::AEi Systems - Analytical HEAVY Lifting::

Introducing NISM for Cadence PSpice

NISM stands for "Non-Invasive Stability Measurement." It was developed and introduced by Steve Sandler (Founder/CTO of AEi Systems and CEO of Picotest) as a method of assessing control loop and filter stability based on output impedance.

The main usage of NISM is to determine the stability margin of regulators, switchers, POLs, and opamps, especially when the control loop is not accessible outside of the device, or if the device has multiple internal loops and only one of the loops is exposed outside the device. It is also an excellent check on the validity of Bode plots. This is now the case with many power ICs both in test and simulation models.

For Power Supply stability testing and simulation, it is indispensable. For Power Integrity the power distribution network (PDN) impedance level and flatness, engineers can now measure the VRM's stability and the PDN performance simultaneously, using the same impedance data.

Since its inclusion in Vector Network Analyzer ('VNA') test instruments in 2010, NISM has proven its ability to accurately predict power system stability.

The output impedance based assessment is superior in many ways to using Bode plots or the transient step load testing. It is analogous to translating the transient step load Q into phase margin (PM), except it is performed in the frequency domain. NISM is the algorithm behind the stability assessment, converting output impedance directly to PM.

And now this capability is available in PSpice, allowing you to simulate stability when a Bode plot is not possible.

To learn more about NISM, how to get it and use it, please see the documentation and other references.

Installing NISM:
NISM is included with PSpice version 23.1 or greater (SPB23.1 hotfix). You don't have to do anything if that is the version you have. Your Probe measurement window should be as shown below.

To install NISM in previous PSpice versions:

Download the NISM_Overlay2.zip using the link below.

Have SPB 22.1 latest ISR installed on your machine. NISM will may or may not work with prior versions.

Overlay the files in "tools" folder (in the zip file) on top of your installation - Cadence\SPB_22.1\tools\bin and Cadence\SPB_22.1\tools\pspice, for instance. Your path might be different. There are three files that go into the \tools\bin folder (pspice.exe, orevalexpr64.dll, and NismDll.dll). There is one file that goes into the \tools\pspice\tclscripts\pspAutoLoad folder (pspinit.tcl). There are two files that go into the \tools\pspice\tclscripts\orPSPNISM\ folder (pkgIndex.tcl, orPspNISM.tcl).

BACKUP THESE FILES BEFORE OVERWRITNG THEM!!

This should make the NISM measurements visible and running inside measurement window as shown below.

Documentation:
NISM Manual v2.0 - PSpice.pdf - NISM Documentation for PSpice Only: Dropbox Link | Box Link
NISM Manual v2.0.pdf - NISM Documentation for All Hardware and Software Platforms: Dropbox Link | Box Link

NISM Downloads:
NISM_Overlay2.zip File with NISM Files: Dropbox Link | Box Link