Connecting an op amp to ground

Discussion in 'The Projects Forum' started by kutalinelucas, Jun 20, 2013.

  1. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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  2. wayneh

    Expert

    Sep 9, 2010
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    I believe you already have a thread on this topic.
     
  3. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    Sorry, but i think that thread has got a little confuddled, I hope this is a simpler question which may help me get on my way...I've been circling around this for 3 days now and starting to give up hope
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    Well first point would be. DO NOT use the 741 type opamp
     
  5. wayneh

    Expert

    Sep 9, 2010
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    OK, well let's start by asking what you really want to do. The short answer is, you cannot connect the circuit shown to a ground without that causing the rails and battery to become referenced to that ground.
     
  6. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    I'm starting to see that now, its just a big 'ol project and being able to measure an accurate output from the op amp really is a huge milestone...do you not think its possible to achieve a split-rail voltage with a common ground this way?

    ah...bugger. Do you have any suggestions on what I could use?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    I didn't say that. The circuit will work exactly the same and just as good (or bad) as being unconnected. The connection to ground only matters if there is another circuit somewhere. This circuit could be connected to a +1000V wire and it wouldn't matter at all - the rails would still be split apart by the battery voltage - unless there is another path to ground somewhere.
     
  8. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    Thats the problem, there are other circuits which need to be interfaced...this is the sensor circuit. http://s23.postimg.org/ej7btm6ij/bre...19_06_2013.png The outputs of each op amp are fed into the ADC of a pic, all aspects of the circuit have a common ground, it is only the split voltage part in the schematic above which is isolated, and I don't know how to bring them together
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Sorry to be dense but can you explain what sensor you want to use, and why you're looking at that virtual ground circuit? Your solution could be as simple as using battery negative as your connection to common ground, or better yet using a (slightly more modern) op-amp such as LM358 that can sense to the ground rail. I can never understand why there are so many folks trying to use the 741, the oldest op-amp possible.
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Just connect the Ltc1015 pin 4 to the ov line as stated in the previous thread. Why do you need a Negative supply??

    If you make a split supply use a better op amp like a TDA2010/ 2040 series.
     
  11. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    I'm sorry but it is more than likley that I am the dense one...I was using the virtual earth circuit to split a 12v supply into +/-6v to power the lt1025 and ltkAo1, which are matched op amp and cold junc. compensator from linear tech. I went this route because I thought it would be quicker than compensating with a thermistor through software...how wrong I was but I'm pretty much committed now. The circuit just amplifies output from two k-type thermocouples (one per op-amp pair). And finally, I chose the 741 because I'm pretty clueless when it comes to analog theories. I do have a box full of LM358AN op amps though
     
  12. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    Both pin 4 and 5 are connected to common ground as per the data sheet. both the 1025 and the op amp both operate on a split supply.
    So far everything has a shared earth, but the -+ power rails are isolated. when you say zero volts, do a put a voltage divider across the split supply and connect the middle to pin 4
     
    Last edited: Jun 20, 2013
  13. wayneh

    Expert

    Sep 9, 2010
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    I'm about 90% sure you could eliminate the negative supply by establishing a reference voltage above the common ground, and use voltages below that reference as the "negative" voltages. But, it would take me a while to figure that out and I've gotta run.

    Thing to remember is, voltages are all just relative and sometimes it's easier to think of there being no such thing as a negative voltage, there is only a voltage lower than some other voltage. So using a split ±6V supply is identical to a +12V supply with a 6V reference in the middle. The difference is just semantics.
     
  14. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    thanks for the advice...but I'm praying there's a way I can amend what I've done so far, as the circuits already been built and EVERYTHING besides the initial 2.5mA of the thermocouple is working great.
     
  15. Gaudeamos

    New Member

    Jun 24, 2013
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    Hi
    Pardon me for getting on board late...

    You may connect the opamp circuit's virtual ground (or any other node in the circuit) to the ground of any ONE other external circuit any time. Just be aware that 'their ground becomes your ground'.

    When you connect to more than one other circuit, you must make sure that each external circuit either (A) is not connected to any of the other external circuits, or (B) is connected such that the ground potential is the same for all external devices.

    Here's an example of how there could be a problem with a virtual ground connection:
    Check out "Why can't a cMoy be powered from a vehicle's +12V accessory supply?" here: http://www.jdslabs.com/faq.php

    hope this helps you
     
  16. LvW

    Active Member

    Jun 13, 2013
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    Of course, R3 should be outside of the unity feedback connection.
     
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