September 26, 2024, Phoenix, AZ: Picotest.com, a leader in high-resolution test and measurement equipment, has announced the availability of the non-invasive stability measurement (‘NISM’) software tool for Cadence® PSpice® from Cadence Design Systems, Inc. NISM is available for PSpice as of version 23.1 at no charge for existing customers. NISM is included as an embedded Measurement Equation. It uses output impedance and is an exact solution based on stability margin; it’s not an approximation. It is an alternative to Bode plots and is especially useful in many newer ICs where the feedback point is not available and a Bode plot test cannot be performed. It works on linear regulators, switchers, POLs, Constant-On-Time converters, and multi-phase converters. It also works for input filter stability and enables power integrity target impedance simulations to predict VRM stability. Previously, NISM was only available for hardware vector network analyzers (VNAs), or as a standalone converter, available at no charge, also via Picotest’s website. NISM enables the evaluation of control loop stability via impedance. PSpice is the first and only SPICE simulator to have NISM embedded.
“It’s time to get NISM into SPICE. Loop stability is a critical design characteristic and there are an increasing number of situations where Bode plots cannot be employed or are not accurate. For power integrity engineers, this is a very important and exciting new capability. While power integrity seeks to assess the PDN impedance level and flatness, engineers can now measure the VRM’s stability and the PDN performance simultaneously, using the same impedance data.”
Steve Sandler, Picotest CEO and inventor of NISM
NISM is a unique alternative to a simulated or tested Bode plot measurement. It allows the user to assess power supply control loop stability, or input filter stability, even when the control loop is not accessible. This is increasingly the case for regulators with multiple internal loops or regulators with fixed output voltages that do not have a feedback pin. In addition, NISM has found great usage in power integrity applications where the power distribution network (PDN) impedance is critical. NISM allows the user to know the stability of the Voltage Regulator Modules (VRM) from the same measurement as the target impedance.
NISM is available for hardware VNAs. Its addition to PSpice, however, is a big leap forward as the stability of power supplies can now be assessed for any power IC model whose output impedance is modeled accurately. Many of these models can be found in the Power IC Model Library (EMA-EDA, AEi Systems), also available for PSpice.
Steve Sandler, Picotest CEO and inventor of NISM, states, “It’s time to get NISM into SPICE. Loop stability is a critical design characteristic and there are an increasing number of situations where Bode plots cannot be employed or are not accurate. For power integrity engineers, this is a very important and exciting new capability. While power integrity seeks to assess the PDN impedance level and flatness, engineers can now measure the VRM’s stability and the PDN performance simultaneously, using the same impedance data.”
NISM determines phase margin from an output impedance measurement. This capability allows the user to measure the stability of opamps, voltage references, voltage regulators, LDOs, switching amplifiers, POLs, input filters, and all types of power converters through their output impedance. The software guides the user through a few simple steps to obtain the stability margin from a circuit’s output impedance.
Testing Stability When There’s No Control Loop Access
The stability of linear or switching regulators, including regulators with no exposed control loops, cannot be assessed using a traditional Bode plot. “Very often you cannot make a Bode plot for several reasons. Either the loop is not exposed, or frequently, newer devices have multiple control loops, which are not all accessible. NISM is the only way to verify stability for these types of circuits that provides a numerical phase margin result,” states Sandler. “A huge benefit of the measurement is that, because it is based on impedance, it can be measured ‘in-situ’ with a one probe measurement and non-invasively, without impacting the result. In fact, given opamp and regulator dependency on loading, and the shrinking size of ICs, as depicted by the TI LDO image next to a kernel of corn, in-circuit is often the only access available. Traditional loop breakage is not possible.”
What is NISM?
Picotest has published many articles and application notes related to the assessment of stability without access to the control loop since this proprietary algorithm was introduced in 2011. This method has proven to be a simple and accurate technique, making it popular for determining the exact level of stability and phase margin. Bode plot measurements and NISM results are generally within 1 degree of each other, up to and including a phase margin of 65 degrees.
Is the Non-Invasive Stability Measurement as Accurate as a Bode Plot?
Yes. In fact, it’s accurate in cases where Bode plots are invalid and it is not subject to some of the issues that plague Bode plots’ accuracy. “In many cases, the NISM is a better assessment of stability than a Bode plot given it is based on Nyquist stability margin,” states Sandler.
Download Software Tools and Examples for PSpice
https://www.picotest.com/support/nism-support/
https://www.aeng.com/NISM.htm
More Information and Reference Material
https://www.picotest.com/non-invasive-stability-measurement.html
About Picotest.com
Picotest.com was founded in 2010 for the purpose of developing high performance test and measurement solutions, particularly the interface between the board being tested and the measurement instrument. Picotest specializes in high fidelity, customized test and measurement probes, injectors, and other equipment, primarily for power supply and power integrity related applications. Picotest.com aims to utilize its strong R&D capability to provide the highest quality test-related products and services to customers worldwide. Picotest.com is located in Phoenix, Arizona and serves customers worldwide. The company is led by Steve Sandler, an award-winning international expert and educator in the field of Power Integrity.
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