circuitry increases the costs and complexity of the overall system. 
     Accordingly, a method and apparatus for the collection of electrical potentials which addresses, at least in part, some of the above-noted shortcomings is desired. 
     SUMMARY 
     In one aspect of the invention there is provided an apparatus comprising an integrated amplifier and electrode into a combined unit for attaching or affixing to a subject (e.g., an animal or a human). Resulting from the extremely small or short connection between the conductive portion of an electrode and the amplifier, significantly less noise is introduced into the signal detected by the amplifier. The amplifier thus amplifies a signal with a much higher signal-to-noise ratio as compared with conventional electrode to lead wire to amplifier arrangements. 
     In an alternate embodiment of the invention, an impedance detection method and apparatus is provided that may be used whenever an amplifier with bipolar transistor inputs is used to detect the signal (i.e., the electrical potential generated by the subject). Bipolar transistor amplifiers, by their nature, introduce an input bias current into each of the differential signal inputs. These bias currents are an inherent property of the bipolar transistor inputs and result in an offset at the amplifier output that is proportional to the difference in impedance between the input leads. The polarity or phase of the common-mode signal can be used to determine which electrode contact is faulty, thus reducing the time-consuming and painstaking process that afflicts current electrode arrangements. This method is ideally suited for applications where the signal of interest is a differential signal and advantageously requires no additional circuitry to generate, filter, and detect the impedance signal. Hence, it reduces the cost, size, complexity, and total noise of the system with compared current arrangements. A further advantage of these impedance-detection method and apparatus is that it is particularly well suited for use in a small space, the type of physical environment in which electrodes are often employed. 
     A further aspect of the invention provides a method and an apparatus comprising mounting at least two signal electrodes to a subject and at least one reference electrode. The at least one reference electrode comprises a differential amplifier directly connected the conductive portion of the at least one reference electrode. The at least two electrodes are each electrically connected to the differential amplifier of the at least one reference electrode via wires with the lengths close to the distances between the connected electrodes. 
     As will be apparent to those of ordinary skill in the art, the methods and apparatus achieve artefact noise reduction in at least three ways. First, at least one lead wire, a significant source of wire-induced noise, is eliminated completely. Second, the remaining lead wires may be as short as allowed by the size of the area of interest on the subject (e.g., the distance between wires mounted to the subject&#39;s head) which is typically much shorter than the typical one-meter length (or greater) used in known arrangements and systems. Third, motion artefacts are significantly reduced since all lead wires, electrodes and the amplifier are each mounted to the subject and all move together significantly reducing differential movement and hence differential artefact noise that otherwise would be induced in the lead wires due to motion through environmental electromagnetic fields. 
     A further aspect of the invention comprises wireless transmission of the electrical potentials amplified by the electrode-mounted amplifier(s) to a signal-processing device. In this aspect of the invention, the invention further comprises electronic circuitry which transforms the amplified electrical potentials into radio waves and transmits them to a remote radio receiver. 
     In a still further aspect of the invention, the wireless transmission of the electrical potentials comprises performing some signal processing enabling wireless transmission of a digital representation of the amplified electrical potentials. A signal processing device is then adapted to receive and use the digital representation of the amplified electrical potentials transmitted from the subject. 
     In a still further aspect of the invention, an amplifier and related circuitry comprise an integrated circuit affixed to an electrode that employs chip-on-board technology enabling the integrated circuit to be directly mounted to the conductive pad or a small printed circuit board (PCB). This arrangement results in a significantly smaller packaging than conventional packaging (e.g., Small Outline Integrated Circuit (SOIC), etc.). The integrated circuit and its associated lead wires electrically connected to the PCB may be encapsulated for its protection, for example in an epoxy resin. 
     In one aspect of the present invention there is provided an electrode module for affixing to a subject to assist in measuring electrical potentials in said subject, said electrode module comprising an amplifier component mounted directly to an electrode. 
     In a further aspect of the invention there is provided a method of amplifying electrical potentials in a subject, said method comprising amplifying a differential electrical potential signal received from first and second signal electrodes, said amplifying is performed near or on one of said signal electrodes and a reference electrode. 
     In a still further aspect of the invention there is provided a system for measuring electrical potentials in a subject, said system comprising a pair of electrodes electrically coupled to an amplifier mounted to a reference electrode, said reference electrode comprising a conductive pad electrically connected to said amplifier, said amplifier for amplifying a differential electrical signal detected by said pair of electrodes. 
