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  #1  
Old 01-18-2012, 03:01 PM
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Default LM741 Offset Null Question

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.



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.
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Old 01-18-2012, 03:58 PM
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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.
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Old 01-18-2012, 04:23 PM
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A question: why are you doing this? Is this a school assignment, for fun, or a real application?
Just to learn about op-amps and offset null.
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Old 01-18-2012, 04:45 PM
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Try putting a 100K-ohm resistor in series with the wiper arm and -V.
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Old 01-18-2012, 04:52 PM
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Originally Posted by MrChips View Post
Try putting a 100K-ohm resistor in series with the wiper arm and -V.
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.
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Old 01-18-2012, 04:56 PM
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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.
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Old 01-18-2012, 05:38 PM
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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 by Adjuster; 01-18-2012 at 05:48 PM.
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Old 01-18-2012, 05:40 PM
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Quote:
Originally Posted by crutschow View Post
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.
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-THE...41tutorial.htm Scroll to the bottom of the page.

Amazing!
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Old 01-18-2012, 06:27 PM
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Quote:
Originally Posted by Adjuster View Post
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.
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.

Quote:
Originally Posted by Adjuster View Post
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).
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.
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Old 01-18-2012, 06:33 PM
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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.
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