Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Strange impedance curve in LTSpice

  1. Sep 18, 2011 #1
    i'm looking at an impedance curve of a non-ideal tantalum capacitor in LTSpice. The test circuit and impedance curve is seen in this screengrab:
    The spice file is here http://www.apspektakel.com/bilder/tantal.asc".
    So, the imdpedance curve shows the expected impedance minimum, but also an unexpected maximum at about 100MHz, where there is also a polarity switch in phase. This feature does not show up if I do an impedance plot in octave:

    Code (Text):
    octave:1> f = [1000:1000:1e9];
    octave:2> C = 2e-3
    C =  0.0020000
    octave:3> R = 2e-3
    R =  0.0020000
    octave:4> L = 1e-9
    L =  1.0000e-09
    octave:5> z = R + f.*2*pi*j*L-1./(f.*2*pi*j*C)
    octave:6> loglog(f,abs(z))
    octave:7> semilogx(f,atand(imag(z)./real(z)))
    Which result in these plots

    The parameters in the octave code are the same as in LTSpice component. Anyone know where this comes from? The phases look pretty different, is there something wrong with the test circuit?
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Sep 20, 2011 #2
    You may want to look at the LTSpice model more closely. That looks like a typical parasitic resonance curve. If the model has a parasitic inductance built into it to model non-idealities, it would look just like that. The Octave simulation is an ideal capacitor so you don't see it.
  4. Sep 20, 2011 #3
    Since I posted I've found out just that. The spice models has a non-zero parallel capacitance as default. It's disabled by setting the parallel capacitance to zero.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook