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Simulating stability of single ended circuit wrt to a local gnd, not gnd!

Bill Toole

I am trying to simulate the stability of an LDO with respect to a local ground on chip. I use a 0V voltage source to break the feedback loop and get fairly normal looking results when I include bondwire inductance on the vcc input line only and local decoupling caps (purple lines in attached plot). When I include bond wire inductance (~ 2nH) on the gnd line though I get very strange results, mainly very significant gain and phase peaking at high frequencies (> 1GHz).  It appears that stb analysis runs with respect to the global gnd! Is there a way to use a probe to run stb wrt to the local gnd? Is it possible to use diffstbprobe for this purpose?  The help file mentions a "localgnd" parameter but I cannot figure out how to add the parameter.

  • Andrew Beckett

    On the stb choose analysis form, click the Options button, and then at the bottom in the additionalParams field, enter localgnd=I1.I2.gndName (where I1.I2.gndName is the hierarchical path down to the node you want to be the local ground - use dots as hierarchy separators).

    Regards,

    Andrew.

  • Frank Wiedmann
    Are there any plans to add the localgnd parameter to the ADE form so that one could simply select the node in the schematic?
  • Bill Toole
    Thank you Andrew, that worked perfectly. My loop gain and phase response changed somewhat, as expected, when I added the inductor to the gnd line but was not stupid like the original results.
  • Andrew Beckett

    Bill, Frank,

    There is an old CCR 534901 asking for this to be added in ADE, but it was automatically inactivated (to Inactive-Never status) a couple of years back because it was old and had no signs of being implemented. I suggest that if you want this, you contact customer support and request this so that it gets a couple of customer requests behind it. It seems a reasonable thing to have on the UI to me! Please reference the existing CCR, as that can then be re-activated.

    Regards,

    Andrew.

  • Frank Wiedmann
    Andrew has submitted two CCRs on my behalf asking to add a localgnd field to ADE. These are CCR 1478078 for stb analysis and CCR 1478087 for pstb analysis. Anyone who would also like to have this should ask Cadence Customer Support for a duplicate of these CCRs on his behalf.

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