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Homework Help: Digital circuit question

  1. Aug 31, 2008 #1
    Hello, I have the following situation. Its probably all very simple, but I want to be sure I understand this correctly.

    http://i67.photobucket.com/albums/h304/john_ukranian/circuit2.jpg [Broken]

    First of all you must know a little about my background. I know what electricity is, how it works, how a battery works and etc very well. I know many basic things very well. But I am very bad at reading circuit diagrams. In fact I was given this on my first day of Digital Logic course, when I only had ONE day of Linear Circuits and Systems an hour before.

    You see nobody taught me how to read circuit diagrams, and yet they expect me to know how to do so in another course which goes together with the one that will teach me how to read circuit diagrams. Its all quite absurd in my opinion.

    So as I read the input binary string, the only resistors which should be on are 4.7, 2.2, and 1.0 Kohms. All the other ones are connected to ground and have a current which is so low, its insignificant in this situation.

    Now comes the part I don't understand. Are these 3 resistors in series or parallel? How do I know?

    According to what I think, they are in parallel, because it looks to me like the original circuit with the resistors 6.8, 3.3, and 1.5 Kohms switched off.

    Here is what I drew.

    http://i67.photobucket.com/albums/h304/john_ukranian/circuit3.jpg [Broken]

    Now of course if I am to determine V-out I need to know whether to combine the resistors in series or in parallel, and to be honest I have no idea.

    Please help.
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Sep 1, 2008 #2
    By the way this is not a homework question, I'm just asking for help in drawing the circuit correctly. If I can do that, I know how to solve the problem.
  4. Sep 1, 2008 #3
    whenever you are analysing an electrical circuit, make sure that you know the functionality of induvidual blocks.
    In your case the data box might be working like a buffer/latch.
    You cant consider the resistors to be in parallel. each of the resistors are driven by their respective latches.
    So the output would be a result of outputs at these data pins, and these resistors are used as current limiters. Also check if the application requires precision resistor values.

    You have consider all these parametrs and more if you want to dig deep.
  5. Sep 1, 2008 #4


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    Staff: Mentor

    That looks like a weighted DAC circuit. You can use wikipedia.org to read about weighted DACs. Check out the R2R ladder DAC explanation at wikipedia.org as well.
  6. Sep 1, 2008 #5
    What the heck is a latch? The professor said the data box only has switches (transistors I assume) which switch the resistor to ground or to the battery. 1= to battery. 0= to ground.

    The professor expects me to draw the corresponding circuit using only the above circuit diagram and the binary string. As I understand it, the binary string just tells me which resistors are on and which are off.

    This is not supposed to be hard, as this is the first assignment of an introductory course. The data box just has switches. Nothing else.

    But like I said I'm extremely bad at reading circuit diagrams, because nobody really taught me how. Not yet... I'm taking the course which will teach me right now.

    I have about 10 such binary strings. If you guys could at least draw the circuit which is resulted from the binary string 010101 for me, I would understand why you did it, and be able to draw the other ones myself.

    Here is the diagram again.

    http://i67.photobucket.com/albums/h304/john_ukranian/circuit2.jpg [Broken]
    Last edited by a moderator: May 3, 2017
  7. Sep 1, 2008 #6


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    Staff: Mentor

    A latch is a switch that holds its state until changed. A gate just drives its output in response to what is at its input.

    In the circuit you show, the implication is that there are 5V logic gates in the box, and when the logical input is "0", the gate output is 0V. When the logical input is "1", the output is 5V.

    So you just need to put 5V or 0V on the left side of each resistor, based on what the logical input string is. Then use KCL to solve for the output voltage. For example, the output voltage sums to 5V for input = 11111, and sums to 0V for input = 00000. You could use an Excel spreadsheet to calculate the intermediate values for other patterns.

    Again, you can read more about this by searching on "weighted dac" at wikipedia.org or at google.
  8. Sep 1, 2008 #7


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    Staff: Mentor

    I'll move this thread to the Homework Help forums, where homework and coursework should be posted.
  9. Sep 3, 2008 #8
    berkerman your explanation doesnt help in this case. I googled weighted dac and that is going much more into specifics than is required for this assignment.

    As I said, this is an introductory Digital Logic course, they assume I know next to nothing about circuits, which is true. I am enrolled in a course which will teach me about circuits right now. But they WILL teach me in a month or two. I know very little currently.

    All I can do is add resistors in series and parallel and capacitors in series and parallel from physics. Thats all.

    And that is all my professor said he wants me to do.

    The professor said that depending on the binary input I get a different circuit diagram, where different resistors are in series and in parallel. All I have to do is add them together to get the equivalent resistance and then solve for V-out from ohm's law.

    My only question is, how did my professor draw this http://img148.imageshack.us/img148/3264/circuit45ti7.jpg [Broken]

    from this http://i67.photobucket.com/albums/h304/john_ukranian/circuit2.jpg [Broken]

    And notice he redrew all the resistors, even the ones which don't have any current in them during the binary string 010101. In what he drew I don't even get where the switch is.

    Which resistors have the 5 volts, which have 0 volts. I cant read the diagram, thats the problem. I don't understand the diagram at all. And I can't tell you why I cant understand it, since I did similar circuits before succesfully. This one is just different and weird. Nobody taught me how to read something like that.

    Can't you guys just explain. Draw some circuit diagrams for me, and explain WHY you drew them the way you did. Thats all i ask. Thank you.
    Last edited by a moderator: May 3, 2017
  10. Sep 3, 2008 #9


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    Science Advisor

    Well, the "simplification" your prof drew is incomplete. There's a point of connection (a node) between every one of the centers of the resistor pairs that's been left out. If you do this, you can see that the 1k, 2k2, and 4k7 resistors are in parallel with each other, and are in series with the parallel resistor formed by the 1k5, 1k3, and 6k8 resistors.

    You ask which resistors have 5 V, and which have 0 V. However, none of these resistors have 5 or 0 V across them (one end of each of the resistors is hooked up to EITHER 5V or 0V, and the other end tied to each other, at the node point), and each carries current. You may be hung up by thinking of the DAC as some complicated device, when the point of this exercise is just to give an application of basic circuit analysis.

    So, forget about the DAC. Where a DAC output (Q0 to Q5) is '1', hook up a 5V supply. Where the DAC output is '0', hook up 0 V. Now, redraw the circuit so that the shared point is in the middle, and all the resistors connected to 5 V are at the top (going to 5 V), and all the resistors connected to 0 V are at the bottom (going to 0 V). Now you've got the (corrected) diagram your prof drew!

    To go above and beyond the scope of the question, the beauty of this circuit is that you can create a range of voltages between 0 V (where all of Q0 to Q5 are at 0V) and 5 V (where all of Q0 to Q5 are at 5 V instead) at the output point using digital outputs! May not mean so much now, but keep this thing in mind going forward in EE school.
    Last edited by a moderator: May 3, 2017
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