LTSpice resistance vs frequency

Discussion in 'General Electronics Chat' started by H19608, Feb 26, 2010.

  1. H19608

    Thread Starter New Member

    Feb 26, 2010
    I am trying to model the resistance of a capacitor with repect to frequency

    How to i get the graph to show resistance (x axis) frequancey (y axis)
  2. Mike33

    AAC Fanatic!

    Feb 4, 2005
    I think this depends on exactly what you're trying to measure. Do you mean in AC analyses, such as filter responses, or are you looking for the ESR of a capacitor?

    In the first case, go to LT Spice, set up an R-C filter or some such thing, click Run, and you'll get a dialog box; go into the AC analysis section, fill out your frequencies etc. and it will run, showing you (in dB) the loss WRT reducing frequency in a nice curve. You could enter the formula for converting dB to V if you like; play around with it.

    If it's the 2nd case (finding ESR), that is a lot more difficult, and not as big an issue unless you're doing something involving really high freq' can simulate it with a 1 to 10 ohm resistor in series with the cap to estimate increasing ESR with age...but each cap's will be different, and actually requires measurement with a frequency generator.
  3. ifixit

    Distinguished Member

    Nov 20, 2008
    You can get impedance on the Y axis and frequency on the X axis as follows...
    1. Run an appropriate AC analysis on the capacitor
    2. Plot the voltage across the cap. e.g. V(N001)
    3. Right click on the V(N001) of the plot and edit the term to divide the cap voltage by the cap current. e.g. V(N001)/I(C1).
    4. Left click on the Y axis dB scale. In the pop-up; change to linear mode instead of Decibel.
    5. You should now have capacitor impedance on the Y axis and frequency on the X axis.
    Good Luck