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Re: Virtual ground issue in LTspice

 

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Hi Andy,

No, in theory it makes no technical difference whether you ground Vref or Vn. After all, voltage is relative, so 0V/12V/24V and -12V/0V/+12V should be identical.

My only reason for doing it one way rather than the other is that it better matches the *reality*. If you're using a 24V single supply and then deriving a midpoint voltage to use as a virtual ground reference, as here, the supply ground would go to the point marked as Vn. If you ground Vref instead, you're essentially providing a full bipolar power supply, in which case, the "virtual ground" part isn't really being tested at all - it's been replaced with a genuine ground. So this may not be the same after all.

I agree that if the input signal is referenced to Vref then the AC coupling is unnecessary. But again, in a real single supply circuit it would be typical for the input to be referenced to the supply ground (as is done here), in which case the AC coupling is required. It's all just trying to keep it as close as possible to what you'll actually *build*.?

Tom


On 31 May 2024, at 01:07, Andy I via <AI.egrps+io@...> wrote:

Tom,

I understand what you're saying, and yes there is pretty good experience simulating them in the way you described, by changing the Vn node to GND.? It fits what I wrote earlier that it should not make much difference which net you choose to call Ground, other than changing how you go about plotting and measuring voltages.? (But sometimes it matters because of the manufacturer's models.)

But it also got me thinking, is there any reason not to ground the Vref node?? Is grounding the Vn node actually better?? I'm not trying to detract from grounding the Vn node.

Grounding the Vn node seems to "feel" better than grounding a node that is being driven by an op-amp's output; but is it really better or does it just "feel" that way?

AC-coupling the input signal would be unnecessary (for the simulation) if it is a voltage source that is referenced to the supply's midpoint, or Vref in this circuit.

Andy

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