Question / Help - XY Plot of a Level detector

Discussion in 'Homework Help' started by Dieverse, Sep 29, 2016.

1. Dieverse Thread Starter New Member

Feb 22, 2016
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I am a student, and I am into my 2nd week of studying Op Amps. Yesterday we had our 1st lab and we briefly used XY Mode on the Oscilloscope for the first time and I am kinda unsure if I totally understand how XY Mode is displaying info........We now have an assignment and we have to draw the XY Plot for a level detector. Vin is 18 VPP the non inverting terminal is 5V and my power supply +/- 10 and thats basically all the info given..So I dont really understand what to draw???... Ive been searching google for a few days trying to find some info to help me, but cant really find what Im looking for....

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2. WBahn Moderator

Mar 31, 2012
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MOD NOTE: Moved to Homework Help

3. AlbertHall Well-Known Member

Jun 4, 2014
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The X-Y mode on a 'scope plots a graph of the two input signals, one against the other. so if the X input is the voltage across, say, a diode and the Y input is the current through it, then the 'scope will display the voltage/current relationship of the diode.

You are asked to draw the X-Y plot of a level detector. I presume that means draw a circuit which would display the plot on a 'scope.
One of the signals to plot is the output of the level detector but I don't understand what the other input is.

4. WBahn Moderator

Mar 31, 2012
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The other input to the scope is the input to the level detector. The problem very clearly shows that Vout is the y-input and Vin is the x-input. Vin is also clearly shown as the signal at the inverting input of the opamp and Vout is clearly shown as the signal at the output of the opamp.

5. AlbertHall Well-Known Member

Jun 4, 2014
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So the op-amp circuit is the level detector, is that right?

6. WBahn Moderator

Mar 31, 2012
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Yep. Notice the wording of the problem: "... for the following level detector."

The use of the term "level detector" in the problem is just eye candy -- it could have simply said, "... for the following circuit."

7. Dieverse Thread Starter New Member

Feb 22, 2016
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Thanks, but the problem wasnt what or where X & Y were or what Vin or Vout were, but was trying to figure out what to actually draw on the graph. Based on the info given Vout would be into saturation. So basically you have to draw a "Z" a straight line horizontally at 5v of the X axis and then straight across vertically at +/- Vsat.... ....

8. JoeJester AAC Fanatic!

Apr 26, 2005
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Draw it and post it.

9. WBahn Moderator

Mar 31, 2012
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Hard to follow your description. Sketch what you are thinking and post it.

10. Dieverse Thread Starter New Member

Feb 22, 2016
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Ok so the first image is an example from a power point slide (Bottom right shows XY plot for the circuit). 2nd picture is my assignment.... Part of the problem has been since the beginning of the semester we have had a supply teacher who just reads power point slides that the actual teacher made, and the power point slides are very vague. Another student and myself tried asking our supply teacher about some of the questions and even they said they were "unsure" what is being asked and would let us know monday (after she talked to the actual teacher..). The joke is this assignment is due tomorrow and then a test tuesday....We are supposed to have our actual teacher back soon. But its been a frustrating start to the school year.......

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11. AlbertHall Well-Known Member

Jun 4, 2014
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What is Vout when Vin = -9V?

12. Dieverse Thread Starter New Member

Feb 22, 2016
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Vout would depend on the gain. For calculations we use an ideal gain of 200,000 (as an Ideal op amp has infinite gain) So Vout = ED (the difference of V+ - V- ) x Aol (200,000) . So Ed = Vref (5V) - Vin (9v) = -4V Vout = -4V x 200,000 = -800Kv which isnt a real world scenario. Vout is impacted by the supply Vcc = +10 Vee=-10 ...So based on the prediction / calculation Vout will be Vsat

13. AlbertHall Well-Known Member

Jun 4, 2014
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Vin goes negative as well as positive. Your graph only shows Vin from 0V to +9V.

14. Dieverse Thread Starter New Member

Feb 22, 2016
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Well that kinda my question, in what I actually draw. If you look closely you can see where my line started at -9 but I erased it....

15. AlbertHall Well-Known Member

Jun 4, 2014
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If this was a real circuit, the input would go negative down to -9V and there would be a corresponding Vout.
Why do you think you should not include this part of the circuit's function?

16. WBahn Moderator

Mar 31, 2012
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It depends on gain no more than it does when Vin = 4 V. In that case you had no problem determining that the output would be +10 V. What is any different from this case and when Vin = 1 V or Vin = -1 V or Vin = -5 V or Vin = -9 V?

17. Dieverse Thread Starter New Member

Feb 22, 2016
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My question was never bout the circuit or its operation I already stated this....My question was about XY plottting and what has to be drawn. My question was also whether drawing XY plot had any relationship to XY mode of a scope....Thats all. Nothing about the circuit.

In the end I figured out what she is asking for, Essentially its visually showing the predicted outcome. In this case being Vout is -Vsat when Vin is most positive.

18. AlbertHall Well-Known Member

Jun 4, 2014
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Yes, what you are drawing is just what would appear on a 'scope in XY mode connected to Vin and Vout.
So where would the dot on the 'scope be when Vin is negative?