# Modelling this differential ended system for SPICE simulations and grounds

Considering above illustration, imagine a voltage measurement by a voltmeter where the voltage is coming from a differential output device i.e a device which sends mirrored voltages according to its own ground. Lets say at this moment the orange device above outputs +1V and -1V differential signals. Now this is coupled to the voltmeter terminals and it will show us 2V i.e the potential difference.

And as you see only the + and - signals are coupled to the voltmeter. Not the grounds.

My confusion is Voltmeter has its own ground. And the diff. out device has also its own ground. But as you see their grounds are not connected to each other.

1-) How is it possible to model/draw this above system for a SPICE simulation? How to model a very simple diff output device and very simple diff. amplifier(representing the voltmeter)? I need a very simple educational model.

2-) In reality their circuit analog grounds are not connected. In simulations should they inevitability be connected ?

• GND is just an arbitrary point in an isolated system we assign 0V to. Generally spice simulations can only work with one system, i.e. one GND point. – PlasmaHH Dec 15 '17 at 13:44
• During operation, if I open the orange diff. out device and mark its analog GND as GND1: and similarly I also open the voltmeter and mark its analog GND as GND2. And finally imagine I now tie their grounds GND1 and GND2 by a copper wire, what happens? – user1999 Dec 15 '17 at 13:50