     In a still further aspect, there is provided an amplifier module for use in measuring electrical potentials in a subject, said amplifier module comprising an amplifier removably mountable to a ground electrode, and electrically coupled to at least two signal electrodes, wherein the amplifier is configured to communicate with a signal processing device, and the amplifier is configured to indicate if one of the at least two signal electrodes is poorly affixed to, or detached from, said subject. 
     In a still further aspect, there is provided an amplifier wherein the at least two signal electrodes comprise a first signal electrode and a second signal electrode, and the amplifier detects differential electrical potential signals presented by the first signal electrode and the second signal electrode and amplifies the differential electrical potential signals by a predetermined gain level to generate an amplified signal, the amplifier transmitting the amplified signal to the signal processing device. 
     In a still further aspect, there is provided an amplifier module wherein the amplifier is configured to indicate which of the at least two signal electrodes is poorly affixed to, or detached from, said subject. 
     In a still further aspect, there is provided and amplifier module further comprising a wireless signal transmitter for wirelessly transmitting the amplified signal from the amplifier to the signal processing device. 
     In a still further aspect, there is provided an amplifier module wherein the wireless signal transmitter is configured to transmit a digital representation of the amplified signal to the signal processing device. 
     In a still further aspect, there is provided an amplifier module wherein the wireless transmitter comprises an analog-to-digital converter for generating the digital representation of the amplified signal. 
     In a still further aspect, there is provided an amplifier module further comprising filter circuitry for filtering the differential electrical potential signals received from the at least two signal electrodes to filter out noise. 
     In a still further aspect, there is provided an amplifier module wherein said filter circuitry filters the amplified electrical signal. 
     In a still further aspect, there is provided an amplifier module wherein the amplifier comprises bipolar transistor inputs for indicating if one of the at least two signal electrodes is poorly affixed to, or detached from, said subject. 
     In a still further aspect, there is provided an amplifier module wherein the amplifier comprises a bipolar transistor amplifier, said bipolar transistor amplifier introducing a bias current into the differential electrical potential signals received from the first and second signal electrodes. 
     In a still further aspect, there is provided an amplifier module wherein the bipolar transistor amplifier generates an output proportional to the difference between the impedance presented to the bipolar transistor amplifier by the first and second signal electrodes to indicate if one of the at least two signal electrodes is poorly affixed to, or detached from, said subject. 
     In a still further aspect, there is provided an amplifier module wherein the amplifier indicates if one of the at least two signal electrodes is poorly affixed to, or detached from, said subject based on a difference in impedance in the differential electrical potential signals presented by the first and second signal electrodes. 
     In a still further aspect, there is provided an amplifier module wherein the amplifier generates a sensory signal for presentation to an operator indicating if one of the at least two signal electrodes is poorly affixed to, or detached from, said subject. 
     In a still further aspect, there is provided an amplifier module wherein the sensory signal comprises at least one of a visual signal, an audible signal and a tactile signal, for presentation to the operator by the signal processing device. 
     These as well as other novel advantages, details, embodiments, features and objects of the present invention will be apparent to those skilled in the art from following the detailed description of the invention, the attached claims and accompanying drawings, listed herein, which are useful in explaining the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following text and drawings, wherein similar reference numerals denote similar elements throughout the several views thereof, the present invention is explained with reference to illustrative embodiments, in which: 
         FIG. 1  is a side view schematic diagram depicting an embodiment of the invention affixed to a subject&#39;s (human&#39;s) head; 
         FIG. 2  is top view schematic diagram of the embodiment of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of the reference electrode illustrated in  FIGS. 1 and 2  embodying aspects of the present invention; 
         FIG. 4  is a more detailed diagram of the components of a portion of the reference electrode of  FIG. 3 ; 
         FIGS. 5 and 6  are simplified circuit diagrams of the reference electrode illustrated in  FIG. 3 ; and 
         FIG. 7  is a side view schematic of an alternative embodiment of the present invention affixed to a subject&#39;s (human&#39;s) head. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the described embodiments of the present invention, reference is made to the subject from which electrical potentials are being detected, measured and analysed. The subject illustrated in some of the figures is illustrated as the head of a human. It is to be noted that other subject areas (i.e., other portions of a human) or other animals could equally be a subject for which the current invention could be employed to detect electrical potentials 
     Referencing  FIG. 1 , an electrical potential system  10  is illustrated. Electrical potential system  10  includes a reference electrode module  12  (which, as is described below includes an amplifier component  38 , not shown in  FIG. 1 ) electrically coupled to two or more conventional electrodes  14 . In  FIG. 1 , two signal electrodes  14  (also referred to herein as simply “electrodes  14 ”) are illustrated—a first electrode  14   a  is illustrated in the foreground while a second electrode  14   b  (shown in dotted line) is in the background. Electrodes  14   a ,  14   b  are electrically coupled to reference electrode module  12  by lead wires  20   a ,  20   b , respectively. 
