Patent Publication Number: US-2022211289-A1

Title: Systems and methods of monitoring electrodermal activity (eda) using an ac signal and discrete fourier transform (dft) analysis

Description:
FIELD OF THE DISCLOSURE 
     The present application relates generally to systems and methods of monitoring electrodermal activity (EDA) in human subjects, and more specifically to systems and methods of monitoring EDA that are suitable for use in wearable electronic devices. 
     BACKGROUND 
     In recent years, activity trackers and other wearable electronic devices have gained increased popularity due to users&#39; desire to monitor, measure, and/or track, in real-time, various aspects relating to their fitness and/or health, including, but not limited to, the number of steps taken by a user, the user&#39;s heart rate and/or heart rate variability, the user&#39;s temperature, the user&#39;s activity and/or stress levels, etc. One known technique for determining a user&#39;s activity and/or stress levels involves monitoring, measuring, and/or tracking the electrodermal activity (EDA) of the user&#39;s skin, which can be performed by measuring the skin impedance or skin conductance. For example, in response to an environmental, psychological, and/or physiological arousal, the user&#39;s skin perspiration may increase as the thickness of the stratum corneum (i.e., the outer layer of the user&#39;s skin) decreases, thereby causing a decrease the skin impedance or an increase in the skin conductance. By measuring changes in the skin impedance or the skin conductance over time, metrics can be obtained relating to the user&#39;s activity level, stress level, pain level, and/or other factor(s) associated with the user&#39;s present psychological and/or physiological condition, allowing the user to take appropriate steps to address the condition based on the obtained metrics, as necessary. 
     A conventional wearable electronic device for measuring the EDA of a user&#39;s skin includes a first electrode for contacting a first area of the user&#39;s skin, a second electrode for contacting a second area of the user&#39;s skin, an excitation signal source, a trans-impedance amplifier, an instrumentation amplifier, an analog-to-digital (A-to-D) converter, and a processing module. The first and second electrodes can each be a dry electrode or a wet electrode. The excitation signal source can be a direct current (DC) signal source or an alternating current (AC) signal source. For example, such a conventional wearable electronic device for measuring EDA of a user&#39;s skin may be implemented in a wristband, a headband, an armband, a foot band, an ankle band, or one or more finger rings. In one mode of operation, the excitation signal source can generate a DC or AC signal, causing a DC or AC current to flow through the stratum corneum of the user&#39;s skin from the first electrode contacting the first area of the user&#39;s skin to the second electrode contacting the second area of the user&#39;s skin. The trans-impedance amplifier can convert the DC or AC current received at the second electrode to a DC or AC voltage, respectively, and the instrumentation amplifier can obtain the DC or AC voltage difference across the first and second electrodes. The A-to-D converter can convert each of the DC or AC voltage from the trans-impedance amplifier and the DC or AC voltage difference from the instrumentation amplifier from analog form to digital form, and provide the DC or AC voltage and DC or AC voltage difference in their digital forms to the processing module. The processing module can analyze such DC or AC voltages and voltage differences to obtain absolute measures of the user&#39;s skin impedance or skin conductance. If the skin impedance is found to have decreased (or the skin conductance is found to have increased) over time, then the user&#39;s activity level, stress level, emotional state, and/or pain level, etc., can be deemed to have increased. If the skin impedance is found to have increased (or the skin conductance is found to have decreased) over time, then the user&#39;s activity level, stress level, emotional state, and/or pain level, etc., can be deemed to have decreased. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with the present application, systems and methods of monitoring electrodermal activity (EDA) in human subjects are disclosed that are suitable for use in wearable electronic devices. In certain embodiments, an exemplary EDA monitoring system can include a first dry electrode for contacting a first area of a user&#39;s skin, a second dry electrode for contacting a second area of the user&#39;s skin, and an alternating current (AC) excitation signal source for producing an AC excitation signal having a predetermined excitation frequency, such as about 100 Hertz (Hz) or 120 Hz. The AC excitation signal can cause an AC current to flow through the stratum corneum (i.e., the outer layer of the user&#39;s skin) from the first dry electrode contacting the first area of the user&#39;s skin to the second dry electrode contacting the second area of the user&#39;s skin. The EDA monitoring system can further include (1) a trans-impedance amplifier for converting the AC current received at the second dry electrode to an AC voltage, (2) an analog-to-digital (A-to-D) converter for sampling the AC voltage at a predetermined sampling frequency, such as about four times (4×) the predetermined excitation frequency, in order to obtain a voltage level sequence, and for converting the voltage level sequence from analog form to digital form, (3) a discrete Fourier transform (DFT) processor for generating a complex frequency domain representation of the sampled voltage level sequence, and (4) a microprocessor or any other suitable integrated circuitry for processing and analyzing the representation of the sampled voltage level sequence in the complex frequency domain in order to obtain relative measures of the user&#39;s