March 19, 2023, 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 circuit simulators.
Picotest offers NISM for the Keysight® PathWave Advanced Design System (ADS) and Cadence® PSpice simulators at no charge. For Cadence PSpice, NISM and for ADS a free software download at the Picotest.com website (https://www.picotest.com/support/) is available now.
Previously, NISM was only available for hardware vector network analyzers (‘VNAs’), which are now also going to be available at no charge, also via Picotest’s website. NISM enables the evaluation of control loop stability via impedance.
NISM is a unique alternative to a simulated or tested Bode plot measurement. It allows the user to assess power supply control loop 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.
NISM is native to the OMICRON Lab Bode 100 software and was formerly a paid software add-on for the Keysight E5061B/E5071C, the Rohde & Schwarz ZNL and ZNLE, and the Copper Mountain VNAs. It is now available for these analyzers at no cost. NISM will also soon be available for the Tektronix MSO series oscilloscopes, making Tektronix the first oscilloscope brand to support this feature.
Steve Sandler, Picotest CEO, states, “It’s time to make NISM free. It is more important to make this broadly accessible to engineers than to generate revenue from it.”
For power integrity engineers, this is a very important and exciting new capability. While power integrity seeks to assess 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 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.
Picotest also offers probes that test output impedance, including 1- and 2-port PDN transmission line probes, the P2104A and P2102A, respectively, which greatly ease the measurement of 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 a number of 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,” stated 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 introducing this proprietary method 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,”
states Sandler.
More information and reference material is located at:
- https://www.picotest.com/solutions/non-invasive-stability-measurement/
- https://www.picotest.com/solutions/solutions-for-rohde-schwarz-equipment/
New software from Picotest enables the Rohde & Schwarz ZNL/ZNLE VNA to make a non-invasive stability margin measurement via output impedance. The Picotest J2102B loop breaker is used to make a 2-port shunt-through impedance measurement of a sample LDO circuit.
The Picotest NISM software enables the Rohde & Schwarz ZNL/ZNLE VNA to make a non-invasive stability margin measurement. Here a 1-port impedance measurement tests the stability of a ~200 MHz opamp. The big “WOW” is that we obtained the (very poor) phase margin from the impedance measurement using NISM (just about 5 degrees). This is a great capability; to be able to accurately assess stability at 100s of MHz or higher without lifting any wires (which would interfere with the measurement).
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