Even a small seemingly innocuous design change can cause a 10X increase of these noise currents. Measuring Common Noise Ground Currents Produced by a Switching Supply Using a Magnetic Loop. The input and output grounds are connected on the test bench with a wire. Be sure to load the scope end of the coaxial cable from the loop in the cable characteristic impedance, usually 50 Ohms. The power supply load should be varied over its rated output current range.
Some power supplies are noisy at low load currents and others at higher load currents.
Another, more direct, way to measure the voltage drop caused by these noise currents is shown in Figure 3. Again, be sure to vary the load current of the power supply. There are a number of other ways to characterize common mode noise currents, but the ones shown here are easy and sufficient in most cases. Measuring Common Noise Ground Currents Produced by a Switching Supply Using A Balanced Probe. Certainly 1 volt induced across 1 cm of wire is too high.