skin impedance or skin conductance. By sampling the AC voltage at the predetermined sampling frequency of about four times (4×) the predetermined excitation frequency of 100 Hz or 120 Hz, and generating a representation of the sampled voltage level sequence in the complex frequency domain, the EDA monitoring system can advantageously obtain relative measures of the user&#39;s skin impedance or skin conductance with reduced electrolysis of the first and second electrodes, reduced computational complexity, and increased signal-to-noise ratio (SNR). Moreover, by obtaining relative measures of the user&#39;s skin impedance or skin conductance, both the structural complexity and the cost of the EDA monitoring system can advantageously be reduced. Such an EDA monitoring system is particularly suited for use in wearable electronic devices configured as wristbands, headbands, armbands, foot bands, ankle bands, or finger rings. 
     In certain further embodiments, a system for monitoring electrodermal activity can include a first dry electrode for making contact with a first area of skin, a second dry electrode for making contact with a second area of the skin, and an excitation channel operative to provide an AC excitation signal at a predetermined excitation frequency, thereby causing an AC current to flow between the first dry electrode and the second dry electrode through a stratum corneum of the skin. The system can further include a reception channel including a trans-impedance amplifier and an A-to-D converter. The trans-impedance amplifier is operative to convert the AC current to an AC voltage signal. The A-to-D converter is operative to sample the AC voltage signal at a sampling frequency of four times (4×) the predetermined excitation frequency, and to convert the sampled AC voltage signal to a digitized voltage level sequence. The system can still further include a DFT processor operative to generate a complex frequency domain representation of the digitized voltage level sequence. 
     In still further embodiments, a method of monitoring electrodermal activity can include providing an AC excitation signal at a predetermined excitation frequency to cause an AC current to flow between a first dry electrode and a second dry electrode through a stratum corneum of skin. The first dry electrode can make contact with a first area of the skin, and the second dry electrode can make contact with a second area of the skin. The method can further include converting the AC current to an AC voltage signal, sampling the AC voltage signal at a sampling frequency of four times (4×) the predetermined excitation frequency, converting the sampled AC voltage signal to a digitized voltage level sequence, and performing a DFT to generate a complex frequency domain representation of the digitized voltage level sequence. 
     In yet further embodiments, a system for monitoring electrodermal activity can include a first dry electrode for making contact with a first area of skin, a second dry electrode for making contact with a second area of skin, and an excitation signal source operative to generate a square wave signal at a predetermined excitation frequency. The system can further include a level shifter operative to level-shift the square wave signal, and a low-pass filter operative to convert the level-shifted square wave signal to an AC sinusoidal signal, and to apply the AC sinusoidal signal to a first capacitor connected to the first dry electrode, thereby causing an AC current to flow between the first dry electrode and the second dry electrode through a stratum corneum of the skin. The system can still further include a trans-impedance amplifier operative to receive the AC current through a second capacitor connected to the second dry electrode, and to convert the AC current to an AC voltage signal. The system can yet further include an A-to-D converter operative to sample the AC voltage signal at a sampling frequency of four times (4×) the predetermined excitation frequency, and to convert the sampled AC voltage signal to a digitized voltage level sequence. The system can still yet further include a DFT processor operative to generate a complex frequency domain representation of the digitized voltage level sequence, as well as integrated circuitry operative to obtain a relative measure of skin impedance or skin conductance based on the digitized voltage level sequence. 
     Other features, functions, and aspects of the invention will be evident from the Detailed Description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments described herein and, together with the Detailed Description, explain these embodiments. In the drawings: 
         FIG. 1 a    is a diagram of a cross-section of the skin of an exemplary human subject, illustrating the stratum corneum (i.e., the outer layer of the user&#39;s skin) having a first thickness and a corresponding first skin impedance or skin conductance; 
         FIG. 1 b    is a diagram of a further cross-section of the skin of the human subject of  FIG. 1 a   , illustrating the stratum corneum having a second thickness and a corresponding second skin impedance or skin conductance; 
         FIG. 1 c    is a diagram of an exemplary electrodermal signal representative of electrodermal activity (EDA) in the skin of the human subject of  FIGS. 1 a    and  1   b;    
         FIG. 2 a    is a block diagram of an exemplary system for monitoring EDA in a human subject, according to the present application; 
         FIG. 2 b    is a block diagram of an analog-to-digital (A-to-D) converter included in the system of  FIG. 2   a;    
         FIG. 3  is a diagram of the system of  FIG. 2 a    implemented in a wristband; 
         FIG. 4  is a diagram illustrating an exemplary environment in which the system of  FIG. 2 a    may be employed; and 
         FIG. 5  is a flow diagram of an exemplary method of monitoring the EDA in a human subject, using the system of  FIG. 2   a.    
     
