As in Figure 1, the switching action of the primary side power switch will induce a magnetic field in the transformer primary winding’s that is coupled by the secondary winding's that produces a voltage that is subsequently rectified and this is the main means of power transfer in any SMPS power supply.
However, there is also a voltage waveform that is impressed across the primary winding's and since the secondary windings are usually wound right on top of the primary  there will be capacitance between the input and output windings. This capacitance can be 50pF in even a small 1 Watt DC/DC converter.
When the voltage is switched on the primary there will then be a current that is induced into the secondary side via this transformer inter-winding capacitance. This current is called the “Common Mode Current” and can be measured with the circuit of Figure 2.
As can be seen in Figure 3, this current is very fast and impulse like in nature and the harmonics extend well into the VHF frequency range. 18 mV peak to peak corresponds to a current of 180 uA peak to peak which is not insignificant considering that the current being common modern nature is flowing on the outsides of all the shielded cables, wires, PCB, and chassis components, etc.
The current in Figure 3 was measured from a cheap USB Isolator (Figure 5) that was meant to be inserted into a USB cable to provide galvanic isolation. And while it may do that job it will also provide a very nice VHF Impulse Generator into all of your other carefully designed circuitry.
Using an SMPS is a great way to get galvanic isolation in any circuit, but as far as Precision Analog goes it may make the “Total Noise” situation worse. As can be seen in Figure 6, adding this ‘Generator / Antenna’ into your design is probably not the desired effect that you were going to be looking for.
The only real mitigation to these Common-Mode Currents is to add inter-winding Faraday shielding inside the transformer to provide a local ground path inside the transformer for the currents or to use common-mode inductors on the input and output of the SMPS.
Perhaps a better way of providing USB isolation in instrumentation circuits is presented in the article:
 Over 25 years ago Jim Williams of Linear Technology did manage to find a Piezoelectric transformer that was an exciter on one end, a receiver on the other end, and a Piezoelectric bar perhaps 2 inches long in between. While this also produced common mode currents between the primary and secondary, they must have been minuscule because of the exceedingly low input to output capacitance. But this is a left-field sort of a device because of the size and price involved, which is so seldom used that it can be considered a ‘laboratory curiosity’ at best.
 Having any transformers primary and secondary winding's inter-wound as closely as possible reduces the Leakage Inductance between primary and secondary Leakage Inductance is where some of the magnetic field is stored but can’t generally be used in providing useful power transfer between input and output circuits . Adding Faraday Shielding between Primary and Secondary windings can reduce the common-mode currents by providing a ground leakage path, it will, however, increase the leakage inductance. Everything is a trade-off.
 There are always exceptions, some resonant converters are cleverly designed to utilize the leakage inductance, but these are not suited for low power designs. Again, everything is a trade-off.
Article By: Steve Hageman / www.AnalogHome.com
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