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Power Supply Rejection Ratio (PSRR) Measurements

Application Notes

Introduction 

The primary measurement tool used to test and characterize power supplies is an oscilloscope. Many of today’s scopes, including the Keysight Technologies, Inc. InfiniiVision X-Series, offer special power measurement options that can help automate many of the most important measurements. Figure 1 shows a list of the power supply characterization measurements that are available on Keysight’s InfiniiVision 3000T and 4000 X-Series oscilloscopes with the power measurements option (DSOX3PWR or DSOX4PWR). Unique to Keysight’s portfolio of measurements are frequency response measurements including power supply rejection ratio (PSRR) and control loop response (Bode). These particular stimulus-response type measurements are typically performed using low-frequency network analyzers. But since Keysight’s InfiniiVision X-Series oscilloscopes come with a built-in function/arbitrary waveform generator, they can also be performed using these scopes.

Table of Contents:

  • Power Supply Rejection Ratio (PSRR)
  • Probing the Input and Output
  • Performing the PSRR Measurement
  • Summary
  • System Requirements

Power Supply Rejection Ratio (PSRR)

Figure 2 shows a block diagram of a PSRR measurement test setup. This test provides a measure of how well a power delivery device, such as a DC-to-DC converter or a low-voltage drop-out regulator (LDO), rejects various frequency components injected at the DC input of the device-under-test. In other words, how much of a disturbance signal injected at the DC input reaches the regulated DC output.

To perform a PSRR test, a sine wave must be injected at the DC input and then swept from a low frequency to a high frequency. A DC + AC network summing device, such as Picotest’s J2120A line injector, is required for this measurement. The measurement system measures both the modulated input and output AC voltage levels and then computes the rejection ratio as 20Log(Vin/Vout) at each frequency within the swept band. Some engineers argue that the formula should be 20Log(Vout/Vin). But this is the formula for gain (A), not rejection. Rejection is the inverse of gain. The built-in waveform generator along with the Power Measurements option in Keysight’s InfiniiVision X-Series oscilloscopes makes this frequency response measurement possible for the first time in an oscilloscope.

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