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When measuring a power source voltage using a differential amplifier, without a common ground with amplifier supply, to which ground should we connect the differential amplifier resistor ground? V1 ground or V2 ground?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Both grounds should be tied together. Or you can try using the ground of each source and check the result. \$\endgroup\$
    – CFCBazar
    Commented Oct 7, 2020 at 8:14
  • \$\begingroup\$ @CFCBazarcom Both grounds can't be connected to each other, it should be isolated. I'm not doing experiments, I'm asking a question! \$\endgroup\$
    – ElectronSurf
    Commented Oct 7, 2020 at 8:20

1 Answer 1

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Your circuit is incomplete. There is no reference between your ground symbol and the power supply. You need this or the circuit will be floating relative to the supply rails and will drift. Connecting it to the mid-point of a pair of potential divider resistors from the PSU might be good enough.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Conventional schematic layout is signal running left to right and voltages arranged with positive to top and negative to bottom. Current generally flows top to bottom.

R1, R2 and C1 form a ground reference relative to the power supply.

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  • \$\begingroup\$ Thanks, is this how industrial PSUs with several isolated outputs measuring the voltages and display it on a single LCD? \$\endgroup\$
    – ElectronSurf
    Commented Oct 7, 2020 at 8:26
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    \$\begingroup\$ If they're using the same LCD then the supplies are unlikely to be isolated. \$\endgroup\$
    – Transistor
    Commented Oct 7, 2020 at 8:30
  • \$\begingroup\$ Here's one product claiming channels are isolated and with a single display, Link. \$\endgroup\$
    – ElectronSurf
    Commented Oct 7, 2020 at 8:34
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    \$\begingroup\$ That's a laboratory PSU rather than an industrial one. They're probably using opto-isolated bi-directional digital communication between the control logic (including the display section) and each isolated power supply. \$\endgroup\$
    – Transistor
    Commented Oct 7, 2020 at 8:39
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    \$\begingroup\$ " They're probably using opto-isolated bi-directional digital communication ..." The commands and readings are converted to digital before transmission. \$\endgroup\$
    – Transistor
    Commented Oct 7, 2020 at 8:50

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