     Reference electrode module  12  is also electrically coupled to signal processing device  18  by way of connector  16 . 
     Reference electrode module  12  and electrodes  14  are affixed or mounted to subject  22  through known adhesives or other fixation methods or mechanisms. Additionally, a conductive substance such as electrode gel, for example, may be used to enhance or ensure electrical conduction between the skin of subject  22  and electrodes  12 ,  14 . 
     Lead wires  20   a  and  20   b  are preferably selected to be taut when electrodes  12  and  14  have been affixed to subject  22 . When lead wires  20   a ,  20   b  are taut the chance of differential motion artefacts resulting from lead wire  20   a  moving in a manner different from lead wire  20   b  is significantly reduced. 
     Connector  16  is preferably a conventional shielded wire allowing amplified electrical potential signals to be transmitted from reference electrode module  12  to signal-processing device  18 . 
     Signal-processing device  18  operates to receive and process signals received from reference electrode module  12  via connector  16 . As will be apparent from the description below, signal-processing device  18  is a conventional signal-processing device that has been adapted to receive amplified electrical potential signals rather than electrical potentials that have yet to be amplified. Signal-processing device  18  may include, for example, a visual display for displaying the received amplified signals, a signal recorder component for recording the signal received for later review and analysis, and various signal-processing circuits and software for processing any amplified signals received. Such signal processing may include circuitry and software for further reducing any noise contained in the received amplified signals. In alternative embodiments, which are described in greater detail below, reference electrode module  12  and signal processing device  18  are adapted to assist an operator of system  10  to determine if an electrode  14  has been poorly affixed to subject  22 . 
     Referencing  FIG. 2 , electrical potential system  10  is illustrated in a top view of subject  22 . As noted above, lead wires  20   a ,  20   b  are tautly and electrically connect electrodes  14   a ,  14   b  to reference electrode module  12 . 
     Reference electrode module  12  is illustrated in greater in  FIG. 3 . In the exemplary embodiment, reference electrode module  12  includes a conventional electrode that has been adapted to include amplifier component  38 . Accordingly, reference electrode module  12  includes adhesive pad  34  that is used to affix electrode  12  to subject  22  and conductive pad  36  mounted to adhesive pad  34  for electrically connecting electrode  12  to subject  22 . 
     In some embodiments reference electrode module  12  may include or be used in conjunction with a conventional conductive substance such as gel  32 , for example, to assist in forming an electrical connection between the skin of subject  22  and conductive pad  36 . 
     As known to those of ordinary skill in the art, conductive pad  36 , which is typically composed of silver, silver-plated tin, silver-chloride, gold or other conductive materials, is adapted to provide an electrical connection between the subject  22  and, ultimately, signal processing device  18  (not shown in  FIG. 3 ). 
     Electrically connected to conductive pad  36  is amplifier component  38 . Amplifier component  38  is also adapted to be electrically connected to lead wires  20   a ,  20   b  and connector  16 . Reference electrode module  12  also acts as the reference electrical ground for electrodes  14   a  and  14   b.    
     Resulting from the inclusion of amplifier component  38  in reference electrode module  12 , electrical potentials detected by electrodes  14  will be passed into amplifier component  38  for signal amplification. The use of short lead wires  20  (usually less than 15-20 cm in length on an adult human&#39;s head and even shorter on an infant&#39;s or small animal&#39;s head) results in far less noise being inductively or capacitively coupled to the lead wires that carries the signal from electrodes  14  to the amplifier component  38  than conventional electrode-lead and wire-amplifier arrangements. Additionally, since lead wires  20  are preferably taut, motion artifacts that induce noise in the lead wires  20  as they move through static (i.e., time invariant) electromagnetic fields are significantly reduced. The motion artifact noise is significantly reduced compared to known arrangements since lead wires  20   a  and  20   b  are likely to move through very similar paths and remain fixed relative to each other through these time-invariant electromagnetic fields. Consequently, there is likely to be only very small differential potentials resulting from these differential motion artifacts that will be detected by amplifier component  38 . 
