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I just noticed a strange issue in LTspice and I am wondering what I am doing wrong. This is about capacitors with series resistance and inductance:

Example 1: A simple bandpass. Behavior seems ok.

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Example 2: same circuit ?! The cap C1 has Lser=1n. Behavior is different

enter image description here

Why?

tobalt
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1 Answers1

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It looks like just by adding Lser=1n inductance the LTspcie decided (by default) to add some parallel capacitance.

\$C_{par} \approx \frac{1}{4\pi^2F^2 * L} \approx\frac{1}{4*\pi^2*16GHz^2*1nH} \approx 0.0989pF\$

To check this try to set Cpar=0 in LTspcie.

G36
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    Yes. That was the reason. In fact, ignoring the parallel capacitance is probably a greater problem for some circuits (in terms of realism and convergence), which is why they added it in the first place. – tobalt Jun 24 '21 at 17:10
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    There's also rlshunt to account for, though I don't see its effects here. Though Rpar for the inductor, alone, might be, since its value is L/Gmin (so for 1n it's 1n/1T=1k). Rlshunt is probably the same. – a concerned citizen Jun 24 '21 at 18:48