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Connect 'virtual ground' to 'common ground'

Discussion in 'General Electronics Discussion' started by osterchrisi, Dec 17, 2012.

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  1. osterchrisi


    Mar 8, 2011
    Hello guys,

    simple question: I have a circuit running off +9V and (ideally) having an output of 8Vpp around +4.5V, so basically somewhere between 0.5V - 8.5V. So this is what is mostly referred to as a 'common ground'.

    Now the question is: If I was to connect the single ended signal (say the output of my last opamp) with a cable that makes that signal refer to an external ground (which would be at 0V seen from my circuit), would that signal then become bipolar with 8Vpp and swinging around -4V to +4V?

    I get it that with AC signals it would be a simple capacitor (+resistor) solving my problem but what about DC voltages? Does that work or not?

    Thanks for all answers!
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    I'd call that a virtual ground since the "true" ground is normally 0V.

    But, anyway, concerning your question in the second part of your post: As long a the true ground of your circuit is not connected to the external circuit, you can connect the virtual gropund of your circuit to the true ground of the external circuit. Your output signal will swing from 0.5V - 8.5V with respect ti the true ground of your circuit. Since the virtuial ground is offset from the tue ground by 4.5V, 4.5V becomes the reference potential for the outputr signal and is connected to 0V of the external circuit. Therefore the external circuit will see a signal between -4 V and +4 V.

    Take care that your internal 0V is not connected to the external circuit (I mentioned that before). Otherwise this connection may short-circuit the virtual ground.
  3. osterchrisi


    Mar 8, 2011
    Oh yeah, sure! I read so much about it before I posted that, so that I simply mixed it up by not thinking too much about it. I basically wanted to write it in apostrophes because I also read vivid discussions about it being really a 'ground' or not...

    But anyway, thanks for your answer! That's what I thought, gonna try it out soon!
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