Tuning for Stability: A Practical Guide to Non-invasive Stability Measurement (NISM) in Keysight ADS

Control loop stability is critical to the performance of any system, influencing everything from closed-loop parameters to system noise. Unfortunately, verifying stability is often a major challenge. For many devices—like voltage references, fixed voltage LDOs, and integrated POLs—traditional Bode plot assessment is simply not feasible because the feedback loop is inaccessible or requires destructive modification of the PCB. Furthermore, Bode plots fail for complex systems, such as those with multiple control loops or non-Linear Time-Invariant (LTI) behavior. For engineers designing small, densely populated portable electronics, accessing control loop elements is almost impossible. In all these cases, the Non-invasive Stability Measurement (NISM) assessment, derived from easily accessible output impedance measurements, becomes the only reliable method to verify stability.

Stability is, therefore, a paramount factor in any Power Delivery Network (PDN), and our SI/PI Library gives you the essential tools to ensure your design is robust and verified pre-layout. Our library, which includes state-space average VRM models like the LTM4650-1, allows you to seamlessly perform this advanced stability analysis directly within Keysight ADS. NISM provides the foundation for accurate PDN analysis by letting you go beyond basic simulations. This video will demonstrate the incredible power of NISM and how it simplifies a historically complex task.

This video demonstrates the incredible power of Non-invasive Stability Measurement (NISM). We'll show you how to quickly check the stability of your PDN using one of our VRM models, enabling rapid and reliable PDN Analysis. But we won't stop there. The video also highlights the convenience of the ADS tuner, showing you how to quickly and easily fix any stability issues that arise. The precision of our high-fidelity models ensures that your tuning adjustments are accurate and reliable. This feature makes it simple to tune your control loop, proving that with our models, correcting your design is as quick as it is effective.

The ability to perform such detailed analysis is a testament to the high-fidelity of our models, a topic we touched on in our first blog on getting started with our PDK. Our models provide both large and small signal analysis, a key feature that we’ve also discussed in Power Integrity Unleashed: Navigating State-Space Average VRM Models for Robust Simulation Designs in Keysight ADS. After mastering stability and PDN analysis, the next step is optimizing for efficiency. In our final blog, we’ll show you how to run an efficiency simulation with one of our models.

Ensuring stability with NISM is the final, critical step in creating a robust product. By integrating these stability checks into an effortless efficiency simulation workflow, you can validate that your maximized performance is also reliably stable: Read our guide, 'Maximizing Performance: Effortless Efficiency Simulation with Our SI/PI Library in Keysight ADS'.

Don't let stability issues derail your design. Our high-fidelity models and NISM tools make it easy to ensure a robust PDN. Contact us to find out more about licensing our SI/PI Library and gaining access to these powerful capabilities within Keysight ADS.

Further reading:

• From Workspace to Simulation: Getting Started with Our SI/PI Library in Keysight ADS

• Mastering Keysight ADS Co-simulation for PI Analysis

• NISM - https://www.picotest.com/measurements/NISM.html