How can I make a dual-supply comparator output 0 to 5V?

Discussion in 'Analog & Mixed-Signal Design' started by David Lin, Mar 8, 2017.

  1. David Lin

    Thread Starter New Member

    Oct 11, 2015
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    Hello,

    I only have dual supply op-amps that require a minimum supply voltage of +/- 10V, and I want to use the comparator configuration. The comparator will either output -10V or +10V, but I want to make 0V and 5V, respectively, from this. What's a simply way to do this?
     
  2. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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  3. ScottWang

    Moderator

    Aug 23, 2012
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    What is the ic needs so much high minimum power?
    Many comparators has open collector(bjt) or open drain(N mosfet), the output is in a floating status when output is high and can be supply with single power, so if your comparator can match above described then it can be supply single +5V,0V, and the output needs to using a resistor as 4.7K or 10K to pull high, so when the output is high has a high level as +5V to output.
     
  4. David Lin

    Thread Starter New Member

    Oct 11, 2015
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    Thanks, do you mean source to ground though?
     
  5. David Lin

    Thread Starter New Member

    Oct 11, 2015
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    Im using lm741. Think I misunderstood the data sheet. It says recommended Supply voltage +/-10V, but i guess that actually means i can supply it with V+ to 10 and V- to 0?
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    The LM741 can be supply ±18V or higher, so how high do you need that is depend on what kind of application are you using.

    And next times when you using op amp as comparator then you have to mention it, otherwise we will thought that you are using the real comparator, I also used the LM324(4 op amp inside) as comparator.

    When you using this old op amp, you should know that it has a higher low level output as 1.4V, but the modern IC almost as 0.2V.

    So now please tell us what do you trying to do for the op amp?
    (Input Voltage/Output Voltage and function, purpose)
     
  7. crutschow

    Expert

    Mar 14, 2008
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    You also didn't read the data sheet for the input range.
    upload_2017-3-8_8-3-20.png
    You can see that the input will only go to within about 2-3V of the supply rails (the value shown is for supply voltages of ±15V.
    Thus with 10V for V+ and 0V for V-, the input voltage range is only between 2v to 3V (or less).

    If you need a comparator, then use a comparator such as the LM339/393 and operate it from V+ and ground (0v).
    Its input goes to 0V
    The output is open collector and you just need an output resistor (10kΩ or so) going to +5V for a 0-5V output.
     
  8. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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    I din;t say that.

    +5====resistor P===Logic tap(0-5)====source=Drain=====Ground

    Logic input==Gate==RESISTOR A==Ground


    Resistor P is the pull-up.
    Resistor A bleeds the gate leakage current to ground. It's not needed if your driving it push/pull which you are.

    There is an internal gate series resistor - so your good to go there.
    There is internal voltage protections for the gate, so your good to go there.
     
  9. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Many comparators are open collector output with external pull up resistor, so I'd start by lifting the op-amps and fitting actual comparators - then its relatively simple to put a current limiting resistor in series with the output so the transistor ratings aren't exceeded and simply clamp the free end of that resistor with Zeners and/or diodes to whatever voltage swing you need.

    Its not impossible if you leave the op-amp in place, but there's a few extra equations to keep an eye on, and comparators work much better as comparators than op-amps do.
     
  10. David Lin

    Thread Starter New Member

    Oct 11, 2015
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    http://www.ti.com/lit/ds/symlink/lm741.pdf
    Na, I can't be bothered to make numbers up.
    upload_2017-3-8_16-12-18.png
    I will try to obtain an actual comparator.
     
  11. David Lin

    Thread Starter New Member

    Oct 11, 2015
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    Oh, I see. Thanks.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Sorry, I don't follow what you are saying. :confused:
     
  13. hp1729

    Well-Known Member

    Nov 23, 2015
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    LM311 or dual LM319
     
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