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I am trying to inject a 1 mA current pulse (10% duty cycle) at a frequency of 10 Hz, across a ball of gel(20 cm in diameter) through rubber electrode pads, using a constant current source (built in house at another university) that is triggered every 100 ms (with a TTL) to generate the desired stimulation waveform. I am monitoring the amount of current I am injecting into the circuit through a series resistance by monitoring the voltage drop across the resistor using an oscilloscope (Va-Vb)method). Please see the below circuit.Circuit Schematic describing the connections.

On starting the stimulation, I observe there is a DC offset to my pulsed stimulation waveform which I can control using a parameter on the current source labeled "Offset control". I can adjust this control to ensure that the baseline of the stimulation waveform coincides with the "zero marker" on my oscilloscope.

I was wondering if someone can explain what might be causing this DC offset and how we can correct for this offset? I do not have access to the specifics of the stimulator and if I want to implement some sort of my own correction (instead of using the "offset control" function), is there a way to do it?

Also, does this DC offset have anything to do with the capacitive effect caused by the ball of gel (since its not entirely resistive)? Any detailed explanation will be greatly appreciated.

Thank you.

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  • \$\begingroup\$ We cannot help you until you provide us with diagrams, photo's, schematic's, etc. Right now we have nothing to look at. Also, please separate your observations from your actions and questions. \$\endgroup\$
    – user105652
    Jun 12, 2016 at 23:17
  • \$\begingroup\$ "...sing a parameter on the current source ..." And the link to the datasheet for 'current source'? \$\endgroup\$
    – Transistor
    Jun 12, 2016 at 23:38
  • \$\begingroup\$ What exactly do you mean by a DC offset? You are setting the current source to supply 1mA for example and you get something different? How are you setting the current source? What happens when you set and measure the current through just the sense resistor? Like others mentioned there's not much else we can help with until we get more details. \$\endgroup\$
    – John D
    Jun 12, 2016 at 23:40
  • \$\begingroup\$ Do you perhaps mean the maximum output voltage that the current source can provide? \$\endgroup\$
    – Sam
    Jun 13, 2016 at 1:58
  • \$\begingroup\$ Hello, I apologize for the limited information. \$\endgroup\$ Jun 15, 2016 at 3:15

1 Answer 1

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schematic

simulate this circuit – Schematic created using CircuitLab

_Figure 1a. A uni-polar squarewave will have an average DC value of 50% of peak. Figure 1(b). A bi-polar squarewave signal will have a DC value of 0 V.

It sounds as though you are using a uni-polar signal. This will have a DC offset of half of peak voltage.

To fix this you need to redesign the current source to alternate the current direction as shown in Figure 1b.

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  • \$\begingroup\$ Thank you very much for your reply. To explain clearly, I am generating a 1 mA current pulse every time I see a TTL trigger so I have a unipolar 10 Hz waveform with 10% duty cycle. When I monitor this waveform across the resistor, I notice I 1V signal and baseline of the waveform coincides with 0V. When I monitor the waveform across the gel, I clearly see a shift in the baseline of the waveform, away from 0V. I was hoping someone could explain this behaviour? May be its shifted across the resistor too but just too small to notice? \$\endgroup\$ Jun 15, 2016 at 14:49

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