Techniques for accurate PSRR measurements

by John Rice, System Engineer, Texas Instruments, and Steve Sandler, Managing Director, Picotest, Vendor website, Oct 01 2013

Focus:
Although conceptually simple, the measurement of power supply rejection ratio (PSRR) is complicated by the sensitivity of the measurement to noise. Specifically, the noise pickup of the scope probes and the PCB layout of the regulator raise the noise floor of the conventional measurement method, making it difficult to measure PSRR accurately, particularly at high values of PSRR. This application note presents two alternative methods for measuring PSRR using proprietary signal injectors (or “link devices”) and a vector network analyzer. After briefly introducing these methods—the line injector and dc bias injector methods—the authors present a series of experiments using the line injector method to demonstrate the noise contributions of scope probes and suboptimal layout and show how these noise contributions can be minimized through the use of 50-ohm coax connections with the appropriate link devices for impedance matching and ac coupling. In the final experiment, these lessons are applied in creating a fully optimized setup, which is then used to measure the PSRR of a very-high (100 dB!) PSRR LDO regulator. Finally, the app note concludes by noting that the dc bias injector method produces similar results to the line injector method. This point is demonstrated with PSRR measurements take on the familiar LM317 regulator.

What you’ll learn:

• How to measure very high values of PSRR
• How to measure PSRR with high accuracy
• How to understand the impact of scope probes and PCB layout on PSRR measurements

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