Calculating resistors values?? Need help!!

Discussion in 'The Projects Forum' started by nenadilic84, Dec 29, 2009.

  1. nenadilic84

    Thread Starter Active Member

    Aug 7, 2009
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  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Is there a DC offset on the generator input?
    Is the - powersupply of the opamp really connected to ground?
    If that is the case you can not have negative voltages at the inputs of the opamp.

    Greetings,
    Bertus
     
  3. nenadilic84

    Thread Starter Active Member

    Aug 7, 2009
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    1. There is no DC offset
    2. Yes, the power supply is connected same way as in the drawing
    3. I don't need negative voltages, just 0-5V pulses.

    Regards,
    Nenad
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When there is no offset on the generator, the voltage will go negative, wich is not allowed.
    You will have to create a DC offset to have always positive voltages.
    What frequency is the generator producing?

    Greetings,
    Bertus
     
  5. nenadilic84

    Thread Starter Active Member

    Aug 7, 2009
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    It is a signal generator and the frequency can be adjusted from 1Hz to 10MHz. I want to implement this in the signal generator so I can have duty cycle adjustable TTL signal. Do you have any suggestions???

    Regards,
    Nenad
     
  6. nenadilic84

    Thread Starter Active Member

    Aug 7, 2009
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    Ok I did a simulation in LTSpice. The circuit is working as I thought, but I'm not sure how optimal it is. Can some one here take a look at a simulation.
    Thank you


    Regards,
    Nenad
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    If you are going to use an input signal that varies around 0v (goes negative) then you will need to use both positive and negative rails (+5v,-5v) to supply power to the LT1016 comparator; otherwise you will destroy the comparator.
     
  8. nenadilic84

    Thread Starter Active Member

    Aug 7, 2009
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    What are you suggesting??? The goal is to have 0-5V pulses, and my input is Sine wave that is not DC offset. If I use both positive and negative rails (+5v, -5v) to supply power of the LT1016, is the output going to be a square wave ±5V? How can I resolve that??

    Thank you.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    If you want the comparator output to vary between 0v and 5v, you'll need to ensure that the input remains above 0v.

    One way to do that is to use a passive summing circuit.
    Change your R1 to 150 Ohms, and add another 150 Ohm resistor between the noninverting (-) comparator input to +5v.
    Remove the -5v from below R2, and replace it with a ground.
     
  10. nenadilic84

    Thread Starter Active Member

    Aug 7, 2009
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    I have made the modifications you suggested and attached the drawing below.The problem now is that the output of is between 0-3.6V instead of 0-5V. Why is this happening??

    Thank you.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Did you read the datasheet for the LT1016?

    3.6v as a high level with no load exceeds the minimum specifications for the device. Connecting a 3.3k resistor from the noninverting output to ground to provide a 1mA+ load still shows the output going above 3.6v.
     
  12. nenadilic84

    Thread Starter Active Member

    Aug 7, 2009
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    I'm sorry I have missed that. Thanks
     
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