    
    
     DETAILED DESCRIPTION 
     Systems and methods of monitoring electrodermal activity (EDA) in human subjects are disclosed that are suitable for use in wearable electronic devices. In one embodiment, an EDA monitoring system is disclosed that can include first and second dry electrodes, an alternating current (AC) excitation signal source, a trans-impedance amplifier, an analog-to-digital (A-to-D) converter, a discrete Fourier transform (DFT) processor, and a microprocessor. The AC excitation signal source can produce an AC excitation signal having a predetermined excitation frequency, such as about 100 Hertz (Hz) or 120 Hz. The analog-to-digital (A-to-D) converter can include a sample-and-hold circuit that operates at a predetermined sampling frequency, such as about four times (4×) the predetermined excitation frequency of 100 Hz or 120 Hz. The DFT processor can generate complex frequency domain representations of one or more digitized, sampled voltage level sequences provided by the A-to-D converter for use in obtaining relative measures of a user&#39;s skin impedance or skin conductance. 
     The disclosed EDA monitoring systems and methods can avoid at least some of the drawbacks of conventional EDA monitoring systems and methods, particularly those that are targeted for use in wearable electronic devices. For example, such conventional EDA monitoring systems and methods often employ so-called “dry” electrodes in conjunction with direct current (DC) excitation signals. Such dry electrodes can be distinguished from so-called “wet” electrodes, which typically require the application of a conductive gel or liquid between each wet electrode and the user&#39;s skin for proper operation, and are therefore generally inconvenient and impractical for use in wearable electronic devices. However, employing dry electrodes (e.g., silver (Ag) electrodes) and DC excitation signals in such conventional EDA monitoring systems may result in electrolysis of the dry electrodes during use, which may lead to corrosion of the dry electrodes over time and/or skin irritation due to the deposit of Ag ions (i.e., Ag + +e − ). Such conventional EDA monitoring systems and methods are also often subjected to environmental noise signals (e.g., 50 or 60 Hz power line noise) and/or noise signals from internal electronics of the wearable electronic devices, which can have adverse effects upon skin impedance or skin conductance measurements. Such conventional EDA monitoring systems typically employ band-pass filtering elements in an effort to mitigate the adverse effects of such noise signals. However, such added filtering elements may contribute to increased complexity and/or cost of the wearable electronic devices. 
     In order to avoid at least some of the drawbacks of conventional EDA monitoring systems and methods, the disclosed EDA monitoring system employs dry electrodes in conjunction with AC excitation signals, which typically cause little or no electrolysis of the dry electrodes during use. Further, the sample-and-hold circuit included in the A-to-D converter of the disclosed EDA monitoring system operates at a sampling frequency of about 4× the excitation frequency of 100 Hz or 120 Hz, allowing the DFT processor to generate complex frequency domain representations of the resulting digitized, sampled voltage level sequences with reduced computational complexity. In addition, by processing and analyzing the representations of the digitized, sampled voltage level sequences in the complex frequency domain, the microprocessor or any other suitable integrated circuitry can separate unwanted noise from the sampled voltage level information to obtain more accurate measures of the user&#39;s skin impedance or skin conductance. Moreover, by obtaining relative measures of the user&#39;s skin impedance or skin conductance, both the structural complexity and the cost of the disclosed EDA monitoring system can advantageously be reduced. 
       FIGS. 1 a  and 1 b    depict the cross-section of skin  110  of an exemplary human subject, showing a stratum corneum  114  (i.e., the outer layer of the skin  110 ), the stratum granulosum  118 , and the stratum lucidum  116  disposed between the stratum corneum  114  and the stratum granulosum  118 . As shown in  FIG. 1 a   , the stratum corneum  114  can have an exemplary first thickness Th 1  spanning from the upper surface of the skin  110  to a potential barrier between the stratum corneum  114  and the stratum lucidum  116 . As shown in  FIG. 1 b   , the stratum corneum  114  can also have at least an exemplary second thickness Th 2  spanning from the upper surface of the skin  110  to the potential barrier between the stratum corneum  114  and the stratum lucidum  116 . The thickness (e.g., Th 1 , Th 2 ) of the stratum corneum  114  can change based on the human subject&#39;s present psychological and/or physiological condition. For example, in response to an environmental, psychological, and/or physiological arousal of the human subject, the human subject&#39;s skin perspiration may increase as the thickness of the stratum corneum  114  decreases (see, e.g., the thickness Th 1  of  FIG. 1 a   ), thereby causing a decrease the skin impedance or an increase in the skin conductance. Further, as the human subject achieves a level of relaxation, the human subject&#39;s skin perspiration may decrease as the thickness of the stratum corneum  114  increases (see, e.g., the thickness Th 2  of  FIG. 1 b   ), thereby causing an increase the skin impedance or a decrease in the skin conductance. 
