How to correctly measure the power ripple index of DC/DC modules

The power ripple index of the DC/DC module is a very important parameter. Clean power supply is the premise of stable operation of digital circuits and an important guarantee for various parameters of analog devices. In order to determine the quality of the power supply, the output ripple of the DC/DC module must be measured. However, the ripple value measured by many people is often hundreds of mV or even hundreds of mV, which is much larger than the maximum ripple value provided in the device manual. This is mainly caused by incorrect measurement methods.

Correct measurement method 1) Limit the oscilloscope bandwidth to 20MHz (most low-end and mid-range oscilloscopes are limited to 20MHz, and high-end products also have the option of 200MHz bandwidth limit), in order to avoid the high-frequency noise of the digital circuit affecting the ripple measurement and to ensure the accuracy of the measurement as much as possible. 2) Set the coupling mode to AC coupling for easy measurement (use a smaller gear to carefully observe the ripple, and don't care about the DC level). 3) Ensure that the probe ground is as short as possible (the main reason for measuring ripple fluctuations of hundreds of mV is that the ground wire is too long), and try to use the original test short pin that comes with the probe. If there is no test short pin, you can remove the ground wire and shell of the probe to expose the probe shell, and wrap the homemade ground wire around the probe shell to ensure that the length of the ground wire is less than 1cm.

Requirements for instruments

Oscilloscope parameter requirements: Support bandwidth limitation function: Generally, oscilloscopes support 20MHz bandwidth limitation, such as SDS1102CM.

Probe requirements: To make the ground wire as short as possible, try to use the original test short pin of the probe. If there is no original test short pin, you must make a short ground wire yourself: remove the probe ground wire cover, and use metal wire to wind the ground short wire yourself. It is recommended to use copper wire from Category 5 wire with moderate strength (still a little soft, please recommend if you have better ones). Other candidates include solder wire and carving knife. Select the 1X non-attenuation gear. Generally, when the passive probe is in the 1X gear, its bandwidth is limited to 6MHz/10MHz bandwidth. In this way, the interference of high-frequency noise can be effectively filtered out at the front end to reduce the impact of ripple measurement.

What should real ripple look like?

The following figures can give you a general intuitive understanding of the appearance of ripple. The DC/DC module under test outputs two voltages of 3.3V and 1.9V.

Source: Shenzhen Dingyang Technology Co., Ltd.