How to Correctly Measure Power Supply Ripple

　　First of all, what exactly is the power ripple we usually talk about? Is it equivalent to power noise? In fact, power ripple is different from power noise. It is a component that appears between the output terminals and is synchronized with the input frequency and switching frequency. It is an AC interference signal superimposed on a stable DC signal. Noise is a high-frequency component other than the ripple that appears between the output terminals.

　　Figure 1 Power supply ripple and power supply noise

　　Now that we understand what power supply ripple is, let's look at how to correctly measure power supply ripple using the ZDS2022 oscilloscope.

　　How to Correctly Measure Power Supply Ripple

　　To measure power ripple correctly, appropriate settings are the key. In order to reduce the noise impact of the oscilloscope itself, the most sensitive range of the oscilloscope should be used as much as possible when measuring power ripple. However, the oscilloscope usually does not have enough offset range to pull the measured DC voltage to the center of the oscilloscope screen for testing at the most sensitive range, so it is usually necessary to use the AC coupling function of the oscilloscope. In addition, power supply tests usually specify testing ripple and noise within a certain frequency range, such as within 20MHz, and the bandwidth of general oscilloscopes is greater than this requirement. Therefore, the bandwidth limit function of the oscilloscope can be turned on during testing, which will also have a better effect on reducing high-frequency noise.

　　Not only the operation settings of the oscilloscope should be paid attention to, but also the probes that directly contact the signal. Since the power supply ripple amplitude is generally very small, if the probe attenuation ratio is set to 10:1, a lot of useless noise will be amplified, which will undoubtedly affect the measurement of the ripple. Therefore, the probe attenuation ratio should be set to 1:1, and it should be noted that the probe ratio of the oscilloscope should be set consistent with the probe attenuation ratio, so that the oscilloscope reading is correct. But is the connection method of the probe still the same as measuring ordinary signals, with the probe cap directly connected to the signal and the alligator clip directly connected to the ground?

　　In fact, this connection method is not applicable to the test of power supply ripple, especially when there is a switching power supply on the board. Since the switching of the switching power supply will generate a large amount of electromagnetic radiation in space, and the long ground wire of the oscilloscope probe is just equivalent to an antenna, electromagnetic interference will be introduced from space into the measurement circuit. A simple verification method is to connect the ground wire and the front end of the probe together, close to the circuit under test (not in direct contact), and you will see a relatively large switching noise on the oscilloscope. Therefore, the shortest possible ground wire should be used during the measurement process. Considering that you may sometimes need to make a short ground connection, a small spring is specially prepared for you in the probe accessories that come standard with the ZDS2022 oscilloscope to replace the alligator clip.

　　After ensuring the correct connection method and function settings, press the [Measure] key, and you will see 51 parameter items in the measurement item selection, and 24 real-time measurement results can be displayed at the same time. When designing the measurement function of the oscilloscope, our R&D team took into account that when you need to measure many parameters, in order to avoid selecting them one by one, we set up convenient settings such as "comprehensive measurement", "voltage measurement" and "time measurement". When we press the voltage measurement, the commonly used parameter items related to voltage will be automatically selected.

　　Power Supply Noise FFT Analysis

　　Sometimes there are some high-frequency components mixed in the power ripple, which are the power noise. These noises will inevitably have a certain impact on the circuit, so we need to analyze this component. The ZDS2022 oscilloscope has an FFT analysis capability of up to 4Mpts sample points, and the frequency resolution can be as low as 250Hz. Press the [Math] key and select FFT. In the 27th 100-episode practical video, we have shared with you the specific steps for FFT analysis of the signal, so we will not repeat them here. Using the spectrum diagram, we can analyze the frequency components of the power ripple and noise in order to further improve the circuit design.