     In order to obtain a measure of the human subject&#39;s skin impedance or skin conductance, a first dry electrode  108   a  and a second dry electrode  108   b  can be placed in contact with a first area and a second area, respectively, of the human subject&#39;s skin  110 , such that the distance between the first and second dry electrodes  108   a ,  108   b  is greater than the thickness Th 1  or Th 2  of the stratum corneum  114 . Further, an excitation signal source  216  (see  FIG. 2 a   ) can generate an AC signal, causing an AC current  112  to flow through the stratum corneum  114  from the first dry electrode  108   a  contacting the first area of the human subject&#39;s skin  110  to the second dry electrode  108   b  contacting the second area of the human subject&#39;s skin  110 . The AC current  112  received at the second dry electrode  108   b  can then be processed to obtain an electrodermal signal, which can be analyzed to obtain a measure of the human subject&#39;s skin impedance or skin conductance. For example, the first dry electrode  108   a  and the second dry electrode  108   b  may each be a silver (Ag) electrode, or any other suitable electrode. 
       FIG. 1 c    depicts an exemplary electrodermal signal  120 , which may be obtained in response to the flow of the AC current  112  through the stratum corneum  114  of the skin  110  of the exemplary human subject. As shown in  FIG. 1 c   , the electrodermal signal  120  includes a low frequency tonic component  120   a  and a higher frequency phasic component  120   b . In the electrodermal signal  120  of  FIG. 1 c   , the tonic component  120   a  generally corresponds to the skin impedance of the human subject in the absence of an environmental, psychological, and/or physiological arousal, and the phasic component  120   b  generally corresponds to the skin impedance of the human subject in response to such an environmental, psychological, and/or physiological arousal, which can involve sight, sound, scent, touch, pain, stress, cognitive processes involving anticipation, decision making, etc. For example, the electrodermal signal  120  may provide a measure of skin impedance at the wrist of the human subject, the absolute value of which can range from about 20,000 Ohms at time T 1  to about 20,000,000 Ohms at time T 2 . Further, the skin impedance at time T 1  may correspond to the exemplary thickness Th 1  of the stratum corneum  114  depicted in  FIG. 1 a   , and the skin impedance at time T 2  may correspond to the exemplary thickness Th 2  of the stratum corneum  114  depicted in  FIG. 1 b   . It is noted that the corresponding absolute value of skin conductance at the wrist of the human subject can range from about 50 micro-Siemens (μS) to about 0.05 μS. 
       FIG. 2 a    depicts an illustrative embodiment of an exemplary system  200  for monitoring EDA (also referred to herein as the “EDA monitoring system”) in a human subject, in accordance with the present application. As shown in  FIG. 2 a   , the EDA monitoring system  200  includes an excitation channel  202 , a reception channel  204 , the first and second dry electrodes  108   a ,  108   b , and a micro-controller  206 . The excitation channel  202  can include an excitation signal source  216 , a level shifter  214 , a low-pass filter  212 , and a capacitor  218 . The reception channel  204  can include one or more ambient sensors  226 , one or more physiological sensors  228 , an accelerometer  230 , a capacitor  220 , trans-impedance amplifier circuitry  232 , a multiplexor (MUX)  236 , and an analog-to-digital (A-to-D) converter  238 . The micro-controller  206  can include a discrete Fourier transform (DFT) processor  240 , a microprocessor  242 , and a memory  244 . For example, the EDA monitoring system  200  may be implemented in a wearable electronic device configured as a wristband, a headband, an armband, a foot band, an ankle band, one or more finger rings, or any other suitable wearable electronic device. 
