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Currently I'm designing a wearable, battery powered module that combines several sensors into a single device. In this device are modules for both ECG measurement (heart beat) and EDA (skin conductance, also known as Galvanic Skin Response, or GSR). Both use electrodes and the ECG electrodes have to be attached to the chest. The main question is, is it possible to use the same electrodes for ECG and EDA measurements simultaniously?

Some background information:

  • ECG is measured using the AD8232 chip. The design is based roughly on the Sparkfun board. For this measurement two electrodes are used. These electrodes are used to pick up a signal of max 1 mV, generated by the heart. The AD8232 has a gain of 100 to amplify and measure this.

  • EDA is measured using a Seeed design. With the EDA circuit roughly 0,5V to 1,0V is passed between two electrodes, through the skin, in a voltage divider setup. Because skin resistance changes, so does the amount of voltage that passes through the skin, compared to the set resistor of the voltage divider. The output of the circuit measures the voltage over the skin and using the known resistor, calculates the skin resistance.

Since both modules use electrodes in contact with a stretch of skin, the idea was to combine the two modules over the same electrodes. However, since the AD8232 has such a large gain, I'm worried that the 0,5V to 1,0V of the EDA will destroy the AD8232 if it passes through the same electrodes. The AD8232 has a max voltage on every pin of 3,9V with a 3,6V power supply. Sub-question 1: Will this happen and if so, is it something that can be prevented? I've been told to look at galvanic isolation of the EDA and ECG modules (different ground planes?), but I'm very unsure how to go about this.

I'm also worried that the EDA voltage will contaminate the ECG measurement. Sub-question 2: Can the galvanic isolation separate the two voltages? Can this even be done at all?

Any advice on any of these points is very welcome.

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    \$\begingroup\$ So you want to measure 1mV across the sensors, while applying a voltage in excess of 0.5V to the same sensors.... Switch off one to do the other. \$\endgroup\$
    – Solar Mike
    Dec 18, 2018 at 14:36
  • \$\begingroup\$ No other way around it, huh? Thank you for the answer. \$\endgroup\$
    – user207636
    Dec 18, 2018 at 15:08
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    \$\begingroup\$ Of course it can be done. But the different subsystems have to be designed to operate with each other. These are not. \$\endgroup\$ Dec 20, 2018 at 2:38

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There are easy but perhaps imperfect solutions to your problem. You can use 2 arduino nanos connected to the same analog input. Since the nanos are small, they can be mounted on the same breadboard. One nano will run the ECG sketch and the other the GSR sketch. One or both will be powered via a 9V or smaller battery. You would probably prefer to visualize the ECG on your serial plotter. You would need to record the GSR data to an SD file for later review. The review of saved data will probably be tedious if, for example, you print it to a spreadsheet and then plot it. Of course, you can also use a laptop on which you have both sketches and you can alternate between them if you are just interested in sampling periodically over an interval of a few minutes to an hour. You should not be plugged into the wall when you run sketches that involve electrodes attached to your body. You might also consider using a USB isolator.

The AD8232 uses 3 electrodes. For the GSR sketch, you could use the same electrodes as for ECG but simply have a different sketch. Since you would be using 2 different Arduinos with the same sensor, it would be possible to run the 2 recordings simultaneously. This has the advantage of not requiring you to jerry rig new electrodes. On the other hand, the electrode distances when the leads are applied for ECG may not be compatible with the GSR application. And applying current to one electrode for GSR might contaminate the ECG. The best way to use the AD8232 for GSR during an ECG would probably be to buy a second AD8232 and apply the electrodes to a different part of the body. Then you could run the 2 sketches simultaneously using 2 Arduinos, both run on battery (or laptop disconnected from mains).

Also, if you use a 2nd AD8232 for GSR as I suggested above, you could record ECG on A0 and GSR on A1 and print both to the serial monitor simultaneously. However, I suspect that the sampling rate you will use for the 2 recordings will be different and that might make it hard to interpret both tracings on the same screen.

I hope these thoughts are helpful.

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