Power supply ripple measurement solution for DC-DC modules

Clean power supply is the premise of stable operation of digital circuits and an important guarantee for various parameters of analog devices. The power ripple index of DC/DC modules is a very important parameter. In order to determine the quality of power supply, the output ripple of DC/DC modules must be measured. However, the ripple values ​​measured by many people are 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.

The right way to measure

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). The purpose is 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 do not care about the DC level).

3) Ensure that the probe grounding wire is as short as possible (the main reason for the measured ripple of hundreds of mV is that the grounding wire is too long), and try to use the original test short needle that comes with the probe. If there is no test short needle, you can remove the probe grounding wire and shell to expose the probe ground shell, and wrap the homemade grounding wire around the probe ground shell to ensure that the grounding wire length is less than 1cm.

4) The oscilloscope ground is suspended, and only the probe ground is used to share the ground with the reference point of the test signal. Do not share the ground with the test equipment in other ways, as this will introduce a lot of ground noise to the ripple measurement. For example: when the oscilloscope and other instruments share the same plug board, the switches of other instruments may bring noise interference to the test through the ground wire.

The third point is the most critical. If the ground wire is too long, its inductance effect will introduce additional noise into the measurement system, as shown in the figure below.

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 (it is still a bit soft, please recommend 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 ripple really 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.