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I am trying to understand how a USB oscilloscope can furnish a proper ground from which to take measurements.

If I'm not mistaken, my benchtop scope which is powered by my wall outlet at AC 120V, has access to actual earth ground through its power supply.

If so, how can a USB scope connected to a battery-powered laptop furnish a ground to the probes? I have a limited understanding of 'virtual-ground' in an electronic device as opposed to actual earth-ground - maybe that applies here.

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    \$\begingroup\$ Laptops, even when connected to a charger, typically don’t have a connection to earth. USB oscilloscopes use USB 0V as ground; in desktop PCs this is actually connected to earth. USB ‘scopes rely on the user connecting the probe’s ground clip to a local reference (which may or may not be earth) on the device that they are inspecting. This is technically desirable as it avoids a ground loop, but can be a safety hazard since the laptop’s 0V could be connected to a high voltage relative to earth. \$\endgroup\$
    – Frog
    Commented Sep 3, 2021 at 20:36
  • \$\begingroup\$ How can a battery powered multimeter measure anything? What if your were in an aeroplane making measurements? \$\endgroup\$
    – Andy aka
    Commented Sep 4, 2021 at 12:07

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The ground wire of the probe on a regular oscilloscope is connected to the outlet ground.

Connecting the two grounds together is not about the signal, though.

The oscilloscope ground is connected to the outlet ground as a safety feature. The point of it is to keep any uninsulated parts of the scope from killing you.

You can (but should not) disconnect the ground pin of the oscilloscope power lead, and the scope will work properly. The metal housing and any exposed metal parts will be at the same potential as your circuit though. If you connect the probe ground clip from the scope to a high voltage source in your circuit, then the high voltage will be on the housing or other metal parts - just waiting for you to touch them so they can kill you.

A scope without a grounded power lead (USB or battery poweted) must be double insulated so that you cannot possibly touch anything that may have high voltage on it.

When you connect the ground clip from the scope to the device, you are providing all the ground that the signal requires.

All an oscilloscope does is to show you the voltage difference between the probe tip and the probe clip. An ungrounded scope can measure the voltage between any two points in a circuit. A grounded scope can only measure voltage between the ground (connected to the power lead ground) and the signal.

All you need to measure a voltage is two points connected to the circuit. To safely measure potentially high voltages, you need either a grounded scope or an insulated and isolated scope.

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  • \$\begingroup\$ "When you connect the ground clip from the scope to the device, you are providing all the ground that the signal requires. All an oscilloscope does is to show you the voltage difference between the probe tip and the probe clip." - JRE - That cleared it up for me - I often go too deep in trying to understand electronics and was overthinking what a scope is actually doing - simply measuring a signal - ie - a voltage between a test point and ground same as with a multimeter - something I do every day. Seems obvious now that no earth ground would be necessary. \$\endgroup\$
    – user205820
    Commented Sep 3, 2021 at 23:45

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