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SPICE capacitance question

Discussion in 'Electronic Design' started by Tim Williams, Dec 7, 2011.

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  1. Tim Williams

    Tim Williams Guest

    Is there any such thing as a box you can put on a node to measure its
    capacitance? XSPICE (or some variants thereof) at least comes with a
    "CMETER" entity, but it only appears to measure passives, not dynamic
    (semiconductor junction or synthesized) capacitances. How useless.

    There's always the old charge-it-with-a-waveform (usually CCS to make a
    voltage ramp, or voltage ramp to make current), then C = I / (dV/dt), but
    that obviously doesn't work if there's bias or leakage current.

    Tim
     
  2. Jamie

    Jamie Guest

    I've been hashing a little with LTspice lately with making a model or
    sort of for an SCR.

    I've succeeded in making a sub circuit connectable in a block using the
    primitive functions of spice, current sources, voltages sources etc..
    but would like to get a better understanding of the model code itself.

    It seems that finding the resources needed for the actual event
    variables that you need to access in your model code and label variables
    for C, R, L on inputs and outputs etc are hard to locate..

    I do programming in various languages so I have a good idea on how
    this should all work. I have looked at various models already and have
    gotten many of my questions answered how ever, It would be nice if there
    was a plain document on the step by step of the primitive variables that
    are involved and the names of them you need to know in the model code.

    Set sizes , cycle count and things of that sort, so that one can
    quickly put together a working step circuit.. if you get my drift.

    The help file, as good as it is, offers you all the math functions and
    the like but it seems to come in short when it wants you to understand
    the under layer of what is to be taken place.

    For example, I saw in one example the use of ro, lo, co, ri, ci, li to
    indicate the basic elements R,L,C in the input or output which makes
    perfect sense how ever, If that is to be primitive variables of spice, I
    can't find that documented? Or, what I saw isn't what it is.

    Years ago in college I didn't use anything like spice, we did it all
    on paper and slide rules. Then I got my first TI si calculator and that
    was a big jump for me :)

    Jamie
     
  3. Tim Williams

    Tim Williams Guest

    AC analysis is the obvious solution, but you have to set the DC operating
    point. Each. And. Every. Time. Remember, I said it's a variable
    capacitance. Sure, you can set a parameter sweep with the bias, but then
    you get, say, a hundred curves with >2 points per curve and no curve
    connecting the curves.

    Tim
     
  4. Tim Williams

    Tim Williams Guest

    This sounds useful, unfortunately it would have to be implemented in a
    scripting language (DelphiScript, JS, etc.) working with Altium's API, if
    it even provides handles to automate this sort of thing in the simulation
    editor.

    Tim
     
  5. Tim Williams

    Tim Williams Guest

    Supposing I run bias from a staircase source (creating the transient
    equivalent of the AC + parameter sweep setup), I have to wait for ALL
    signals to stabilize, both bias AND the c-meter reading. This will take
    more than a few cycles of whatever the c-meter is doing, and a hundred
    points will take several minutes to simulate.

    Then if I want to incrementally tweak the model I'm testing, I have to
    wait several minutes again.

    Plus the time of designing and building the setup in the first place,
    which is probably a few hours, depending on how involved the c-meter is
    made to be. It doesn't have to be fancy since it could be e.g. current
    sources, switches, charge pumping, that sort of thing, but those all cost
    simulation time.

    Tim
     
  6. legg

    legg Guest

    Semiconductor junction capacitances are currently characterized base
    on curve fiting of static data, and the characteritics are applied
    dynamically. Are you suggesting that this does not reflect real
    situations?
    As in real life, you'd have to couple the 'tester' to the node in
    question, to minimize loading effects or interference in function. The
    issue isn't avoided, just because it's a simulation.

    Complex impedance measurements can be made, conventionally, to extract
    reactive components.

    It's a little ironic, as the solution to any spice node's
    characteristics should be solvable mathematically - it is a construct,
    after all. As we adopt models and tools that are more complex - that
    we don't completely understand - we require more complex test features
    to 'simplify' the process of understanding.

    I wonder if, in the end, we know more, or less?

    Constructing the spice equivalent of test equipment is an activity
    that is being tried from a number of directions in the LTspice yahoo
    group. This has included curve tracers and impedance/network
    analysers.

    RL
     
  7. Tim Williams

    Tim Williams Guest

    That's all fine and dandy, but like I said to John, if I implement it
    conventionally, I have to wait for it to stabilize. I'd love to do it in
    real time, because heck, I can generate that fast enough to sweep a CRT
    scope, let alone generate some X-Y points on the DSO in under a second.
    But doing it in sim is a lot more painstaking. I was hoping someone might
    have an elegant solution which takes advantage of the environment.

    Tim
     
  8. legg

    legg Guest

    If only Tim would read postings to the bottom of the page, he might
    get somewhere, intead of stewing.

    There are all sorts of ways to reduce simulation time. Establishing
    initial operating conditions is only one of them, though in my line of
    work it would avoid useful hints about relevant disasters-in-waiting.

    RL
     
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