     A schematic of the elements included in amplifier component  38  is shown in detail in  FIG. 4 . Amplifier component  38  includes, in the present exemplary embodiment, a power supply  42 , gain-setting resistor  48  and amplifier  44 . Power supply  42  and gain-setting resistor  48  are both electrically connected to amplifier  44 . Additionally, amplifier  44  is electrically connected to connector  16  (which also connects to signal-processing device  18 —not shown in  FIG. 4 ) and lead wires  20   a  and  20   b  (which are also electrically connected to electrodes  14   a  and  14   b , respectively and not shown in  FIG. 4 ). Amplifier component  38  may also include optional protective coating  50  to provide physical protection and additional electrical isolation of the various components. Epoxy or silicone resins known in the art may be appropriate for such a protective coating. 
     In the exemplary embodiment, amplifier  44  is an AD620 Instrumentation Amplifier available from Analog Devices of Norwood, Mass., USA (the data sheet for which is available from Analog Devices&#39; web site at http://www.analog.com/UploadedFiles/Data_Sheets/37793330023930AD620_e.pdf, the contents of which are hereby incorporated herein by reference). Alternative embodiments may employ different amplifiers. For example, it is believed that the INA128 or INA129 amplifier from the Burr-Brown Corporation (part of Texas Instruments) of Tucson, Ariz., USA may be appropriate in some circumstances. As persons of ordinary skill in the art will appreciate, other amplifiers that could be employed in alternative embodiments will have different pin-outs resulting in slightly differing wiring from that illustrated in  FIG. 4 . 
     In the exemplary embodiment, the AD620 amplifier (amplifier  44 ) has its gain adjusted through use of different levels of resistance (R G ) between pins  1  and  8 . A single resistor  48  connected between these pins can be used to set the level of gain (G) of amplifier  44 . In the exemplary embodiment, gain is determined in accordance with equation (1) (where R I  is internal resistance of amplifier  44  and is approximately 49.4 kΩ for the AD620 amplifier):
 
 G =1 +R   I   /R   G   (1)
 
     Resistor  48  may be a variable resistor or circuitry allowing for an operator to vary the level of resistance presented to amplifier  44  thus allowing for the modification of the level of gain applied to any differential potentials detected by amplifier  44 . Typically, many operating environments will require a level of gain (G) exceeding 100 and preferably closer to 1000 (the maximum level of gain offered by the AD620 amplifier). Accordingly, resistor  48  would, in the exemplary embodiment, require a level of resistance between approximately 499.0Ω and 49.5Ω. 
     Power supply  42 , which can be provided through use of a conventional (although preferably small) battery and any required and related circuitry known to those of ordinary skill in the art, is electrically connected to pins  4  and  6  of amplifier  44 . 
     Lead wires  20   a  and  20   b  are electrically connected to pins  2  and  3  of amplifier  44 . 
     Pin  8  of amplifier  44  is electrically connected to conductive pad  36  ( FIG. 3 ) of reference electrode module  12 . As a result of the electrical connection between amplifier  44  and conductive pad  36  (which, in turn, is connected to subject  22  during use), amplifier  44  will be provided with a reference electrical ground. 
     Referencing  FIGS. 1-4 , in operation of system  10 , an operator affixes reference electrode module  12  and electrodes  14  to a subject in the relevant areas of interest in a manner known to those of ordinary skill in the art. The operator also electrically connects, by way of a lead wire  20 , each electrode  14  to reference electrode module  12 . In the exemplary embodiment, electrode  14   a  is connected to reference electrode module  12  by way of lead wire  20   a  and electrode  14   b  is connected to reference electrode module  12  by way of lead wire  20   b . Lead wires  20  may be connected to electrodes  12 ,  14  prior or after fixation to the subject. As noted above, it is preferable that once electrodes  12 ,  14  have been affixed and lead wires  20  have been connected thereto, lead wires  20  are relatively taut. An operator also electrically connects reference electrode module  12  to signal processing device  18  by way of connector  16 . 
