Non-invasive Stability Measurement

What is Non-Invasive Stability Measurement ('NISM')?

Control loop stability is critical to the performance of all systems, as it influences all closed loop parameters, as well as system noise. Unfortunately, in many instances, particularly in the cases of voltage references, fixed voltage LDOs, and integrated POLs, a Bode plot assessment is not feasible because there is no feedback loop access to the part. In other cases, the feedback loop is difficult to access because the hardware is integrated or would require cutting a PCB trace. In yet other cases, the devices either contain multiple control loops, with only one of them being accessible, or the order of the control loop is higher than 2nd order, in which case the Bode plot is a poor predictor of relative stability. A further complication is that in many portable electronics, such as cell phones and tablets, the circuitry is very small and densely populated leaving little in the way of access to the control loop elements. In these cases, the non-invasive stability margin (‘NISM’) assessment, which is derived from easily accessible output impedance measurements, is the only way to verify stability. Click here for a list of test methods and equipment that you can use to measure impedance which utlize Picotest Probes.

The mathematical relationship that allows the precise determination of the control loop stability from output impedance data was initially incorporated into the OMICRON Lab Bode 100 VNA software in late 2011. Now it is available in many VNAs and EDA simulation tools. The mathematical solution is based on minor loop gain theory, which makes it equally applicable to measuring the stability of switching converters with input filters (i.e. Middlebrook stability criteria). This method is simple, precise, and can be performed in simulation and using a VNA to measure either the 1-port reflection impedance or 2-port shunt through impedance. Both of these methods are well established and Bode plot measurements and non-invasive results generally agree to within 1 degree of each up to the approximately 65 degrees.