digital analog conversion

Discussion in 'VHDL' started by Veronica Matthews, Nov 11, 2004.

  1. Dear newsgroup community,

    recently I came across the following challenge. There are several digital
    values which I want to convert to analog signals. Ok then, no problem.
    Simply D/A conversion! But after converting the signals the general set up
    requires that these values should be held for about - let's say - a period
    of 5 minutes with practically no droop (decay of the analog value) at best!
    The D/A conversion itself takes place in a 1 MHz period, the values to be
    set have to pend for about 5 minues. I guess a hold-element (capacitor and
    op-amp) would be the obvious choice. But how should I dimension the
    capacitance and how can I affect the droop? Is it realistic to expect
    virtually no droop assuming an optimal configuration ? Isn't it, that with a
    large time constant the charging time would be endless, too? Please help me,
    if you can. I am almost become desperate. I need this for my graduation
    report.

    Thank you in advance and many greets
    Veronica
     
    Veronica Matthews, Nov 11, 2004
    #1
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  2. Veronica Matthews

    rickman Guest

    Veronica Matthews wrote:
    >
    > Dear newsgroup community,
    >
    > recently I came across the following challenge. There are several digital
    > values which I want to convert to analog signals. Ok then, no problem.
    > Simply D/A conversion! But after converting the signals the general set up
    > requires that these values should be held for about - let's say - a period
    > of 5 minutes with practically no droop (decay of the analog value) at best!
    > The D/A conversion itself takes place in a 1 MHz period, the values to be
    > set have to pend for about 5 minues. I guess a hold-element (capacitor and
    > op-amp) would be the obvious choice. But how should I dimension the
    > capacitance and how can I affect the droop? Is it realistic to expect
    > virtually no droop assuming an optimal configuration ? Isn't it, that with a
    > large time constant the charging time would be endless, too? Please help me,
    > if you can. I am almost become desperate. I need this for my graduation
    > report.


    What you are looking for is called a track-and-hold circuit. You use a
    switch to remove the driving voltage from the cap. So the RC constant
    is very small when the switch is closed and very large when the switch
    is open. If you need the cap voltage as a signal source, you will need
    to buffer with an op amp or instrumentation amp to provide a very high
    input impedance.

    Is there a reason that you can't provide a DAC for every output? Then
    you don't need a cap to hold the voltage. Oh, I almost forgot, with
    that long of a hold time, you will need a cap with a very low leakage
    current. I don't remember which caps are best, but they will not have
    high capacitance values. It is a trade off where the high value caps
    have high leakage. With a 300 sec hold time you will need a 100 uF cap
    and over 300 Mohms for the total leakage to ground. I am not sure if
    this is practical.

    --

    Rick "rickman" Collins


    Ignore the reply address. To email me use the above address with the XY
    removed.

    Arius - A Signal Processing Solutions Company
    Specializing in DSP and FPGA design URL http://www.arius.com
    4 King Ave 301-682-7772 Voice
    Frederick, MD 21701-3110 301-682-7666 FAX
     
    rickman, Nov 11, 2004
    #2
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  3. Veronica Matthews

    Pete Fraser Guest

    "rickman" <> wrote in message
    news:...

    >
    > Is there a reason that you can't provide a DAC for every output? Then
    > you don't need a cap to hold the voltage. Oh, I almost forgot, with
    > that long of a hold time, you will need a cap with a very low leakage
    > current. I don't remember which caps are best, but they will not have
    > high capacitance values. It is a trade off where the high value caps
    > have high leakage. With a 300 sec hold time you will need a 100 uF cap
    > and over 300 Mohms for the total leakage to ground. I am not sure if
    > this is practical.
    >


    A DAC for every output is the cleanest.
    If only one DAC is available, use track and hold circuits
    as suggested by Rickman, but refresh them frequently
    so you don't need exotic capacitors and very high impedances.
     
