How to Correctly Measure Power Supply Ripple Using an Oscilloscope

　　Power ripple test is a very important parameter in power quality detection, but how to accurately measure power ripple has become a difficult problem in the minds of engineers. How can we solve this problem? In fact, after searching for it for thousands of times, when you look back, you will find the method is in the dim light.

　　Since a DC regulated power supply is generally formed by an AC power supply through rectification, filtering, and voltage stabilization, it is inevitable that there will be some AC component in the DC voltage. This AC component superimposed on the DC regulated voltage is called ripple.

　　1. Improper ripple test

　　Connect a 3.3V power signal to the ZDS2024 Plus oscilloscope, use the probe gear X10, and measure the power ripple. After clicking [Auto Setup], adjust the horizontal time base, vertical gear and vertical offset, and you can get the result shown in Figure 1 below.

Figure 1 Incorrect ripple measurement method

　　As can be seen from the figure, the measured waveform is mixed with a lot of noise and clutter, and the DC and AC waveforms are mixed together, so it is impossible to clearly observe the ripple, resulting in the inability to accurately measure the ripple value. Many engineers measure ripples in this way because they do not master the correct ripple measurement method.

　　2. Correct Power Supply Ripple Test Method

　　1. First, select the appropriate gear of the probe. If the voltage is relatively large or the bandwidth requirement is relatively high, you can use the X10 gear. Under normal circumstances, it is recommended to use the X1 gear to avoid unnecessary noise attenuation affecting the ripple measurement.

Figure 2 Probe position selection

　　2. Ripple is an AC component, so the "channel coupling" method can use the "AC" method to limit the input of DC signals, as shown in Figure 3.

　　3. You can use the "bandwidth limit" function appropriately and select the "20MHz" bandwidth limit to filter out unnecessary high-frequency noise, as shown in Figure 3.

Figure 3 Channel interface parameter settings

　　4. In addition, it is more important to avoid interference with the signal caused by electromagnetic radiation, so it is recommended to use a "grounding spring" for grounding during measurement to avoid unnecessary interference caused by long grounding wires.

Figure 4 Comparison of grounding methods

　　5. The trigger mode can be edge triggered, and the trigger mode can be in Auto/Normal state.

　　6. Appropriately adjust the horizontal time base, vertical scale and vertical offset so that the waveform signal is displayed in the center of the screen with better effect.

　　The specific captured ripple is shown in Figure 5 below.

Figure 5 Correctly captured ripple

　　As can be seen from Figure 5, the correct ripple measurement method can clearly capture the normal ripple and reduce the impact of unnecessary noise and clutter on the ripple, which is basically a clean ripple. Power supply ripple measurement based on this can more accurately measure the ripple value and thus accurately estimate the power quality.

　　Ripple test is generally expressed in peak-to-peak value. You can use [measure] to perform automatic measurement, as shown in the following screenshot. The ZDS2000 series oscilloscope supports 51 true parameter measurement statistical functions, and measures ripple parameters based on the full storage depth. You can also use the "one-key cursor" for manual measurement, as shown in Figure 6 below.

Figure 6 Ripple measurement data

　　3. Analysis of measurement results

　　From the measurement, we can see that the peak-to-peak value of the power supply ripple is 18mv. Intel stipulates in the ATX12V specification that the peak-to-peak value of the +12V output ripple shall not exceed 120mv, and the peak-to-peak values ​​of the +3.3V and +5V ripples shall not exceed 50mv. The smaller the ripple, the higher the power supply quality.