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1、Keysight Technologies Power Supply Rejection Ratio (PSRR) Measurements Using Keysight InfiniiVision X-Series Oscilloscopes Application Note Introduction The primary measurement tool used to test and characterize power supplies is an oscilloscope. Many of todays scopes, including Keysights InfiniiVis
2、ion 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 Keysights InfiniiVision 3000T and 4000 X-Series oscilloscopes with the power measurement
3、s option (DSOX3PWR or DSOX4PWR). Unique to Keysights 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
4、analyzers. But since Keysights InfiniiVision X-Series oscilloscopes come with a built-in function/arbitrary waveform generator, they can also be performed using these scopes. Figure 1: Power supply characterization measurements available in Keysights InfiniiVision X-Series oscilloscopes. 03 | Keysig
5、ht | M9037A PXIe Embedded Controller - Data Sheet 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 linear regulator or a low-voltage drop-out regulator (LDO), rejects vario
6、us 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 fre
7、quency. A DC + AC network summing device, such as Picotests 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.
8、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 gen- erator along with the Power Measurements option in Keysights InfiniiVision X-Series oscilloscopes makes this frequency re
9、sponse measurement possible for the first time in an oscilloscope. Figure 2: Block diagram of a Power Supply Rejection Ratio (PSRR) measurement on a low-voltage drop out regulator. 04 | Keysight | M9037A PXIe Embedded Controller - Data Sheet Probing the input and output Although a standard 10:1 pass
10、ive probe can be used to probe the modulated DC input, a 1:1 passive probe must be used on the output. Assuming that the LDO under test has lots of rejection, the amplitude of output will be very small (sub millivolts). In addition, prop- erly grounding the probe near the DC output test point is cri
11、tical. If you use the standard ground lead that comes with your 1:1 passive probe, it will act as an antenna and pick up a significant amount of noise in the air and thereby reduce the dynamic range of your measurement. A short spring clip ground adapter (usually shipped as an accessory with the pro
12、be and shown in the inset image of Figure 3), or better yet, a soldered-in probe socket will provide the best low-noise signal fidelity. Figure 3 shows a photo of actual test set up of a PSRR measurement on a Picotest 3.3V linear regulator evaluation board using a Picotest J2120A Line Injector as th
13、e input sum- ming device. Note the probe socket used on the output probing point for a solid ground connection (no antenna!). Figure 3: Probing the DC-to-DC converter using two channels of the oscilloscope. 05 | Keysight | M9037A PXIe Embedded Controller - Data Sheet Performing the PSRR measurement
14、After making all connections to the DUT, first select the PSRR measurement in the Power Analysis menu. In the Signals sub-menu, define which channels of the scope are probing the input and output. For example, Input = channel-1 and Output = channel-2. Next, select the Settings sub-menu and define th
15、e test parameters including Start Frequency, Stop Frequency, and WaveGen amplitude. Although a higher input disturbance signal amplitude can provide a higher dynamic range measurement, if the amplitude is set too high, non-linear distortions can occur. In the Settings menu you can also define the ma
16、x- imum plot ratio in dB. Although the default setting is 100 dB, oscilloscopes dont have this much dynamic range. To begin the PSRR measurement, simply touch Apply. During this one-sweep test, the scope triggers internally on the WaveGen signal for a very stable trigger. At each tested frequency, the scope automatically optimizes vertical scaling of the input and output waveforms and takes eight averages at each frequency in order to eliminate random noise and provide maximum dynamic r
