Reading a Opto coupler data sheet.

Discussion in 'General Electronics Chat' started by Vasanthkini, Oct 13, 2010.

  1. Vasanthkini

    Thread Starter Member

    Jun 9, 2010
    40
    2
    Hey,

    Could some one help me in figuring out the maximum operable frequency of a opto coupler from the data sheet?How exactly will it be mentioned in the datasheet?


    Thanks:)
     
  2. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Are you replacing another opto, or are you designing the driver and receiver from new? What will the frequency response influence?
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    Well frequency is only part of the picture.

    Propagation delay will have a much larger affect on performance where it matters. Also the CTR or current transfer ratio is important because it degrades over time.

    What frequency did you have in mind?
     
  4. Vasanthkini

    Thread Starter Member

    Jun 9, 2010
    40
    2
    Helo,

    Basically,there is a frequency limit above which i feel the opto coupler does not operate.The maximum frequency that the opto coupler can be operated at.Lets for example say,the device(Opto) has a square wave as the input for the photo diode.The frequency of this square wave is what i am talking about !


    Thanks for your time guys !

    :)
     
  5. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    98
    What maximum frequency are you looking to isolate?
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    I know what you are asking and I'm saying that you are asking the wrong question. Let me be more precise. An inexpensive opto like say a 4N35 will have a propagation delay time of 3-5 microseconds, and a current transfer ratio of 40%. These two facts are what you should be concerned about. A really expensive opto like the HCPL-7710 has a propagation delay of 40 nanoseconds and I forget the CTR. They were fine for data signals up to 500 kHz but not 1M because the delay did not allow for a positive cable length given the contraints of the network physical layer which had absolutely nothing to do with frequency.
     
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  7. Vasanthkini

    Thread Starter Member

    Jun 9, 2010
    40
    2
    This information was pretty useful.Between,is there a relation between Propagation delay,CTR and operating frequency?

    Thanks
    :)
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    The relationship, if one exists is probably not useful or illuminating. What you are really interested in is how well does the opto preserve the essential elements of the signal that you are trying to pass through it. The simple LED/Photo-transistor types have the additional problem of requiring an external pullup in common-emitter configuration or an external emitter resistor in the emitter follower configuration. That in conjunction with stray capacitance will have a substantial influence the integrity of the signals that you are trying to pass. The more sophisticated types like the HCPL-7710 have an actual push-pull amplifier output.

    I'd like to suggest that you explain the requirements of your application and ask: "what kind of opto-isolator should I be looking at?"
     
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