What if you use your second (or indeed first, or indeed any of all the other devices) with a device made by someone thinking the same as you with a shared supply in some larger encompassing device? Call the person and ask if the one ground is the same as the other ground? i.e. Is signal ground the same level as power ground?
I can tell you, in "cheap" designs many do it the same way as you, but often use the full power input to divide. So you are going to compete. Violently, if their reference is 12V/2.
If you want to make a good design that's universally compatible, you make sure that the external grounds are all actually the same hardwired level. So if you output a +5V and a 0V only, that 0V should be the same as the 0V on all the audio plugs. That's a good design.
So in this set-up, what would have been better is to output your original 5VDC as a +/- 2.5VDC balanced around your 'weak' virtual ground in a three pin plug. Then suddenly you have a balanced power supply to your second box and you could even devise a system where if the middle pin is not lifted to half VCC, then you make it yourself if needed. Or for separate use make a second DC plug with a switch built in that activates the divider and disconnects the 5VDC lines (for safety).
The high-end or 'common rail' design thing if you have a DC input jack, is to use a voltage inverter to obtain your negative rail, +9VDC --> -8VDC ; +9VDC --> +5VDC ; -8VDC --> -5VDC (or 2.5V for each, but then, use 5VDC in, for efficiency). Or even better a fully isolated balanced DC. Unfortunately nobody else who makes $10 gadgets does this, so you can't even assume it.
Want to use a divided VCC as a virtual ground with an external DC adapter and stay safe and compatible to "shared power situations"? One of two options:
- You'll have to decouple and route the hard external ground through input and output or just force your own ground on the output again. It's not awesome, but it's what it is.
- Add the DCDC isolation on the DC input I discussed before. If your consumption is as low as 1W DC/DC with +5V and 0V out or +3.3V, 0V and -3.3V out is about $5. Using the balanced with two low-drop 2.5V regulators, one negative one positive, will even get you much better thermal stability on the ground and its relation to your supplies. Plus, it sources and sinks the maximum available current with no problems or aberrations. In fact, doing that same trick with your original DC input would have been possible by first dividing the 9V to virtual ground and then dropping the original power lines to +2.5V and -2.5V.
If you go down the rabbit hole of "Oh, I'll just assume it's only a 0.1V difference", in six months you'll be kicking yourself, because you have a 9VDC system with a 4.5V virtual ground, etc.
Yes. I've used the term "virtual ground" for a node used as reference for signals and that was generated by making roughly a mid point between two supplies. I think that's pretty common usage.
I also agree that "virtual ground" can be used to describe the negative input of a opamp in inverting amplifier configuration, but only when the positive input is tied to ground.
This is a loose enough term that it should be defined before use, unless it is clear from context. Talking to someone about virtual ground without context or a definition is wrong.
Best Answer
All power supplies that are isolated from the power line will have some form of transformer.
There will be capacitance between primary and secondary of that transformer.
There will be "displacement current" coupled across that transformer.
That current will explore all possible paths, to return to the source of the charge.
Switching power supplies will produce 100 nanosecond edges (if not faster), and your opamp needs to harness that energy.
Or you must use bypass capacitors.
Otherwise ugly "ground loops" form, as that transformer-provided current makes its way back to the source.