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Loop gain block for keeping DC

robert 21

Hello , I was told that there is a block in Cadence Virtuoso which keep the same DC value which is used for braking the loop and calculate loop gain.

In the past inductance and capacitors where used for the purpose of keeping the same Vdc .

What is the name of this Cadence Virtuoso block?

Thanks.

  • This is probably referring to either the AC Resistance parameter of a "res" in analogLib (so you can have a resistor with a very large value during the AC, and thus make it open loop) or the spt1switch (and other similarly named switches which have 2, 3 or 4 throws) which can be in different positions in different analyses.

    However, this is a very poor way of measuring stability. When the loop is opened, the load the other side of the loop is removed, and so the conditions are not the same.

    This is precisely why you should use the stb analysis, because that avoids the need to break the loop. It measures the loop gain whilst keeping the loop closed and preserving the loading at the point you are measuring the loop gain.

    The approach described in your picture above (using spt1switch or AC resistance) is the old-school way of doing it which is fraught with accuracy problems and should be avoided.

    Andrew.

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