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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 '18 at 14:36
  • \$\begingroup\$ No other way around it, huh? Thank you for the answer. \$\endgroup\$ – user207636 Dec 18 '18 at 15:08
  • \$\begingroup\$ Of course it can be done. But the different subsystems have to be designed to operate with each other. These are not. \$\endgroup\$ – Edgar Brown Dec 20 '18 at 2:38

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