Oscilloscope Calibration Unit with LTC6655

Discussion in 'The Projects Forum' started by ELECTRONERD, Feb 2, 2010.

  1. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
    1,146
    16
    Hey everyone,

    I built this o'scope calibration unit I thought everyone would like to see, in case any of you would like to build one as well. First of all, it uses the LTC6655 voltage reference IC and will only have 775nV of noise max, and with an initial accuracy of ±0.025% max. See the datasheet for the circuit schematic (Pg. 1):

    http://cds.linear.com/docs/Datasheet/6655f.pdf

    It is possible to reduce the noise level to 160nV max, but to do that, more circuitry is required. If your interested in that noise level, see this document:

    http://cds.linear.com/docs/Application Note/an124f.pdf

    As you may have noticed, I've used a BNC connector for the output of my unit. This will feed directly into your o'scope if you add a female to female connector to make the connection. I also have a DC connector that you can see as the yellow rectangle in the attached pictures. The IC has a range of 3<Vin<13.2V, but I have around 5V going powering the unit from a cell phone wall-adapter. I didn't bother to fasten the PCB to the enclosure since it is so lightweight and because the wires hold it in place. Probably the most difficult portion of building this project was soldering to the IC. The IC is in a SMT MSOP package; and it's hard to get a stubby iron tip in there without shorting those miniscule pins! In fact, I accidentally broke off a pin from my first IC I used.

    Enjoy!

    Austin
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,648
    632
    Thank you for the high resolution photographs. Do you have any construction details or calbration hints that you wish to share?
     
  3. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
    1,146
    16
    Hello,

    Thanks for the comment!

    One construction detail that everyone should be aware of is the short wire length. Make sure that you have very short wires when you are connecting things, otherwise you'll get some extra noise. You could add coax if you want the minimal noise possible, but as long as you keep the wires a diminutive length you should be fine.

    That's all! Any comments, tips, or questions are encouraged!

    Austin
     
    Last edited: Feb 3, 2010
  4. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
    1,146
    16
    Good news! I recently calibrated my scope so that it's right on target; no problem whatsoever! My amplitude was off by quite a bit, so it's a good thing I adjusted it.

    Austin
     
  5. KenH

    New Member

    Feb 4, 2010
    3
    0
    Where to buy the LTC6655 chip? I've checked Mouser, DigiKey, and Newark, not of those had the chip.

    Ken
     
  6. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
    1,146
    16
    Hi Ken,

    You might have to request some samples from their website :D Go to http://www.linear.com/ and type "LTC6655" in the search bar. Once you arrive to the product page, scroll down until you see the package diagram. Under there, you'll see a "Request Samples" icon; click on that. You'll have to register as a member though to receive them.

    Austin
     
    Last edited: Feb 4, 2010
  7. KenH

    New Member

    Feb 4, 2010
    3
    0
    Thanks for the info Austin - I just hopped over and looked the site over - I'll do the request tomorrow from work... that email might look better than my gmail account?

    I guess you've done this? How many chips will they send - I saw they said no more than 5 part numbers of 2 samples each?

    Ken H>
     
  8. ELECTRONERD

    Thread Starter Senior Member

    May 26, 2009
    1,146
    16
    Hi Ken,

    Yes, a work email would certainly look more proffesional, and they only send two samples max.

    Austin
     
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