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I'm building a device which includes oscilloscope, signal generator and multimeter. I have trouble deciding how to properly ground each part, so my questions are:

Can oscilloscope ground be connected to the signal generator ground? What happens if these grounds are then connected to different potentials when in use?

If device is powered by 5V external power supply (like phone charger), does this mean that device ground is "virtual"?

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    \$\begingroup\$ If you connect two "grounds" together and then connect them to different potentials, you're shorting tho potentials together and bad things will happen. The classic example is trying to probe a circuit powered by a bridge rectifier and no transformer. \$\endgroup\$
    – DoxyLover
    Commented Sep 18, 2016 at 17:43
  • \$\begingroup\$ @DoxyLover with an oscilloscope. \$\endgroup\$
    – Bradman175
    Commented Sep 19, 2016 at 0:04

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It is a fairly safe bet that an oscilloscope input has one side connected to earth through its power chord. If it has a BNC connector as input, the shell of the BNC will likely connect to ground. Its easy to measure with your multimeter "ohm" function or "continuity" function.
A battery-powered oscilloscope is not ground-connected, but you should assume that significant capacitance appears from it to ground, possibly through the user's own body. The "hot" 'scope input is usually expected to be 1 Meg ohm in parallel with tens of picofarads capacitance.
Function generators are far less predictable. Some have their cold-side grounded to earth (to their power-plug ground) while others float. A floating generator is more versatile, allowing a few to be stacked in series. The manual should tell you how many volts above or below ground you may stretch its cold-side connection. The rare oscilloscope that is ungrounded should state a similar spec in its manual
You may certainly connect oscilloscope and function generator grounds together. A common fault is mis-identifying which function generator or 'scope output is "ground-side". Cross-connecting the hot terminal of one to the cold terminal of the other may cause you to believe that one instrument is faulty. Connecting the ground of a grounded 'scope to the ground of a grounded generator creates a ground-loop that can cause some very subtle noise problems when measuring very small signals.
Multimeters are universally ungrounded, even when powered from a wall plug.
When using a 'scope plus function generator with other equipment, it is wise to consider these measurement tools to be grounded, and that any connection you make to their cold side will ground it.
There are sections of your wall-connected phone charger that cannot stand to have a ground-connection made - the circuit will fail. Other sections of a phone charger can accept being grounded with no problem. Moving the ground-connection around while probing an unfamiliar circuit should be done with considerable forethought - are you grounding a forbidden circuit area? It is a very easy mistake to make, but the spectacular results usually ensure that it is not made often in future.

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Can oscilloscope ground be connected to the signal generator ground? Yes, and most oscilloscope grounds are connected to signal generator grounds if they are built in. It depends on the result you want, there may be advantages to having them separated to avoid common mode noise. Make sure your ground is low enough resistance to avoid common mode noise.

What happens if these grounds are then connected to different potentials when in use? Nothing will happen provided that you connect ground to ground. If the ground is at a different potential (ie lets say one is 10mV above the other) then current will flow between the two grounds.

If device is powered by 5V external power supply (like phone charger), does this mean that device ground is "virtual"?

Isolated = virtual. Isolated means the ground is not connected to mains ground, it also means you can 'float' the ground. This is usually done via a transformer.

Keep in mind that volt meters don't have a ground, you measure the potential between any two points, and most multimeters these days will do 100's of volts. A multi meter is a differential measurement and not connected to ground and is isolated.

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