Digital Ramp ADC Analysis difficulty

Discussion in 'Homework Help' started by desecrate, Jan 14, 2011.

  1. desecrate

    Thread Starter New Member

    May 26, 2010
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    I have been using this website for a few years now. I decided to do a degree in Electronic Engineering last year, and I am already struggling :)

    I have attached a circuit that is part of a report I must do on a fuel level indicator. V_fuel ranges from 0 to 3V, where 3V is a full tank. OA1, OA2, and OA3 are HCT logic family op-amps (i.e. TL074). Vp=5 and Vm=-5.

    I have more questions than answers about this circuit to be honest. We are only just learning this material, so it is a big step for me to absorb it all.

    I'll begin with my attempt at explaining the circuit. I think that the resistors R1 to R4 will give out a stepped signal, ranging over time, from greatest voltage to least. Then they go into a summing amplifier? Which will then invert the signal too. The next op-amp I am uncertain of, I know it has a gain of 1 due to R6=R7, but I don't know what it's doing except inverting the signal? The next op-amp I believe is a comparator, that will compare V_fuel with the stepped signal. It will give out a 5V signal for when the analogue signal is closest to the step that corresponds to the binary equivalent. The diode is to make sure that a negative value never gets through? I don't know why the logic inverter is there.

    I must calculate the values of R1 -> R5, the assignment says that they should give out voltages between 0V to 3.75V. I am not certain how to do this as I dont know what the op-amp is going to do. I believe the resistor network, and the next two op amps are combined to be as what is normally considered a DAC?

    I must also draw graphs of points J, K, and L in a complete cycle. L should be trivial, I think J is simply the inverted steps. K back to the non-inverted steps again??

    My initial thinking was that the summing amplifier is level shifting down to 0 to 3.75V and the next op amp is simply inverting the signal. Anyway, sorry about this big lump of text, but I have so many questions and very little answers; if it is too much to decipher what I mean, then if you can, please try to explain what the first two op amps are doing and why they are doing it.
     
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  2. bertus

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  3. Georacer

    Moderator

    Nov 25, 2009
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    The middle OpAmp is for inversion and isolation (unneeded) purposes only. What you have to do is read Bertus' link to DAC converter in order to build a signal that will fluctuate from 0V to 3.75V when given the numbers from 0000 until 1111. That is as far as your assignment goes, as I see it.

    What I don't understand is the use of the counter in the first place and the purpose of the 7-segment display. Any thoughts?
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I assume that the latch is meant to hold the count value corresponding to the moment when there is a transition at the output of the last op-amp, as the voltage ramp reaches the sensor level. Displaying this as a number shows the fuel level.
     
  5. Georacer

    Moderator

    Nov 25, 2009
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    Ah, I see it now...
     
  6. desecrate

    Thread Starter New Member

    May 26, 2010
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    Things have cleared up a little bit after reading that link, Bertus. This is what I think is happening in order from counter to decoder:-


    • The counter counts from 0 to 15. It counts once every clock tick.
    • The 5V out is translated into a step by the resistor network.
    • This 'step' goes into the first op-amp which is setup as a summing amplifier. The summing op-amp shifts the voltage from 0->5V to 0->3.75V? The assignment says this is what the voltage range should be at point J.
    • The next op-amp is for isolation and inversion, so at point K, it will be exactly the same as J but inverted?
    • The comparator now compares with the analogue voltage, v_fuel. It will send +5V through the diode if the step voltage is equal to v_fuel. If not, it will send out -5V, which will be supressed by the diode? Giving around 0V.
    • The inverter is there so that the comparator, upon sending out a '1', i.e. +5V, will then be reset on the next count, as the shift register operates only on a rising clock edge?
    I am still uncertain about the resistor network/summing amp. If the resistor network can give out 0->3.75V then why does the summing amp need to exist?

    This is the only part of my course I struggle on, even the difficult math, programming, logic, physics, systems design, lab work, software engineering are a cakewalk compared to circuit analysis. It f***s me off and I feel like giving up on the course because of this alone, it's the most esoteric part of E.Eng.
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you can get 0 - 5 Volts from the DAC, you can attenuate the voltage to get the 0 - 3.75 Volts.

    Bertus
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    R1 to R4 are different values.

    From the counter, all outputs are 0V or 5V. What each resistor does is apply the "weight" of the change per binary digit. The LSB will have the smallest voltage change out of the summing amp, while the MSB will influence the largest voltage change from the summing amp. The two in between are in between.

    You need to balance R1-R4 with R5 to create 0-3.75v at the output. R5 will scale the output's maximum voltage.

    Here is a link describing the formula

    Bottom of this page shows it better
     
  9. desecrate

    Thread Starter New Member

    May 26, 2010
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    Oh man that bottom link was an excellent explanation, thank you thatoneguy. I think that may have cleared up all my problems in one go haha. I think however it's bedtime now, I'll finish this tomorrow and let you know if my equations simulate properly.
     
  10. Pacman2k

    New Member

    Jan 16, 2011
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    What would be a sensible clock frequency for this circuit?
     
  11. Georacer

    Moderator

    Nov 25, 2009
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    It really depends on the OpAmp, but all should handle KHz easilly. Not that you need such a high frequency. I don't think you'd expect the indicator voltage to change by the second, so I guess to get a "real time" result, a sampling time of 100 or 50 ms would be adequate.
     
  12. desecrate

    Thread Starter New Member

    May 26, 2010
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    All sorted. Cheers lads.
     
  13. krazh

    New Member

    Nov 22, 2013
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    Hello 'desecrate'.
    I find myself in a similar situation as you were in. Only this time, required to analyse a freezer indicator circuit.
    Is there a way I can contact you (if you don't mind), just to clear a few doubts ?
    Would appreciate it.

    Cheers !!
     
  14. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Hello krazh,

    You probably have noted the OP has been inactive since 2011. This doesn't necessarily mean they don't still monitor AAC. You could try sending a PM direct to them.
    Otherwise why not begin a new thread and then you will get plenty of suggestions from interested members.
     
  15. krazh

    New Member

    Nov 22, 2013
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    Hello t_n_k,

    Thanks for your guidance.
    I am new to AAC and for some reason am unable to find the menu-option to PM the OP directly. Maybe it has been deactivated by the user ? !!

    Indeed, your idea of a new thread sounds practical. But would be great if I were able to atleast attempt to contact the OP. Any suggestions ?

    Kind regards.
     
  16. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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  17. WBahn

    Moderator

    Mar 31, 2012
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    Or, when you click on their username on one of their posts, the second option is to send a private message to them. I just tested it for descrate and it works (brings up the PM page ready for you to enter the message).
     
  18. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The PM system is disabled for new members (post count less than 10).
    As soon as you reach 10 post counts, the PM system will be available.

    Bertus

    PS the last activity of the OP was 10 feb 2011.
    He also disabled the possibility to email him.
     
  19. WBahn

    Moderator

    Mar 31, 2012
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    Ah. I forgot about that. I've seen it pointed out several time and I can never, for some reason, remember it.
     
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