-How to self-check before oscilloscope wiring measurement?

In the previous video, we learned about the improper operation of using the oscilloscope. Today, let's see how we can judge whether the ground on the probe can be directly connected to the ground of the board under test before wiring? Very simple, three steps to achieve self-test:
Preparation tools: board under test, multimeter, oscilloscope
1. Preparation: Turn on the multimeter and select the maximum AC file;
2. Wiring: The oscilloscope and the board under test are respectively connected to the power supply, and the power is not turned on for the time being; one end of the multimeter's test leads is connected to the ground of the oscilloscope calibration signal, and the other end is connected to the ground of the board under test. The simple wiring diagram is shown in Figure 3;
3. Measurement: power on each and measure the AC value.
At this time, if the measured value is not 0, it means that there is a voltage difference between the board under test and the ground of the oscilloscope, as shown in Figure 4. Then you cannot directly connect the ground on the board under test with a single passive probe ground clip.
It is best to use a high-voltage differential probe for floating measurement and mains measurement, which is both safe and accurate.