Oscilloscope probe grounding and channel crosstalk analysis

When engineers use an oscilloscope to measure the output signal of a switching power supply, they often find that the signals of the two measurement channels interfere with each other (crosstalk). If the measurement method can be changed, this confusion can be solved. Below PRBTEK teaches you how to do it:

I. Overview

When engineers use an oscilloscope to measure the output signal of a switching power supply, they often find that the two measurement channels interfere with each other (crosstalk). This situation is usually related to factors such as channel isolation and test methods, and can be compared and analyzed through a small experiment.

Test principle: Channel 1 measures small signals (100mv/div), channel 2 measures large signals (5V/div), and observes the mutual influence of the signals.

First, use a standard signal source to verify the factors affecting channel isolation;

Different measurement methods are then used to verify the influencing factors of crosstalk.

2. Channel isolation test

As shown in Figure 1, the signals input from the two channels of the oscilloscope are directly output from the signal generator. It can be found that the signal from channel 2 has no effect on channel 1, and the channel isolation is good. Of course, there are more rigorous testing methods for the channel isolation index, so we will not make a detailed analysis here.

Figure 1 CH1 and CH2 measuring signal generator output, good grounding

3. Impact of measurement methods

The measurement method is mainly related to the use of the probe, especially the connection method of the probe ground wire. The measurement process is as follows, and the crosstalk of this test mode is recorded at the same time:

1. Use an alligator clip to ground the probe of channel 1, and close channel 2 (to prove that the signal of channel 1 is in its original state, see Figure 2)

2. Use alligator clips to connect the probes of channels 1 and 2 to the same ground (crosstalk is very serious, see Figure 3);

3. Use alligator clips to ground the probes of channels 1 and 2 respectively (severe crosstalk, see Figure 4);

4. Channel 1 is grounded using a spring ground, and channel 2 is grounded using an alligator clip (crosstalk is minimal, Figure 5);

5. Use spring ground to ground the probes of channels 1 and 2 separately (minimum crosstalk, see Figure 6).

Note: Although the signal amplitude of channel 1 is very small, the interference at the 600K frequency can still be located through FFT analysis.

Figure 2 CH1 uses a clip to measure the power output, CH2 is left floating

Figure 3: CH1 uses a clip to measure the power supply output, CH2 uses a clip to measure the inductance, and single-point grounding

Figure 4: CH1 uses a clip to measure the power supply output, and CH2 uses a clip to measure the inductance, and each is grounded

Figure 5 CH1 uses a ground spring to measure the power supply output, and CH2 uses a clip to measure the inductance, each connected to ground

Figure 6 CH1 uses a ground spring to measure the power supply output, and CH2 uses a ground spring to measure the inductance. Each is grounded

IV. Conclusion

1. When the measuring probe and ground wire are well connected, the interference between channels of the oscilloscope is very small;

2. The interference comes from the influence of grounding parasitic parameters at the test probe, such as lead inductance;

3. When measuring two signals at the same time, separate grounding is required to avoid mutual interference of ground loops;

4. When measuring sensitive signals, use a grounding spring and, if necessary, an interface terminal.

In summary, through the above test comparison, it is found that the direct source of interference in channel 1 is introduced by the probe ground wire of channel 1. Therefore, when measuring multiple signals at the same time, in order to avoid mutual interference of ground loops, it is strongly recommended to separate the grounding.