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I have two voltage sources. The first one is 230VAC to 5VDC cell phone battery charger while the another is output from Raspberry Pi's Ground and 3V3 pins. I feed the 5VDC output for an Arduino. What I am planning to do is to connect RPI's Ground pin to the corresponding pin of the 5VDC's Ground pin. That's how I would make a reference 0V voltage for data connection between RPI and Arduino. I don't have a voltage level shifter (these allow two different Ground references to be used) to do that easily.

However, I measured a voltage difference of 0.06 Volts between the Ground pins. I think it's little, but where does it come from and is it safe to connect the ground pins? I measured voltages between RPI's 3V3 pin and 5VDC's Ground pin, and vice versa and results were similar in these situations as well.

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    \$\begingroup\$ It comes mostly from your voltmeter imprecision. \$\endgroup\$ – Eugene Sh. Jun 23 '16 at 18:25
  • \$\begingroup\$ If both are fed from isolated supplies it is right to connect the grounds together if you want them to communicate. The measured voltage is just what the meter is reading of stray capacitively coupled and static voltages between the two supply outputs. \$\endgroup\$ – KalleMP Jun 23 '16 at 21:40
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If the RPi is floating, then your measurements are meaningless. That is because the voltmeter is providing the only connection between the two.

You haven't said where the 3.3 V of the RPi come from, but if from a isolated power supply, like a wall wart, then it is floating. In that case you have to connect the two grounds for the signal of one to be meaningful and not hurt the other.

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  • \$\begingroup\$ Yes, RPI is fed by an AC adapter. So it's safe to connect them? It's unclear to me how would the connection be harmful in theory. \$\endgroup\$ – Laotse Jun 23 '16 at 19:46

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