APPARATUS FOR ELECTRODERMAL ACTIVITY MEASUREMENT WITH CURRENT COMPENSATION
An apparatus for measuring electrodermal activity can include a first electrode in contact with a first portion and a second electrode in contact with a second portion of a stratum corneum, and in electronic communication with the second electrode through the stratum corneum. A processing module is electrically coupled to the first electrode and the second electrode and is operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V− (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current (d) measure a resulting current producing an amplified output voltage (e) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage and the compensation current to desaturate the output voltage.
1 . An apparatus for measuring electrodermal activity, comprising:
a first electrode in contact with a first portion of a stratum corneum of skin;
a second electrode in contact with a second portion of the stratum corneum and in electronic communication with the first electrode through the stratum corneum; and
a processing module electrically coupled to the first electrode and the second electrode and operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V−, (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current, (d) measure a resulting current and produce an amplified output voltage, (e) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage, and the compensation current to desaturate the output voltage.
2 . The apparatus of claim 1 , wherein the processing module is further configured to adjust the compensation current if the conductance of the stratum corneum is too low.
3 . The apparatus of claim 1 , wherein the processing module is further configured to measure a compensated value of conductance from which a tonic level conductance is removed.
4 . The apparatus of claim 1 , wherein the processing module is configured to reverse the polarity of at least one of the first electrode and the second electrode after a predetermined period of time to substantially reduce electrolysis.
5 . The apparatus of claim 1 , wherein the processing module is configured to allow a tuning of the compensation current that is subtracted from the current between the first electrode and the second electrode prior to amplification.
6 . The apparatus of claim 1 , wherein the stratum corneum is a stratum corneum of any portion of person's skin.
7 . The apparatus of claim 6 , wherein the apparatus is configured to measure a tonic level conductance of the stratum corneum in the range of about 0.05 μS to about 50 μS.
8 . The apparatus of claim 1 , wherein the apparatus is configured to measure the conductance of the stratum corneum with a resolution of 0.0001 uS.
9 . A wearable device for measuring electrodermal activity, comprising:
a housing configured to be removably associated with a user;
a first electrode, at least a portion of the first electrode disposed outside the housing and configured to contact a first portion of a stratum corneum of skin when the housing is associated with the user;
a second electrode, at least a portion of the first electrode disposed outside the housing and configured to contact a second portion of the stratum corneum of the skin when the housing is associated with the user;
a processing module disposed in the housing and electrically coupled to the first electrode and the second electrode, the processing module operable to (a) bias the first electrode at a first voltage V+ and the second electrode at a second voltage V− (b) measure a current flowing between the first electrode and the second electrode, the current corresponding to the conductance of the stratum corneum, (c) subtract a compensation current from the measured current, (d) measure a resulting current and produce an amplified output voltage, (c) measure a conductance of the stratum corneum, and (f) adjust at least one of the first voltage, the second voltage, and the compensation current to desaturate the output voltage;
a communications module disposed in the housing, the communications module coupled to the processing module and configured to at least one of a display an electrodermal activity of the user, and communicate electrodermal activity data from the processing module to an external device; and
a power source disposed in the housing and configured to provide electrical power to the processing module and the communications module.
10 . The wearable device of claim 9 , wherein the processing module is further configured to decrease the compensation current if the conductance of the stratum corneum is too low.
11 . The wearable device of claim 10 , wherein the wearable device is a wrist band.
12 . The wearable device of claim 11 , wherein the processing module is configured to measure a tonic level conductance of the stratum corneum of the wrist of the user in the range of about 0.05 μS to about 50 μS.
13 . The wearable device of claim 19 , wherein the wearable device is at least one of a head band, an arm band, a foot band, an ankle band, and a ring.
14 . The wearable device of claim 9 , wherein the processing module is further configured to reverse the polarity of at least one of the first electrode and the second electrode after a predetermined period of time to substantially reduce electrolysis.
15 . The wearable device of claim 9 , further comprising a heart beat sensor.
16 . The wearable device of claim 15 , further comprising at least one of an accelerometer, a temperature sensor, a blood oxygen sensor, and a glucose sensor.
17 . The wearable device of claim 16 , wherein the communications module is configured to communicate at least one of heart beat data, an accelerometer data, a temperature data, a blood oxygen data, and a glucose data to the external device.
18 . The wearable device of claim 17 , where the communication module is configured to communicate with the external device via at least one of a USB, USB 2.0, IEEE 1394, BLUETOOTH®, low powered BLUETOOTH®, and Wi-Fi.
19 . A method for measuring electrodermal activity, comprising:
disposing a first electrode and a second electrode on a stratum corneum of a user;
biasing the first electrode at a first voltage and a second electrode at a second voltage;
measuring a current flowing between the first electrode and the second electrode;
subtracting a compensation current from the current flowing between the first electrode and the second electrode to get an input current;
transforming the input current into an amplified output voltage;
measuring an output voltage;
measuring a conductance of the stratum corneum;
determining if the output voltage is saturated; and
increasing the compensation current to change the output voltage such that the output voltage is not saturated.
20 . The method of claim 19 further comprising:
determining if the conductance of the stratum corneum is too low; and
decreasing the compensation current if the conductance is too low.
21 . The method of claim 20 , wherein the measured conductance is a tonic level conductance having a value in the range of about 0.05 μS to about 50 μS.
22 . The method of claim 20 , wherein the measured conductance is a phasic level conductance having a resolution up to 0.0001 μS.