Linear Ramp Generator w/High Slew Rate

Discussion in 'The Projects Forum' started by AmberJ, Dec 21, 2010.

  1. AmberJ

    Thread Starter New Member

    Dec 21, 2010
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    I am needing to build a linear ramp generator with a high slew rate. It will be powered by a DC voltage of up to ±15V (can be less). I have found two designs that could work:
    1. http://www.allaboutcircuits.com/vol_6/chpt_6/8.html
    2. http://pdfserv.maxim-ic.com/en/an/AN3655.pdf
    With the 2nd one I will need to also make a square wave generator.

    I was wondering if anyone knew which one would be best to get the highest slew rate. Or, if there is a better design that I have not found. Thanks,

    Amber
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    There are differences, even among different models of 555's. A TLC555 (the TI version of a CMOS 555) can go to 2 Mhz. Pick the appropriate transistor and it can be very good.
     
  3. AmberJ

    Thread Starter New Member

    Dec 21, 2010
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    Thanks for the reply.

    I am not so much concerned with the frequency of the output; more the slope of the drop. I am using it to power a linear positioner kind of like this one:
    http://www.attocube.com/nanoPOSITIONING/ANPz51/ANPz51.htm
    so I need a sharp voltage drop inorder to pull my piezo back quickly.

    Do you have any sugesstions as to which model I should use? Thanks again.

    Amber
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Well, I'll admit to being impressed with the STM Image of Silicon(111)

    What is your use for these, out of pure curiosity. It just seems, well, amazing that using a 555 timer for imaging atoms is possible.

    What V/nS are you trying to find for the falling edge?
     
  5. AmberJ

    Thread Starter New Member

    Dec 21, 2010
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    I am doing research studying the optical properties of semiconductor quantum dots. The linear positioners are used to move an optical fiber over the surface of our QD sample and collect light emitted from the QDs.

    We (my research group; I am a physics graduate student) have been having problems getting some of our positioners to work because they are homemade, we are working at low temperatures (~7K), in high vacuum, and we needed to stack six ontop of eachother. This is a bit more complicated than what we have done in the past.

    We are not really sure what slew rate we need or really what voltage we need to drive the piezos at. But increasing both of these parameters should help.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    You refer to high slew rate. Can you put a value on what you are calling a high slew rate?

    Also it seems to me that you would need to know what the real-time position of your moving platform happens to be.

    hgmjr
     
  7. AmberJ

    Thread Starter New Member

    Dec 21, 2010
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    I need a slew rate of at least 0.3V/μs.

    The platform does not always have consistent step sizes, so I am not sure how we could track its position. Currently, we use a microscope objective to position our fiber visually.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Have you breadboard the 555 design and looked at it?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    What is your target PRF or PRT for the sawtooth input?

    The Attocube positioners you referenced have an effective capacitance anywhere from 150nF (0.15uF) to 1050nF (1.05uF), which is quite significant, and must be taken into account when designing a driver for it.

    Do you have the exact manufacturer and model of your positioner available, or can you find out what it is?

    One possibility might be to use current feedback opamps (like video drivers) to build a solution. You're probably going to need to source/sink a good bit of current to charge/discharge the positioner in order to get the times you're looking for.

    There's also the inertia of the positioner to deal with, but I have no idea how to calculate that.
     
  10. AmberJ

    Thread Starter New Member

    Dec 21, 2010
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    I constructed the 555 design and, after changing some resistors and capacitors, I got a signal that should work. Thanks for the help.

    Amber
     
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