    Pete Fraser, Nov 11, 2004
    #3
  4. Thanks for your numerous answers so far. For a better understanding let me
    elaborate on my intention.
    What I want to do is to handle several outputs (with the analog
    representation of the digital value) with just one single D/A converter.
    That means: feed the digital values through a single D/A converter and
    switch the converter output to one analog hold circuit per channel.
    Therefore the goal is to hold the analog values! The analog values to be
    hold are DC, that's true.
    Again, would it be advisable to use a capacitor and op-amp? How should I
    dimension the capacitance and how can I affect the droop? Is it realistic to
    expect virtually no droop? Isn't it, that with a large time constant the
    charging time would be endless? Maybe the solution is nearer as I can see?
    Maybe there is another way to solve the problem. But this "one D/A converter
    for multiple output channels"-configuration should be seen as basic
    condition!!!

    Many greetings, Veronica
     
    Veronica Matthews, Nov 12, 2004
    #4
  5. Just to put my aim in perspective: I'm neither trying to fool you nor trying
    to get my homework solved (like a given individual presumed). Why I am
    talking about a basic condition with respect to the "one D/A converter for
    multiple output channels"-configuration is that this single D/A converter
    already exists in hardware. It is there, physical, for me to touch, already
    bought... And now I want to use this very D/A converter to feed several
    output channels. Of course I could buy a DAC for every channel but that's
    not my intention. The hardware setup does not allow to solder other devices
    on the board. So PLEASE just take it as it is! I want to solve the problem
    that way. So don't try to proselytize me like that jehovah's witnesses
    guys... ;-)

    Hope you come up with more constructive suggestions!

    Veronica
     
    Veronica Matthews, Nov 12, 2004
    #5
  6. But obviously you have a board to place the analog-value-holders.
    The best way to store values are digital registers.
    So, why not use one ADC-Register-DAC chain for every channel (plus some
    control logic)?
    There are 8-channel ADC's and DAC's on the market.

    Hope this helps.
    Markus.



    Veronica Matthews wrote:
    > Just to put my aim in perspective: I'm neither trying to fool you nor trying
    > to get my homework solved (like a given individual presumed). Why I am
    > talking about a basic condition with respect to the "one D/A converter for
    > multiple output channels"-configuration is that this single D/A converter
    > already exists in hardware. It is there, physical, for me to touch, already
    > bought... And now I want to use this very D/A converter to feed several
    > output channels. Of course I could buy a DAC for every channel but that's
    > not my intention. The hardware setup does not allow to solder other devices
    > on the board. So PLEASE just take it as it is! I want to solve the problem
    > that way. So don't try to proselytize me like that jehovah's witnesses
    > guys... ;-)
    >
    > Hope you come up with more constructive suggestions!
    >
    > Veronica
    >
    >
     
    Markus Michel, Nov 12, 2004
    #6
  7. "Veronica Matthews" <> schreef in bericht
    news:cn1vot$r8b$-berlin.de...
    > Just to put my aim in perspective: I'm neither trying to fool you nor
    > trying
    > to get my homework solved (like a given individual presumed). Why I am
    > talking about a basic condition with respect to the "one D/A converter for
    > multiple output channels"-configuration is that this single D/A converter
    > already exists in hardware. It is there, physical, for me to touch,
    > already
    > bought... And now I want to use this very D/A converter to feed several
    > output channels. Of course I could buy a DAC for every channel but that's
    > not my intention. The hardware setup does not allow to solder other
    > devices
    > on the board. So PLEASE just take it as it is! I want to solve the problem
    > that way. So don't try to proselytize me like that jehovah's witnesses
    > guys... ;-)
    >
    > Hope you come up with more constructive suggestions!
    >
    > Veronica
    >
    >


    If your concern is the number of output signals for each DA converter then
    just consider a delta-sigma (or PWM) DAC for each channel. This will only
    cost you one pin for each output channel, together with a simple R-C lowpass
    filter.

    Mark
     
    Mark van de Belt, Nov 27, 2004
    #7
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