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How to measure node to node capacitance?

Hi everyone,

is there a way to measure node-to-node capacitance in version IC6.1.8-64b.500.14? For instance in following feature "A" to "VSS". I mean, I want to measure the equivalent capacitance of the dashed border block to consider it as a load capacitor.

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  • Dear Holz,

    The capacitance, as I am sure you are aware is somewhat bias dependent. Hence, there is no single value that is applicable. The capacitance between those two nodes can be simulated as a function of bias.

    This may be not the answer you were hoping to read! If you are only looking for the parasitic capacitance between those two nodes, an extracted view from a layout of the devices can provide that number in spectre.

    Did I understand your question Holz, and does this help answer it at all?

    Shawn

  • This question is not that trivial, in spite of its apparent simplicity.
    To measure the capacitance between nodes A and B (let's say, on a post-layout netlist), you can ground (or apply a DC bias to) node A, apply small-signal AC voltage to node B, and measure AC current through voltage source on node A.
    The imaginary part of the current is omega*C_AB*dV, so you can easily get C_AB from such a measurement.

    The load capacitance seen by the circuit terminated at (e.g.) node B, however, is very different - it includes capacitances (form net B) to other nets, such as to gates of the two MOSFETs (that are grounded, on your diagram), capacitance drain-to-bulk of the MOSFET, it is bias-dependent, as Shawn noted, etc.

    I think that the coupling between A and B will be dominated by the interconnect parasitic capacitance (in which case, it's voltage-independent).
    However, if the well where these two transistors are sitting in, is floating, then you have four (or three, for shared diffusion case) back-to-back p-n junction diodes, that will contribute to C_AB, and that will be bias-dependent.

  • Dear Maxim,

    This question is not that trivial, in spite of its apparent simplicity.

    Thank you very much for adding to my comments to Holz Maxim! Your insights are much appreciated!

    I did not go into further detail in my response as I was not sure exactly the kind of answer Holz is looking to find. I definitely agree that a layout and operating conditions are really necessary to provide a good estimate of the capacitance. I think many of the points you noted might be answered if there was a layout to.

    Anyway, I am sure your insight will also provide Holz with some good insight into his question to ponder!

    Thank you Maxim!

    Shawn

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