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I am applying AC current to a capacitive electrode and recording the change in voltage over time. I have observed a voltage drift that does not follow any consistent pattern. Sometimes, the drift is upwards, other times it is downwards, and at times it alternates between upwards and downwards. My experimental setup involves a capacitive electrode immersed in phosphate-buffered saline, with an AC amplitude of 100µA applied. I can confirm that the electrodes are clean before the start of each experiment. There is minimal or no mechanical vibration. The electrode surface appears intact, and the solution does not seem contaminated. Additionally, there is no visible formation of bubbles. I have attached a picture for reference. Can someone please help me explain what is going on?

AC voltage vs time constant AC current of 125uA applied to electrode AC voltage vs time zoomed in

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    $\begingroup$ 1/ Verify if there is any voltage drift without applied AC. 2/ Verify if AC has DC component big enough to cause the drift. $\endgroup$
    – Poutnik
    Feb 19 at 15:00
  • $\begingroup$ Thanks for your comment. So there is no applied DC component. In other words, the DC voltage has been set to 0V. $\endgroup$
    – Simon
    Feb 19 at 15:08
  • $\begingroup$ I mean residual DC component. Zero is zero in math, but seldom in physics. $\endgroup$
    – Poutnik
    Feb 19 at 15:11

1 Answer 1

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Clearly, from the images and data, the amplitude of the alternating current (AC) is not varying, but the direct current (DC) voltage offset is. Since you state the electrodes are capacitively coupled, they could only transmit AC, not DC.

Therefore, the DC drift you observed is in the amplifier following the electrodes.

  • Since you're measuring AC, then the output of the amplifier should also be capacitively coupled, removing that DC offset.
  • A well-designed AC amplifier has DC negative feedback to increase stability. Can that be improved?
  • High-impedance instrumentation amplifiers can pick up currents electrostatically. Just walking past the leads can cause DC (actually, very low frequency AC) fluctuations. Shield (earth) the apparatus.
  • A well-designed amplifier has temperature compensation. Can that be improved?
  • An amplifier can be put in a temperature-controlled oven, keeping it at a stable temperature, a few degrees above ambient.
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