Selecting & Using a fast Optocoupler

Discussion in 'General Electronics Chat' started by Power_user_EX, Dec 23, 2012.

  1. Power_user_EX

    Thread Starter New Member

    Dec 23, 2012
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    Hi all , I m working on a parallel port breakout board for CNC controller which will send step/direction pulses to stepper driver.

    The controller software(Mach3) will generate pulses @ 25Khz , 35Khz , 45Khz , ... as per my setting. Personally I think 25Khz is enough for me and I probably wont go over 45Khz ever. Also the pulse width of these pulse is between 3us(min) to 15us(min) which can be set by user.

    After googling around I found some optocoupler ICs like PC817/PC827 , CNY117 , Ltv847 , 6N139 , H11L1M.

    Ltv847 , 6N139 , H11L1M seem to be fast. H11L1M has in-built schimmt trigger while the other 2 don't .. so H11L1M seems to be the candidate but its costly as compared to LTV847 , PC847.

    I would like to know about better &/or cheaper alternatives for the above ICs considering my design requirements.

    At the moment I am stuck between LTV847 and PC847.

    I found some design guidelines in C11 parallel breakout board manual @ http://www.kelinginc.net/C11Manual.pdf

    As shown in Fig 1 ,2 (attached) they have used buffers before and after optocoupler. I guess they have used buffers on output because of low current transfer ratio(CTR) which might not be able to drive the LEDs .. am I correct ? Can some one please explain the use of buffer before optocoupler ?

    I need help in determining correct values for R and C in Low pass the filter.
     
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    Last edited: Dec 23, 2012
  2. Papabravo

    Expert

    Feb 24, 2006
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    1,789
    You need to define "fast". Frequency response is one thing, and delay is quite another. Which one is more important? You can have cheap, or you can have "fast", but you are unlikely to get both in the same part.
     
  3. Power_user_EX

    Thread Starter New Member

    Dec 23, 2012
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    0
    Hi Papabravo ,

    Fast in my case will be the one having repsonse time (low to high) <= 1us or 2us.

    The BOB ,that I mentioned, has typ signal delay of 3us. So delays in this range might not be issue i guess. Also many of the online BOBs for CNC are using PC847/LVT847. So , ill be most probably using LVT847 or variant. Also I don't yet completely understand 'frequency response'. I more into digital than analogue :( Atm I need help with designing a Low pass RC filter for schmitt trigger.
     
    Last edited: Dec 23, 2012
  4. Papabravo

    Expert

    Feb 24, 2006
    10,140
    1,789
    For an opto to work at say 50 MHz, you need to know that if you put a 50 MHz. signal in that you will get a 50 MHz. signal out. Now there may be a 10 microsecond delay between input and output, but in some applications that does not matter.

    For an opto to have a low propagation delay, say 40 nanoseconds, like an HCPL 0710, says nothing about the frequency of the signal at the input being reproduced at the output. Again if delay is what you care about then the frequency response matters less.

    http://www.avagotech.com/pages/en/o...coupler/high_speed_cmos_logic_gate/hcpl-0710/

    Everything is analog if you want to stay out of trouble in design. There should be nothing missing from your toolbox.
     
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