# Split supply virtual ground questions

I'm trying to make a simple op-amp inverting amplifier. If I assume I have a split-supply, everything works fine, but I'm very confused about how split supply designs can actually work in the real world.

Let me show a simplified schematic for what I'm trying to make:

In this circuit, VGND is the virtual ground, giving me a split supply generated by R1 and R2 with an op-amp buffer.

It seems to me this would work fine if I was building an entirely self-contained circuit, but it seems like this type of split supply (where VGND is actually 15V off GND) limits what I can connect the output to.

For example, if the power supply (USB power supply) was mains referenced, wouldn't that mean I couldn't probe it with an oscilloscope as shown in the diagram, because the oscilloscope would be shorting 15V?

If it's powered by USB (and therefore 15V off USB ground,) wouldn't that mean I also couldn't connect the output to an Arduino, because when I tried to connect to the grounds I would again be shorting 15V?

It seems to me like this type of virtual ground circuit can never be connected to other circuits, so this sine wave amplifier could never be connected to an Arduino, right?

Is there a term for kind of problem?

Ideally, I would love to figure out how to get a split supply that makes VGND = GND, instead of making VGND = V/2, but I can't seem to find anything on the internet about this problem, so even just some terms to search would be helpful.

• Are you sure you want VGND=GND? because in that case, the output will be swinging from -3V to +3V. You can't feed a negative voltage to the Arduino. BTW, if you want to connect an oscilloscope, why not use capacitive coupling feature? That will eliminate the DC component of the output and hence you do not need to connect to VGND. I think the term used for this is not split supply. It is called setting the common mode voltage at the output. You can set VGND=GND but for that you need a negative power supply.
– sai
Commented Mar 11, 2023 at 6:46
• Hey @sai, I think I do want VGND=GND because this circuit is part of a function generator (which already has built in bias adjust). I do want it to be able to generate true AC (swinging above and below GND) though so I could at-least connect the ground to other USB powered devices without shorting 15V. Commented Mar 11, 2023 at 6:50
• you can keep VGND=GND and add a negative power supply for the OPAMP instead of using GND for the OPAMP.
– sai
Commented Mar 11, 2023 at 7:10
• you mention that USB supply is 30V but as far as I know, it is only 5V. Are you referring to something different than the USB ports that we have on our laptops?
– sai
Commented Mar 11, 2023 at 16:14

• Thanks sai, I think I really do need a -15V source here, do you have any suggestions on how to get -15V from a positive supply? I have been looking at some chips like the LMC7660 that used a switched capacitor. I was also wondering if an isolated transformer would work AC->DC transformer could work (since it presumably wouldn't be mains referenced anywhere) Commented Mar 12, 2023 at 0:55