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Problem With OPAMP

Discussion in 'General Electronics Discussion' started by sid2286, Oct 28, 2016.

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  1. sid2286

    sid2286

    102
    1
    Aug 24, 2011
    Dear All,

    I have designed a circuit and it is working ok in parts: Attached Circuit diagram:

    working part:
    1. I am getting output at TP5,
    2. when i connect the output from TP5 to TP7 directly my XTR111 and Comparator section is working ok.

    problem:
    1. I have used an OPAMP since when i Set zero (say 1V) and while doing calibration using SPAN i set (say 2V) when I remove the calibration set up then reading does-not come back to 1v but comes to 1.5V. ( 1st How do i resolve this problem)

    2. So i tried using the OPAMP 604, but i am not getting any output at TP6, for 1v , ideally i should get -1V which is not happening.

    3. next OPAMP is also not working.

    4. what i noticed is at the input of R11 i am getting 1v but at the otherend of R11 it is getting reduced to 450mV (Not sure why)
    when i remove R14 i do get 1v at R11. but i am clueless how why that is happening.

    please help me reslove this problem
    Thanks,
    Sid
     

    Attached Files:

  2. davenn

    davenn Moderator

    13,552
    1,850
    Sep 5, 2009
    do you understand how op-amps work ?
    I suspect, not quite

    What did you expect to see at the inverting input side of R11 ?

    lets take a look at just that section of your circuit
    have deleted a bit of irrelevant stuff to make the layout easier to read

    upload_2016-10-29_15-5-30.png


    do you know the 2 rules of Op-Amps, keeping in mind that they are for an idea op-amps, but are applicable for general situations

    just in case you don't .....

    Rule 1) no current flows in or out of the 2 inputs
    Rule 2) The op-amp tries to keep the inputs at the same voltage

    now keeping rule 2 in mind, that should answer your Q I quoted


    so if you disconnect the feedback resistor, how do you think that affects Rule 2 ?


    Dae
     
  3. sid2286

    sid2286

    102
    1
    Aug 24, 2011
    What did you expect to see at the inverting input side of R11 ?​

    since the R13 is connected to ground there should have been no voltage at pin 2 i.e. to say both the pins inverting and non inverting pins are at 0 potential for me to get a -1V at the output. but that is not happening, I dont the reason where i am going wrong.

    request you to help.
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    I don't see any of these OpAmps powered from a split (±) supply. What I do see is the (-) power pins at GND. So how can the OpAmp's output swing below zero? :confused:

    Chris
     
  5. davenn

    davenn Moderator

    13,552
    1,850
    Sep 5, 2009
    It cant but that may not be an isdue depending on how its being used

    That was going to be a future Q from me once we got the basic op amp operation sorted out

    Maybe its time for that Q now ....

    A negative output cannot be generated from a positive only supply
    (Not that im aware of any way)

    Dave
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,359
    2,755
    Jan 21, 2010
    Without inductors or switched capacitors (but I don't think that's relevant here)
     
    davenn likes this.
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    648
    May 8, 2012
    Maybe we should be asking Sid why he thinks he should measure -1V on the output pin?

    Chris
     
  8. sid2286

    sid2286

    102
    1
    Aug 24, 2011
    Oh now I got it., I am extremely sorry for posting this one. I simply read the datasheet which say the opamp works on 0 to +5ve supply, without furthermore thinking I used it.

    But now that I have used it, is there anyway I can tweak it or should I be replacing the opamp and use 7660 for negative supply ?
     
    davenn likes this.
  9. davenn

    davenn Moderator

    13,552
    1,850
    Sep 5, 2009

    yeah you really should be using a split rail supply op-amp

    maybe play with some TL072's or 074's
     
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