LM741 Offset Null Question

Discussion in 'The Projects Forum' started by tracecom, Jan 18, 2012.

  1. tracecom

    tracecom Thread Starter Well-Known Member

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    At this website, it says "Many OP-AMPs have two pins labeled OFFSET NULL. When both inputs are connected to the same voltage, the output should be zero. If the project requires a zero output under these conditions, the OFFSET NULL should be adjusted by adding a 10k pot between the Offset Null pins with the centre of the pot connected to 0v. By adjusting the pot, the output will produce 0v."

    And it gives this diagram.

    [​IMG]

    I am trying to duplicate this. I am using an LM741CN with a dual power supply (two 9V batteries,) but I cannot get 0V out. When I adjust the pot with the wiper connected to -V, the voltage changes and even goes from positive to negative, but the change is so abrupt that it is impossible to stop at 0V.

    I see in the diagram that the center of the pot is connected to -V instead of 0V, and I have tried it both ways. Which is correct, and what could I be doing wrong?

    Thanks.

    Attached Files:

  2. mcasale

    mcasale Member

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    A question: why are you doing this? Is this a school assignment, for fun, or a real application?

    The 741 op amp is ancient. Depending on what offset you are trying to achieve (don't say zero - what do you really need?) there are MANY newer op amps with very low input offset voltage.
  3. tracecom

    tracecom Thread Starter Well-Known Member

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    Just to learn about op-amps and offset null.
  4. MrChips

    MrChips Moderator Staff Member

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    Try putting a 100K-ohm resistor in series with the wiper arm and -V.
  5. tracecom

    tracecom Thread Starter Well-Known Member

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    Thanks. I tried a 100k, 10k, and 1k. With any of these, the pot became totally ineffective, i.e., no voltage change at all on pin 6. I must be doing something really dumb.
  6. crutschow

    crutschow AAC Fanatic!

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    You cannot short the inputs and adjust the null to give zero output. The high open loop gain will cause the output to abruptly change, as you observed, due to the finite resolution of the pot.

    Normally the pot is used when the op amp is connected in a circuit where the closed loop gain is lower. Alternately if you adjust the offset in your circuit just at the point that the output switches, you will be at or very close to the minimum offset point.
  7. Adjuster

    Adjuster Well-Known Member

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    This data-sheet shows the pot connected to -Vcc. http://www.ti.com/lit/ds/symlink/ua741.pdf

    The offset be easier to adjust if a multi-turn pot is used. The adjustment will be very fierce on a simple 270° pot.

    Also, adding equal (value to be determined) fixed resistors in series with each end of the pot. may help, but a balance will not be possible if these resistors are made too big.

    Even with these measures, adjusting the offset will be difficult because the gain is large. As has been mentioned, nowadays this may be more useful as student exercise.

    Edit: Following from what crutschow has said, this is more easily done in the context of negative feedback. For circuits using no feedback, or even positive feedback (such as comparison functions) it may be more convenient to trim the reference voltage so as to obtain the correct threshold voltage, omitting the offset null altogether. Offset null adjustment can impair performance, particularly Power Supply Rejection Ratio (PSRR).
    Last edited: Jan 18, 2012
  8. tracecom

    tracecom Thread Starter Well-Known Member

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    Thanks. That certainly is supported by my experiment.

    So the information that I referenced in my first post in this thread is a bunch of hogwash written by someone who never tried it. And that's not the only place I have seen the same info. Here's one that even gives step by step instructions: http://www.hamradioindia.com/HRI-THEOR/General/741tutorial.htm Scroll to the bottom of the page.

    Amazing!
  9. tracecom

    tracecom Thread Starter Well-Known Member

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    Thanks.

    I tried a 10 turn pot, but to no avail; the voltage still switches abruptly from -8.xx to +8.xx. I also tried to insert some resistance; with 10 ohms in each side, the results were unchanged. At 22 ohms and above, the pot was ineffective. Maybe the answer lies somewhere between 10 and 22 ohms, but I have lost my enthusiasm for additional searching.

    Using a voltage divider with a pot in one side feeding one of the inputs seems to be the standard method. Someday, maybe I'll see a real LM741 schematic with pins 1 and 5 being used.
  10. MrChips

    MrChips Moderator Staff Member

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    Oops. I had not noticed that the op has no feedback resistors and he is using the 741 in open loop gain. You cannot do that.
  11. crutschow

    crutschow AAC Fanatic!

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    Unfortunately there are a lot of dry-lab circuits on the internet that have never actually been built or tested. Caveat emptor.:rolleyes:
  12. tracecom

    tracecom Thread Starter Well-Known Member

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    Well, at least it was worth every cent I paid. :)
  13. SgtWookie

    SgtWookie Expert

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    Always look in the datasheet for the part in queestion first, as errors abound on the Internet.

    While the schematic shown essentially agrees with the National Semiconductor (now TI) datasheet, you need to realize that their is essectially NO feedback path, which causes the opamp to operate open loop, or maximum gain. OPamps run open-loop usually have a gain of 100k or more, so the output will slam from one rail to the other.

    If you added some divided feedback, then you would be able to control the output voltage far more easily.

    Have a read through this section of our E-book:
    http://www.allaboutcircuits.com/vol_3/chpt_8/3.html

    In a nutshell: an opamp attempts to keep the voltage on the inverting (-) input the same as the noninverting (+) input by adjusting the output.

    Once you understand this concept, opamps will be a whole lot easier to work with. Please do a lot of experiments with your opamps until you realize that the above is indeed the case. Use values of feedback resistors in a range of 3k to 1 MEG to avoid excessive loading of the opamp output.
    Last edited: Jan 18, 2012
  14. SgtWookie

    SgtWookie Expert

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    Gee, I guess I should've hit REFRESH as I'd loaded the page around 11:AM and replied finally at 3pm.... :rolleyes:
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