By John Rice, System Engineer, Texas Instruments and Steve Sandler, Managing Director, Picotest

In theory, measuring the power-supply-rejection ratio (PSRR) is relatively simple. A variable-frequency signal modulates the power-supply input, and the attenuation of that signal is measured at the output. However, the measurement is highly sensitive to setup noise, including noise from the probe-loop area and the layout of the printed circuit board (PCB). This article explores commonly encountered setup issues that limit PSRR measurement and offers a method to overcome them using high-fidelity signal injectors and a highly sensitive/selective vector network analyzer (VNA).

Input-signal modulation
The easiest way to modulate the input to a regulator is with a line injector, such as the Picotest J2120A. This device accommodates 50 V at the input and an input current of 5 A. Coupled with a VNA, the J2120A directly modulates the input voltage while the VNA measures the input/output attenuation. The drawbacks of this method are the need to break into the input lines and the need to accommodate a voltage drop across the injector. While these drawbacks are generally not issues for bench testing, they can be troublesome when the measurement is performed in circuit.

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