     Amplifier  44 , powered by power supply  42 , will begin to detect differential electrical potential signals presented by electrodes  14   a  and  14   b . Amplifier  44  then amplifies these detected signals by the set level of gain (G)—where, as noted above, the level of gain (G) is determined by resistor  48  and the inner components of amplifier  44 . Since lead wires  20   a ,  20   b  connecting electrodes  14   a ,  14   b  to amplifier  44  are considerably shorter than the lead wires in known arrangements (i.e., 20 cm vs. 100-250 cm), the amount of electrical noise inductively or capacitively coupled to the lead wires is significantly reduced. Accordingly, amplifier  44  is presented with electrical signals having a much greater (i.e., improved) signal to noise ratio than in known arrangements. Additionally, since lead wires  20   a ,  20   b  are substantially fixed relative to each other (especially, if lead wires  20   a  and  20   b  are taut), motion artifacts created by the movement of lead wires along different physical paths through electromagnetic fields (a source of considerable noise in known systems) are also significantly reduced. 
     Once electrical potentials detected by amplifier  44  have been amplified (resulting in an amplified signal having considerably less noise than known systems), the amplified signal is transmitted to signal processing device  18  via connector  16 . The amplified signal can then be further processed, recorded and analysed to provide the required diagnostic test being performed on subject  22 . 
     As will be appreciated by those of ordinary skill in the art, the resulting significant reduction in noise presented to the amplifier of system  10  results in a reduction of signal processing that needs to be performed to eliminate or reduce noise in any signal detected as compared to known systems. Consequently, time averaging techniques which are presently employed to reduce the effects of noise in a detected signal and which require multiple and/or lengthy tests to be conducted may be reduced in many cases. 
     An exemplary simplified circuit diagram for system  10  is illustrated in  FIGS. 5 and 6 . Resulting from the arrangement and the selection of the components therein, system  10  can also be used to assist in determining if one of electrodes  14   a  or  14   b , has become detached from subject  22  and, if so, provide assistance in determining which one of the electrodes has become so detached. System  10  includes an impedance detection that may be used whenever an amplifier with bipolar transistor inputs (e.g., the AD620 amplifier described above) is used to detect the signal (i.e., the electrical potential generated by the subject). As those of ordinary skill in the art will appreciate, a bipolar transistor amplifier will introduce an input bias current into each of the differential signal inputs. These bias currents are an inherent property of the bipolar transistor inputs and result in an offset at the amplifier output that is proportional to the difference in impedance between the input leads (e.g., the impedance presented by the lead wire-electrode-subject arrangement). Adapting signal-processing device  18  to determine the polarity or phase of the common-mode signal, signal-processing device  18  can be used to determine which electrode contact is faulty thus reducing the time-consuming and painstaking process that afflicts current electrode arrangements. An operator would then be presented with some form of sensory feedback or signal indicating which one of the electrodes  14  has a faulty or poor connection to subject  22 . The sensory feedback presented to the operator may be one or more of the following: a visual signal or indicator (e.g., a text and/or graphical message), an audible signal (e.g., a warning buzzer with, for example, different tones and/or volumes to indicate which electrode has a poor/faulty connection), and/or a tactile or other sense of touch signal (e.g., a vibration generated by a device—such as, for example, a pager-like device—worn by operator, with different types of vibrations associated with each of electrodes  14 ). In the preferred embodiment, the sensory signal is a combination of an audible alarm or warning coupled with a visual signal output on a display screen forming part of signal-processing device  18 . The audible alarm provides an indication that one of the electrodes  14  has a poor or faulty connection to subject  22  and prompts the operator to review the display screen of signal-processing device  18 . The visual indicator displayed by signal-processing device  18  provides to the operator data (text and/or graphics) indicating which one of the electrodes  14  is the source of the problem. 