     In an exemplary mode of operation, the EDA monitoring system  200  (see  FIG. 2 a   ) can be implemented in a wearable electronic device configured as a wristband, which may be worn on a wrist  210  of a human subject (such as the human subject  401 ; see  FIG. 4 ). In this exemplary mode of operation, the excitation signal source  216  can generate a 100 Hz or 120 Hz square wave signal. Such a square wave signal can be level-shifted by the level shifter  214  to mitigate ground noise before being converted to an AC sinusoidal signal by the low-pass filter  212 , which can be a 2 nd  order low-pass filter or any other suitable filter. The capacitor  218  included in the excitation channel  202 , as well as the capacitor  220  included in the reception channel  204 , can provide capacitive isolation for the wrist  210  of the human subject. The AC sinusoidal signal is provided via the capacitor  218  to the first dry electrode  108   a , which can be in contact with a first area of skin on the human subject&#39;s wrist  210 . The AC sinusoidal signal at the first dry electrode  108   a  can cause an AC current to flow from the first area of skin on the human subject&#39;s wrist  210 , through the stratum corneum of the skin, and ultimately to the second dry electrode  108   b , which can be in contact with a second area of skin on the human subject&#39;s wrist  210 . The AC current at the second dry electrode  108   b  is provided via the capacitor  220  to the trans-impedance amplifier circuitry  232 , which can convert the AC current to an AC voltage signal. The MUX  236  can selectively provide, under control of the micro-controller  206 , the AC voltage signal from the trans-impedance amplifier circuitry  232  to the A-to-D converter (ADC)  238 . 
       FIG. 2 b    depicts a detailed view of the A-to-D converter (ADC)  238 . As shown in  FIG. 2 b   , the A-to-D converter (ADC)  238  can include a sample-and-hold circuit  246  and a quantizer/encoder  248 . Each of the sample-and-hold circuit  246  and the quantizer/encoder  248  can receive a clock (CLK) signal from the micro-controller  206 . In this exemplary mode of operation, the sample-and-hold circuit  246  can sample the AC voltage signal from the trans-impedance amplifier circuitry  232  at a predetermined sampling frequency, such as about four times (4×) the frequency (e.g., 100 Hz or 120 Hz) of the square wave (also referred to herein as the “excitation frequency”) generated by the excitation signal source  216 . Having sampled the AC voltage signal at the predetermined sampling frequency, the sample-and-hold circuit  246  can provide a corresponding sampled voltage level sequence to the quantizer/encoder  248 , which can quantize and encode the sampled voltage level sequence in order to provide a corresponding digitized, sampled voltage level sequence (e.g., a sequence of binary encoded numbers) to the micro-controller  206 . 
     As described herein, the skin impedance or the skin conductance of a human subject can change based on the human subject&#39;s present psychological and/or physiological condition. In order to better track certain factors that can affect the human subject&#39;s psychological and/or physiological condition, the reception channel  204  can include the ambient sensor(s)  226 , the physiological sensor(s)  228 , and the accelerometer  230 . For example, the ambient sensor(s)  226  may include one or more sensors for gathering environmental data relating to the external temperature and/or humidity, and the physiological sensors  228  may include one or more sensors for gathering physiological data relating to the human subject&#39;s heart rate, heart rate variability, temperature, autonomous nervous system activity, and/or any other suitable physiological condition(s). Further, the accelerometer  230  may gather acceleration data relating to the human subject&#39;s motion, such as the number of steps taken by the human subject over a specified time period, or any other suitable motion of the human subject. The ambient sensor(s)  226 , the physiological sensor(s)  228 , and the accelerometer  230  can provide the environmental data, the physiological data, and the motion data to the MUX  236 , which can selectively provide, under control of the micro-controller  206 , the environmental, physiological, and/or motion data to the A-to-D converter  238 . The A-to-D converter  238  can convert the environmental, physiological, and/or motion data from analog form to digital form, and provide the environmental, physiological, and/or motion data in their digital forms to the micro-controller  206  for processing, analysis, and/or storage in the memory  244 . 
     Having received one or more digitized, sampled voltage level sequences corresponding to the AC voltage signal provided by the trans-impedance amplifier circuitry  232  at the micro-controller  206 , the DFT processor  240  can generate one or more complex frequency domain representations of the respective sampled voltage level sequences. As described herein, the sample-and-hold circuit  246  can sample the AC voltage signal at the predetermined sampling frequency of about 4× the excitation frequency. By sampling the AC voltage signal at 4× the excitation frequency, the DFT processor  240  can generate the complex frequency domain representations of the respective sampled voltage level sequences with reduced computational complexity. 
     For example, the general expression for the discrete Fourier transform (DFT) may be formulated, as follows: 
     
