Spring 2016 A-TeChToP Analog Electrodermal Signal Processing

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By: Robin Yancey (Systems Engineer)

A highly accurate electrodermal activity signal, within a precise frequency range and voltage level, is obtained by developing a simple circuit for analog pre-processing [1]. This circuitry is used to limit the current to the electrodes to a safe amount, measure the resistance between the electrodes, and amplify the signal for input to the ADC of the BTLC1000 chip.

Although the determination of the official EDA sensor that is to be implemented is left to the discretion of the electronics and control engineer, the stages of a proposed design are listed below.

  1. A voltage divider which holds a measured voltage of 0.5 volts above the resistor going to ground, can be used to maintain a safe voltage level at the electrodes, and an ideal signal to noise ratio at the output.
  2. A Wheatstone bridge at the 0.5 V node could measure resistance across the electrodes [1]. The two electrodes would take the place of the variable resistor. The parallel remaining bridge resistors should be estimated for similar values on each side to get the best measurement range.
  3. Either an instrumentation amplifier, or two buffers and a differential amplifier will work to amplify the difference in voltage between the two electrodes to within the maximum range of the BTLC1000 (-0.3 and 1.5 volts), without drawing current from the load.
  4. A summing amplifier may then be used to bring the negative voltage difference between the electrodes to above -0.3 volts.
  5. Finally, an RC low-pass filter with a cutoff frequency of 2.5 Hz or a band-pass filter between 0.5 and 2.5 Hz, can help limit the noise artifacts to the ADC input.

 Works Cited:

  1. Boucsein, Wolfram. (2012). Electrodermal Activity: Methods of Electrodermal Recording. Springer US: Springer Science+Business Media, LLC.