I want to Read external signal with the microcontroller.

But the problem is that ADC does not work because the signal GND and the microcontroller are not Common,

My question is, have you encountered such a problem? Is there a way to do it?

  • you are trying to measure the voltage difference between two nodes with only one node attached ....... try measuring the voltage of a battery with only one probe
    – jsotola
    Feb 4, 2020 at 20:16
  • How about You can explain more ? @jsotola
    – navid
    Feb 4, 2020 at 20:19
  • jsotola means: You need a common ground, no way around that. So connect both grounds together
    – chrisl
    Feb 4, 2020 at 20:22
  • So there is no solution. I tried to read one of the ECG electrode signals to the ADC until I had trouble. @chrisl
    – navid
    Feb 4, 2020 at 20:32
  • you could use an opto coupler if needed. i've DIY'd "analog" ones with an LED glued to a CDS then wrapped in tape, adjusting the LED resistor to "calibrate" the range. After tweaking, i was able to get a fairly linear response throughout 75% of the range.
    – dandavis
    Feb 4, 2020 at 21:27

1 Answer 1


The solution is to always use at least 2 electrodes. You need 1 electrode, that you connect to ground. You can put this electrode to somewhere neutral (this instructable suggest connecting it to your leg, but I don't know, if that position is really good).

An ADC measures voltage and voltage is the difference in potential between 2 points, so to measure voltage you by definition need 2 points/connections.

Note: The voltages from the EEG electrodes are normally very low, so that they completely vanish in the noise. To really get something useful, you need to amplify the signal. I think there are many tutorials out there, that explain how to do this.

  • This is true . But I want to send each 10 leads wirelessly. I need to read each of the leads with ADC. Can the microcontroller ground be common with the shield lead ground to work the ADC? @chrisl
    – navid
    Feb 5, 2020 at 13:27
  • There are many things unclear in that question. What shield? (You didn't mention a shield before) And how do you plan to "send leads wirelessly"? An analog voltage cannot just simply send wirelessly. The normal way is to measure it wired (since you need it wired for measurement), and then send the measured value over a wireless transmit technology of your choice. Do you already have a complete project plan? If yes, you need to share it for me to answer. If no, then you first need to think about the overall plan. How everything should be working
    – chrisl
    Feb 5, 2020 at 15:15
  • OK . Let me explain the project to you. I first wanted to wire 10 leads of ECG each with #WiFI sended. But because the ECG device works differently, it didn't work !!!!So I wanted to put all the leads in a small control panel and send it with #WiFI. I'm confused now and I don't know what to do. What do you think should be done? @chrisl
    – navid
    Feb 6, 2020 at 20:18
  • "But because the ECG device works differently" Do you already have some kind of device, that does the actual measurement of the ECG? Or do you have to build one? It is unclear to me, what you already have. You seem to need general project guidance. Unfortunately this site is not good for guidance, since it's a simple Q&A site. Maybe you get better results and help for your project when posting in the project guidance section of the Arduino forum (you can simply google it). A forum is a better fit for project guidance.
    – chrisl
    Feb 6, 2020 at 21:19

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