       
         
           
             
               
                 
                   
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                         W 
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     in which 
         W   N   nk   =e   −j2πnk/N   (2)
 
     and “k” is a non-negative integer ranging from 0 to N−1. 
     By employing a sampling frequency that is 4× the excitation frequency (e.g., 100 Hz or 120 Hz), the general expression for the DFT can be reduced, as follows: 
     
       
         
           
             
               
                 
                   
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     in which 
     
       
         
           
             
               
                 
                   
                     
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     As shown in equation (3) above, the real and imaginary parts of the reduced DFT expression each include two (N/4)-point subsequences, namely, the two (N/4)-point subsequences “x(4n)” and “x(4n+2)” in the real part of the reduced DFT expression, and the two (N/4)-point subsequences “x(4n+3)” and “x(4n+1)” in the imaginary part of the reduced DFT expression. As a result, the real and imaginary parts of the reduced DFT expression (see equation (3)) can each be evaluated using basic arithmetic operations of addition and/or subtraction, thereby reducing the computational overhead and complexity of the DFT processor  240 . 
     For example, in certain embodiments, the DFT processor  240  may evaluate the real (“real”) and imaginary (“imag”) parts of the reduced DFT expression (see equation (3)) by executing computer code in the C/C++ language (or any other suitable computer language), such as the following exemplary computer code: 
                                            For(uint8_t i=0; i&lt;(NSAMPLES/4);i++)               {                real=real+(int32_t)auxp[4*i]−(int32_t)auxp[(4*i)+2];                imag=imag+(int32_t)auxp[(4*i)+3]−(int32_t)auxp[(4*i)+1];               }                    
As indicated in the exemplary computer code above, the evaluation of the real (“real”) and imaginary (“imag”) parts of the reduced DFT expression (see equation (3)) involves arithmetic operations of addition and/or subtraction, and avoids more computationally intensive arithmetic operations such as multiplication and/or division.
 
     Having obtained, in accordance with equations (3) and (4) above, complex frequency domain representations of the sampled voltage level sequences corresponding to the AC voltage signal provided by the trans-impedance amplifier circuitry  232 , the microprocessor  242  can process and analyze the representations of the sampled voltage level sequences in the complex frequency domain in order to obtain measures of the human subject&#39;s skin impedance or skin conductance. For example, the microprocessor  242  may obtain the magnitude, |Z SKIN |, of the human subject&#39;s skin impedance in the complex frequency domain, as follows: 
       | Z   SKIN |=|APPLIED_AC_VOLTAGE|/|OBTAINED_AC_VOLTAGE|,  (5)
 
     in which “|APPLIED_AC_VOLTAGE|” is the magnitude of a complex frequency domain representation of the known voltage of the AC sinusoidal signal applied to the skin on the human subject&#39;s wrist  210 , and “|OBTAINED_AC_VOLTAGE|” is the magnitude of the complex frequency domain representation of the AC voltage signal obtained by the trans-impedance amplifier circuitry  232 . Further, the microprocessor  242  may obtain the phase, Phase(Z SKIN ), of the human subject&#39;s skin impedance in the complex frequency domain, as follows: 
       Phase( Z   SKIN )=180+(Phase(APPLIED_AC_VOLTAGE)−Phase(OBTAINED_AC_VOLTAGE)),  (6)
 
