High Speed ADC ?

Discussion in 'General Electronics Chat' started by CVMichael, Mar 4, 2010.

  1. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    I need a high speed ADC, that can output the bits as individual pins same as ADC0808 (for example), except I need an ADC that can go up to 500MHz.

    I want to interface it with my new LA1034 LogicPort so I can measure analog inputs also.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Something simple, right?

    No mention of how many bits resolution? What do you need, 10-bit, 12-bit, 16-bit, 24-bit?

    You realize that trying to build an ADC that'll run at such a high speed is going to be a problem, right? Parasitics will really start coming into play. A multilayer 0806-size SMT 100pF ceramic cap that measures 100pF at 1MHz will measure more like 200pF at 0.5GHz. The larger the cap, the worse the parasitics are.

    A 10mm piece of straight wire measures around 15nH; not a problem with low frequency stuff, but at high frequency watch out. Trace layout is going to be a challenge.
     
  3. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    Anything from 8 to 12 Bit is OK

    Even though I said "that can go up to 500MHz", I doubt that I will really measure higher than 100MHz... I just said that because the LA1034 LogicPort goes that high.

    But really, I just want to get anything higher than the ADC8080 wich goes up to 640K (from what I read on the forums).

    And of course since i'm no expert in electronics, I would like something simple to impletent, with not too many components outside of the chip...
     
  4. SgtWookie

    Expert

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    Well, you're wanting to measure digital signals, right?

    Keep in mind that high speed ADC's are still pretty expensive. I don't know what your budget is. If you're not willing to spend $100 or more, there's not much point in even looking for an IC.
     
  5. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    If you are measuring digital signals do you even need a adc, would not a ddc do a well?
     
  6. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    LA1034 LogicPort has 34 digital inputs, and it has the capability to convert digital bits to display as analog wave.

    For example, here is a sample posted by BMorse in another thread:
    [​IMG]

    So I want to make a small ADC to convert analog to digital so that I can display it in LA1034 LogicPort.

    As I said, I want something like ADC0808 but faster. I am not asking for a particular frequency, I want want anything that is faster than the ADC0808 (something in the MHz range)
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
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    You may want a frequency or function generator.. They will allow you to create different varying outputs in many different forms.

    An ADC converter will not convert a sine wave into a "digital sine wave" to be viewed by the Logicport.

    It will convert the inputed sine-wave into a square wave, which will look just like the other square waves on the screen.

    I think you want to use the logicport as an oscilloscope, and I do not believe it will do what you want.
     
    Last edited: Mar 4, 2010
  8. hgmjr

    Moderator

    Jan 28, 2005
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    One of the fastest a-to-d converter topologies is the flash ADC. There is an introduction to the basics of such an ADC here in the AAC ebook.

    hgmjr
     
  9. retched

    AAC Fanatic!

    Dec 5, 2009
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    It may be possible, put the resolution would be very low.

    The sine wave you were looking at on BMorses' post was a conglomeration of 10 logic waveforms being shown as an average. So it wouldn't show very small changes in an analog waveform.

    Here is the expanded group that made up the sinusoidal wave.

    [​IMG]
     
  10. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    Hi retched,

    I found an ADC0804 through my integrated chips, and I put together a simple ADC on a test board.

    I set the LogicPort the "Pre-Fill Time Limit: 10sec", and "Post-Fill Time Limit: 10sec", then I started recording the samples, and quickly moved the variable rezistor down and up, and stopped recording.

    See attached pictures with the results.

    So now... is it so hard to ask if you know a simple chip that does the same thing but faster ?
     
    Last edited: Mar 4, 2010
  11. SgtWookie

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  12. SgtWookie

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    Last edited: Mar 4, 2010
  13. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    Thank You SgtWookie !

    In terms of price, up to $50 is OK....

    But the problem is I never worked with QFN package chips before, and when I see so many pins it scares me. (but i am open to learning, that is the whole point of this)

    And on top of that I took a quick look at the data sheets, and I don't see any schematic examples of the minimum componets you have to add to get it to do anything, and I would definitely not know what other components it needs to get it to work on my own.

    Anything more simple, even if the speed is lower ? (even as low as ~ 50MHz ?)
     
  14. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    Last edited: Mar 4, 2010
  15. SgtWookie

    Expert

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    Well, now you're looking at ADCs that are an order of magnitude slower than you originally requested. That's OK though.

    You're going to be hard-pressed to find anything in a DIP package that will be anywhere near your original specification.

    When you find something that looks enticing, look on the manufacturer's site for application notes. The datasheet is merely a bare-bones summary of what the IC will do; you have to find the application notes. You will have a lot of reading to do.

    The more research you do on the subject, the more you will realize why I sounded rather pessimistic in the beginning of the thread. Performing an accurate conversion at the speeds you desire is technically very challenging, and parasitics can wreak havoc even with apparently well-thought-out plans.

    Manufacturers have gone to packages like QFN because the traces can be shorter. A 10mm piece of wire has roughly 15nH of inductance, which is no problem at low speeds, but if you're talking broadband digital, the cumulative effect of inductance in the traces is like a brick wall to further progress. The only way they can make things like this faster is to make them smaller.
     
  16. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    I did that and I found even better ones:

    This one has a "Demonstration board" on page 21 which is VERY useful.
    ADC1006S055/070
    Single 10 bits ADC, up to 55 MHz or 70 MHz
    http://www.nxp.com/documents/data_sheet/ADC1006S055_070.pdf

    This one has a "Application diagram" on page 17, nice stuff !
    ADC1207S080
    Single 12 bits ADC, up to 80 MHz with direct/ultra high IF sampling
    http://www.nxp.com/documents/data_sheet/ADC1207S080.pdf

    Both of them need a CLK in, and I wouldn't know how to provide that to the circuit.

    Another thing I don't understand, is (if you look at the attached image), the input goes through a transformer?... filter... thinghy..., so this means that the input has to be sinusoidal, so if I measure diferent kind of signal will it work ?

    I wish I could find this already built somewhere ? or if someone could do it for me, but that would probably be expensive...
     
  17. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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    This one is awesome, On EBay: LTC2260CUJ-14#PBF C ADC 14-BIT 105MSPS 40-QFN, only $45

    Datasheet

    There is a "TYPICAL APPLICATIONS" diagram on page 28, and also a list of related parts in the last page.

    Someone needs to build this for me :(

    [edit] Evaluation kit is $200 :eek: looks much more complicated than what I originally imagined...

    [edit 2] I am so tired of looking through datasheets, etc... I need a break
     
    Last edited: Mar 5, 2010
  18. CVMichael

    Thread Starter Senior Member

    Aug 3, 2007
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