     Referring to  FIG. 6 , Z 1  represents the impedance presented to amplifier  44  by the connection between the subject  22  and electrode  14   a  and Z 2  represents the impedance presented to amplifier  44  by the connection between subject  22  and electrode  14   b . The bias current flowing through the subject-electrode connections is represented by i offset1  and i offset2 , respectively. The offset voltage (V offset ) follows equation (2) set out below:
 
(( i   offset1   −i   offset2 )( Z   1   −Z   2 ) G )= V   offset   (2)
 
     If the impedances of the subject-electrode connections are the same or similar (i.e., both are well adhered or affixed to the subject) the second term of equation (2) will be zero or very small resulting in a very small offset voltage. If one of the two electrodes is poorly affixed to subject  22  (or has become disconnected), then the offset voltage will be relatively large. If electrode  14   a  is disconnected V offset  will be much greater than zero and this value can be displayed (or some other signal generated) to an operator of system  10  by signal processing device  18 . Consequently, the operator of system  10  will be provided information identifying the electrode which has been poorly connected or affixed to subject  22  saving considerable time and effort that would otherwise be expended. Similarly, if V offset  is much less than zero, this value is indicative of electrode  14   b  being disconnected or poorly connected to subject  22  and the operator can be informed of this situation. As a result of this operation of system  10 , an operator of system  10  can spend much less time making a determination of which of the electrodes needs to be re-attached or better attached to subject  22 . 
     As will be appreciated, in alternative embodiments of the present invention having multiple pairs of electrodes  14  affixed to a subject, the assistance provided to an operator of system  10  in determining which electrodes  14  have been poorly attached to the subject will result in significant time and cost savings. 
     This advantage of the present invention is suitable for applications where the signal of interest is a differential signal. Beneficially, such an advantage requires no additional circuitry to generate, filter and detect the impedance signal and results in a reduction of the cost, size, complexity, and total noise of the system compared current arrangements. A further advantage of the impedance detection method and apparatus is that it is particularly well suited for use in a small space; the type of physical environment in which electrodes are often employed. 
     Some alternatives to the exemplary embodiment illustrated as system  10  will now be described. 
     In one alternative embodiment, system  10  is adapted to transmit amplified signals from the subject to signal processing device  18  using a wireless connection as illustrated by system  70  in  FIG. 7 . Similar to system  10  ( FIG. 1 ), system  70  includes a pair of conventional electrodes  14   a ,  14   b  electrically connected by way of lead wires  20   a ,  20   b , respectively, to reference electrode module  12 . Reference electrode module  12 , which also includes the amplifier component  38  described above, is electrically connected to connector  72  rather than connector  16 . Connector  72  electrically connects reference electrode module  12  to signal transmitter  74 . 
     Signal transmitter  74  is adapted to receive the detected and amplified signals (as described above) processed by amplifier component mounted to reference electrode module  12 . However, rather than transmit the detected and amplified signal over a wire to signal-processing device  18  like system  10  of  FIG. 1 , signal transmitter  74  transmits the signal via radio waves to signal-processing device  18 . In turn, signal processing device  18  has been adapted to receive the transmitted radio signal by inclusion of radio-receiving device and antenna  76 . Those of ordinary skill in the art will appreciate that signal transmitter  74  will modulate (either in the amplitude or frequency domains, or both) a radio signal of selected frequency and, thus, will include circuitry and power sources (e.g., a battery) to perform this function. Additionally, signal transmitter  74  may include some filtering circuitry to remove some of the unwanted (although limited) noise included in the amplified signal generated by reference electrode module  12 . 
     In a further alternative, signal transmitter  74  may transmit a digital representation of the amplified signal generated by reference electrode module  12 . In this alternative embodiment, signal transmitter would include a conventional analog-to-digital processor (A/D). The digital representation could then be transmitted using known wireless transmission protocols (e.g., BlueTooth, 802.11a, b or g, or the like). In this instance, receiving device and antenna  76  would also require some modification so that the digitally transmitted signal can be received and processed as required. 
     While the preferred embodiment includes the amplifier component mounted directly on the underlying electrode, a further alternative embodiment includes the amplifier component affixed to subject  22  and near to the underlying electrode (i.e., near to conductive pad  36  of reference electrode module  12 ). For example, amplifier component could be included in the circuitry of signal transmitter  74  ( FIG. 7 ). 
     In a still further alternative embodiment, the amplifier component is mounted on or near a signal electrode (rather than mounting the amplifier component on or near the reference electrode) to form an electrode module. 
     In view of the many possible embodiments to which the principles of this invention may be applied, it should be recognized that the embodiments described herein and shown in the drawing figures is meant to be illustrative only and should not be taken as limiting the scope of invention. For example, those of skill in the art will recognize that the elements of the illustrated embodiment can be modified in arrangement and detail without departing from the spirit of the invention. Therefore, the invention as described herein contemplates all such embodiments as may come within the scope of the following claims and equivalents thereof.