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ADC Calculations and sampling

Discussion in 'Electronic Design' started by john, Nov 17, 2008.

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  1. john

    john Guest

    Hello All,

    I have following confusions and questions about ADC based system: I am
    trying to find the two ADCs for the following specifications


    1. The specifications of input analog signal are as follows
    a. Frequency range # 1 : 500Hz to 5MHz
    b. Frequency range # 2 : 30 KHz
    c. Both signals are biphasic square wave
    d. Amplitude : +/- 5 volts

    2. Digital interface will be USB2.0 (480 Mbps). Planning to interface
    the ADC digital bus with USB2.0.

    I need to choose an ADC that can sample the input analog signal
    efficiently and generate the desired data bit rate (which I do not
    know exactly now).

    Sampling Calculations:

    I have worked with an ADC from Maxim (MAX 187). This chip can do
    75Ksps. So, it might be ok with the input signal of frequency of
    40KHz. The Nyquist theorem says that I can not read the frequency
    higher that (fs / 2). So, the maximum output analog frequency that I
    can read if 37.5 KHz. Am I correct?
    Can anybody suggest the part for the Frequency range # 1?
    My other question about sampling is that the square wave consists of
    many frequency harmonics, what harmonic should be chosen for sampling
    the fundamental, second or third?


    Data Rate Calculations:

    The Data rate of the USB2.0 IS 480Mbps and the MAX 187 is 12 bit /
    sample ADC at 75Ksps. So, the digital output rate of the MAX187 will
    be 900,000 bits/sec. Is this correct?
    Now, if I divide 480 Mbps with 12 bits / sample, then USB2.0 can
    support 40 Mbps. Is this correct?

    I will appreciate of someone suggest some parts and clear few
    designing principles regarding ADC for me.


    Thanks
    John
     
  2. Guest

    Hi John, I'm not going to be much help to you, but I was wondeirng
    why you need an ADC if you have square waves as an input signal. What
    do you want to know besides the frequency?

    George
     
  3. john

    john Guest

    Hi,

    Yes, you are right! The Input signal could be discountinuous, square
    wave or any other type of signal. But I still need verification of my
    calculations.

    Thanks
    John
     
  4. MooseFET

    MooseFET Guest

    Actually the Nyquist is the first frequency you can't measure not the
    last one you can. You can have a sine wave a Nyquist crossing zero
    exactly at the sample times and thus read zero.
    This also doesn't leave you any room for an anti-alias filter.
    The LTC2203 or LTC2286 will go fast enough to do 5MHz. How many bits
    do you need?

    That is the peak rate don't expect to maintain that rate for the whole
    time.
    It is sort of correct. You will have some overhead.

    Can you actually obtain the MAX 187?

    Again expect some overhead.
     
  5. Guest

    If the OP wants to measure a square wave why does he need any more
    than one bit? I'm confused.

    George
     
  6. Guest

    That is the peak rate don't expect to maintain that rate for the whole

    I have managed to get about half the 480Mbps over usb 2.0 using the
    cypress ez-usb fx2. The gnu software radio project has a lot of info
    on how to do it. I found that the custom drivers were not actually
    necessary and the user mode wrapper from cypress will do just fine. In
    my experience multiple end points don't get you any more bandwidth and
    isochronous transfers are also a waste of time. However, IO completion
    ports are an absolute must. The real gotcha is that some usb host
    controllers will do there best to reorder every single packet you
    send, while others won't.

    Good luck, Jon Pry
     
  7. john

    john Guest

    Hello,

    Sampling Calculations:

    I have worked with an ADC from Maxim (MAX 187).  This chip can do
    75Ksps. So, it might be ok with the input signal of frequency of
    40KHz. The Nyquist theorem says that I can not read the frequency
    higher that (fs / 2).  So, the maximum output analog frequency that I
    can read if 37.5 KHz. Am I correct?

    Tim wrote: Nyquist didn't say that.http://www.wescottdesign.com/
    articles/Sampling/sampling.html.

    So, I am wrong about sampling the 40KHz signal with 75Ksps ADC. And
    the reason is ???



    Can anybody suggest the part for the Frequency range # 1?
    My other question about sampling is that the square wave consists of
    many frequency harmonics, what harmonic should be chosen for sampling
    the fundamental, second or third?

    Tim wrote:If you are saying that you have a 5MHz square wave, then
    you'll have
    harmonics going well above 5MHz -- the sharper the edges, the higher
    the
    harmonics.

    So , what harmonic should I sample, The highest frequency range one,
    middle frequency range ??


    Data Rate Calculations:

    The Data rate of  the USB2.0 IS 480Mbps and the MAX 187 is 12 bit /
    sample ADC at 75Ksps. So, the digital output rate of the MAX187 will
    be 900,000 bits/sec. Is this correct?
    Now, if I divide 480 Mbps with 12 bits / sample, then USB2.0 can
    support 40 Mbps. Is this correct?

    I will appreciate of someone suggest some parts and clear few
    designing principles regarding ADC for me.

    Tim wrote: 480Mbps is the absolute maximum raw data transfer rate, not
    the rate
    that can be sustained by any one channel, and possibly not (I haven't
    read the fine print of the USB specification) something that you
    could
    ever actually achieve in practice.
    A big part of this is that you can't get all the bus bandwidth in one
    channel -- it's simply not in the standard.  You want to do a bit of
    digging and find out the maximum data rate that you can sustain using
    isochronous transfers with your high-speed USB.


    Lets suppose the USB 2.0 has stable 480Mbps. Then are my calculations
    true??



    Tim wrote: You never mentioned your precision requirement -- are you
    just assuming
    that 12 bits is both necessary and sufficient, or might you get away
    with less (or absolutely need more)?

    Yes, I am looking for 12 bit precision.

    Regards.
     
  8. john

    john Guest

     
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