Virtual ground issue

Discussion in 'The Projects Forum' started by Shashi_PMTC, Feb 21, 2011.

  1. Shashi_PMTC

    Thread Starter New Member

    Nov 22, 2010
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    Hi everyone,

    Now am working on measurement of resistance using AC wheatstone bridge circuit and display it on LCD. I used LM358 to amplify signal ,to which i gave dual supply +12V & -12V to get full ac waveform. After rectifying ac signal to dc using precision rectifier , i give it to PIC16F72 analog input.

    But while measuring voltage at input its floating (Ref voltage issue). I think it is because of grounding issue. I have some doubts in this circuit.

    Is it possible to connect virtual ground to real ground?

    So kindly help me on this issue.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
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    Post your circuit, please.
     
  3. Shashi_PMTC

    Thread Starter New Member

    Nov 22, 2010
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    0
    Dear sir,

    Can you give your mail id i will send circuit diagram... here am unable to attach any doc.


    Moderator's note: Please confine questions and answers to the public forums.
     
    Last edited by a moderator: Feb 21, 2011
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
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    No you can add attachments using the paper clip symbol [​IMG]in the menu. Post your document in pdf or png format. If you can avoid it. Do not use office doc format.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You must click the "Go Advanced" button near the bottom of the thread before you will see the paper clip or "Manage Attachments" button.
     
  6. Shashi_PMTC

    Thread Starter New Member

    Nov 22, 2010
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    Kindly checked the circuit and provide a solution.

    Thanking you,

    Regards
    Shashikant V Vadavi
    Lucas-TVS,Padi Chennai-50
     
  7. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    The purpose of the virtual ground is not to bais the opamp into saturation!

    The purpose of the virtual ground is to allow splitting of a single voltage power supply, by creating a voltage that is half the voltage of the input power supply, which, by the way, floats.

    The right way: disconnect your voltage source from ground. Connect the "virtual ground" output of your opamp to ground. Disconnect the inverting input of the opamp in the full wave rectifier from ground (only the three summing resistors should be connected to that input). Now everything should work.

    If this is a SPICE simulation, you might have to add a 100 Meg Ohm resistor from one battery terminal to ground (depending upon the opamp model) in order to get it to converge.

    See how much better things are when you post a schematic?

    AN ALTERNATIVE, PREFERRED IF NOT USING A BATTERY

    Get rid of the "supply splitter", opamp that makes the virtual ground and instead use separate positive and negative power supplies.
     
    Last edited: Feb 23, 2011
  8. Shashi_PMTC

    Thread Starter New Member

    Nov 22, 2010
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    0
    Dear Sir,

    Thanks for our schematic diagram analysis.

    We have quite confusion with your suggestion as disconnect inverting input of fullwave rectifier opamp from ground.

    Kindly explain is this non-inverting input ?.

    Thanking you,

    Best Regards,
    Shashikant V Vadavi
     
  9. Shashi_PMTC

    Thread Starter New Member

    Nov 22, 2010
    6
    0
    Dear Dickchappel,

    We have tried out your suggested modification in our circuit ( ie. disconnected voltage source from ground and connected opamp virtual ground to ground.)

    We observed that output of opamp is not coming as required. Its giving output voltage as open circuit voltage.

    Once again i list out our problem.

    When i check signal conditioning and micro-controller LCD interfacing circuit separately, both are working fine.
    I use virtual ground for signal conditioning circuit and real ground 0V for micro-controller circuit.
    When i connect the conditioned (Amplified,rectified) signal to micro-controller analog input pin, its not working ie. shows max. +5v input.
    Is it because of virtual ground and real ground reference?.
    Can you help me in this regards?

    Regards
    Shashikant Vadavi
     
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