     in which “Phase(APPLIED_AC_VOLTAGE)” is the phase of the complex frequency domain representation of the known voltage of the AC sinusoidal signal applied to the skin on the human subject&#39;s wrist  210 , and “Phase(OBTAINED_AC_VOLTAGE)” is the phase of the complex frequency domain representation of the AC voltage signal obtained by the trans-impedance amplifier circuitry  232 . 
     It is noted that the frequency of the phasic component  120   b  of the exemplary electrodermal signal  120  (see  FIG. 1 c   ) is typically less than 0.5 Hz, and that the frequency of the tonic component  120   a  of the exemplary electrodermal signal  120  is lower than that of the phasic component  120   b . Obtaining measures of the magnitude and phase of the human subject&#39;s skin impedance (Z SKIN ) in the complex frequency domain, as in equations (5) and (6) above, therefore allows the frequencies of interest (e.g., the frequencies of the tonic and phasic components  120   a ,  120   b ; see  FIG. 1 c   ) to be easily separated from undesirable noise frequencies, such as environmental noise frequencies (e.g., 50 or 60 Hz power line noise) and/or noise frequencies from the internal electronics (e.g., greater than 100 Hz) of the EDA monitoring system  200 . In this way, the signal-to-noise ratio (SNR) of the EDA monitoring system  200  can be increased over conventional EDA monitoring systems. It is noted that measures of the magnitude and phase of the human subject&#39;s skin conductance in the complex frequency domain can be obtained in likewise fashion. 
       FIG. 3  depicts an illustrative embodiment of the EDA monitoring system  200  of  FIG. 2 a    implemented in a wristband  300 . As shown in  FIG. 3 , the wristband  300  can include a housing  302 , a first wrist strap  304   a , and a second wrist strap  304   b , which, while the wristband  300  is being worn on the wrist  210  of a human subject, can be secured to the first wrist strap  304   a  by a snap, a buckle, a Velcro® fastener, or any other suitable fastener. The housing  302  is configured to house various components of the EDA monitoring system  200  including the excitation channel  202 , the reception channel  204 , and the micro-controller  206 , as well as a display  306  and a transmitter/receiver  308 . In certain embodiments, the housing  302  can be configured to incorporate the first and second dry electrodes  108   a ,  108   b  so as to allow the respective dry electrodes  108   a ,  108   b  to make proper contact with the skin on the wrist  210  while the wristband  300  is being worn by the human subject. In certain further embodiments, one or both of the wrist straps  304   a ,  304   b  can be configured to incorporate one or both of the first and second dry electrodes  108   a ,  108   b , again allowing the respective dry electrodes  108   a ,  108   b  to make proper contact with the skin on the wrist  210  while the wristband  300  is being worn by the human subject. 
       FIG. 4  depicts a typical environment  400  in which the EDA monitoring system  200  implemented in the wristband  300  may be employed. In the typical environment  400 , the EDA monitoring system  200  is operative to engage in bidirectional communications over wireless communication paths  410  with a smartphone  402 , or any other suitable communications device. The smartphone  402  is operative to engage in bidirectional communications over wireless communication paths  412  with a communications network  404  (e.g., the Internet). The smartphone  402  is further operative, via the communications network  404 , to engage in bidirectional communications over wireless communication paths  414  with cloud-based data storage  406 , and to engage in bidirectional communications over wireless communication paths  416  with emergency services  408 . 
     The operation of the EDA monitoring system  200  will be further understood with reference to the following illustrative example, as well as  FIGS. 2 a   ,  3  and  4 . In this illustrative example, the human subject  401  (see  FIG. 4 ) straps the wristband  300  (see  FIGS. 3 and 4 ) with the EDA monitoring system  200  (see  FIGS. 2 a    and  4 ) implemented therein onto his or her wrist  210 , and engages in walking, running, dancing, bicycling, and/or any other suitable activity. While the human subject  401  walks, runs, dances, bicycles, etc., the EDA monitoring system  200  gathers or collects skin impedance or conductance data, as well as environmental data, acceleration data, and/or other physiological data from the human subject  401 . For example, the ambient sensor(s)  226  may gather or collect environmental data relating to the external temperature and/or humidity, and the accelerometer  230  may gather or collect acceleration data relating to the human subject&#39;s gait or other motion. Further, the physiological sensor(s)  228  may gather or collect physiological data relating to the human subject&#39;s wrist-based heart rate and/or heart rate variability, temperature, and/or autonomous nervous system activity. As described herein, the trans-impedance amplifier circuitry  232  can provide an AC voltage signal corresponding to an AC current flowing between the first and second dry electrodes  108   a ,  108   b , and the micro-controller  206  can process and analyze the AC voltage signal to obtain skin impedance or conductance data relating to the human subject&#39;s motion and/or environmental, psychological, and/or physiological arousal. 
     In this illustrative example, the micro-controller  206  can further process and analyze the environmental data, the acceleration data, and/or the physiological data in addition to the skin impedance or conductance data, and store the environmental data, the acceleration data, the physiological data, and/or the skin impedance or conductance data in the memory  244 . Moreover, the micro-controller  206  can provide the environmental data, the acceleration data, the physiological data, and/or the skin impedance or conductance data in displayable form to the display  306  and/or the transmitter/receiver  308 , which can transmit the various data and associated metrics over the wireless communication paths  410  to the smartphone  402 . The smartphone  402  can then transmit, via the communications network  404 , some or all of the data/metrics over the wireless communication paths  412 ,  414  to the cloud-based data storage  406  for subsequent downloading by the human subject  401  and/or a healthcare professional for monitoring and/or tracking purposes. In certain embodiments, based on the environmental data, the acceleration data, the physiological data, and/or the skin impedance or conductance data indicating a potential emergency situation involving the human subject  401 , the smartphone  402  can further transmit, via the communications network  404 , an alert over the wireless communication paths  412 ,  416  to the emergency services  408 , notifying them of the potential emergency situation. In response, the emergency services  408  can dispatch appropriate emergency personnel and/or equipment to the human subject&#39;s geographical location, which may be determined using a global positioning system (GPS) receiver (not shown) included in the smartphone  402  or the EDA monitoring system  200 . 
     An illustrative method of monitoring the EDA in a human subject, using the EDA monitoring system  200 , is described below with reference to  FIGS. 2 a , 2 b   ,  3 ,  4 , and  5 . In this illustrative method, the EDA monitoring system  200  is implemented in the wristband  300 . As depicted in block  502  (see  FIG. 5 ), the wristband  300  (see  FIG. 3 ) is strapped onto the wrist  210  (see  FIG. 2 a   ) of the human subject  401  (see  FIG. 4 ). As depicted in block  504 , a square wave signal having a predetermined excitation frequency is generated by the excitation signal source  216 . As depicted in block  506 , the square wave signal is level-shifted by the level shifter  214 , and converted to an AC sinusoidal signal by the low-pass filter  212 . As depicted in block  508 , the AC sinusoidal signal is provided, via the capacitor  218 , to the first dry electrode  108   a  making contact with a first area of skin on the human subject&#39;s wrist  210 , thereby causing an AC current to flow from the first area of skin on the human subject&#39;s wrist  210 , through the stratum corneum of the skin, and ultimately to the second dry electrode  108   b  making contact with a second area of skin on the human subject&#39;s wrist  210 . As depicted in block  510 , the AC current at the second dry electrode  108   b  is provided, via the capacitor  220 , to the trans-impedance amplifier circuitry  232 . As depicted in block  512 , the AC current is converted, by the trans-impedance amplifier circuitry  232 , to an AC voltage signal. 
     As depicted in block  518 , the AC voltage signal from the trans-impedance amplifier circuitry  232  is selectively provided, by the MUX  236 , to the A-to-D converter (ADC)  238  (see  FIGS. 2 a  and 2 b   ), which includes the sample-and-hold circuit  246  and the quantizer/encoder  248 . As depicted in block  520 , the AC voltage signal from the trans-impedance amplifier circuitry  232  is sampled, by the sample-and-hold circuit  246  (see  FIG. 2 b   ), at a predetermined sampling frequency equal to four times (4×) the predetermined excitation frequency in order to obtain a corresponding sampled voltage level sequence. As depicted in block  522 , the sampled voltage level sequence corresponding to the AC voltage signal is quantized and encoded by the quantizer/encoder  248  (see  FIG. 2 b   ). As depicted in block  524 , the quantized and encoded voltage level sequence is provided to the micro-controller  206 , which includes the DFT processor  240  and the microprocessor  242 . As depicted in block  526 , a complex frequency domain representation of the quantized/encoded voltage level sequence is generated by the DFT processor  240 . As depicted in block  528 , the representation of the quantized/encoded voltage level sequence is processed and analyzed, by the microprocessor  242 , in the complex frequency domain in order to obtain a measure of the human subject&#39;s skin impedance or skin conductance, each such measure including a magnitude and a phase of the skin impedance or skin conductance in the complex frequency domain. 
     Having described the above illustrative embodiments of the disclosed EDA monitoring system, other alternative embodiments or variations may be made and/or practiced. For example, it was described herein that the excitation signal source  216  can produce an AC excitation signal having a predetermined excitation frequency of about 100 Hz or 120 Hz. In certain alternative embodiments, the excitation signal source  216  can produce an AC excitation signal having an excitation frequency ranging from about 50 Hz to about 1,000 Hz. It was further described herein that the sample-and-hold circuit  246  within the A-to-D converter  238  can operate at a predetermined sampling frequency of about four times (4×) the predetermined excitation frequency of 100 Hz or 120 Hz (i.e., at a sampling frequency of about 400 Hz or 480 Hz). In certain alternative embodiments, the sample-and-hold circuit  246  can operate at a sampling frequency of about five times (5×), six times (6×), seven times (7×), eight times (8×), or any other suitable number of times the predetermined excitation frequency. 
     It will be appreciated by those of ordinary skill in the art that modifications to and variations of the above-described systems and methods may be made without departing from the inventive concepts disclosed herein. Accordingly, the invention should not be viewed as limited except as by the scope and spirit of the appended claims.