Common mode noise is caused by a common mode current. In this case noise current flows along both the outgoing lines in the same direction and returns by some parasitic path through system ground that is not part of the design, the so-called “sneak circuit” discussed earlier. In many cases, common mode noise is conducted through parasitic capacitance in the circuit. Common mode currents flow in the same direction in or out of the switching supply via the power leads and return to the source through ground. Common mode currents will also flow through the capacitance formed between the case and ground. Differential mode noise results from a differential mode current in a two wire pair. The differential mode current is the expected current on the two wire pair, i.e., current leaves at the source end of the line and comes back on the return side of the line. The noise is measured on each line with respect to a designated reference point. The resultant measurement would be the difference in the noise on the two lines. Differential mode currents flow between the switching supply and its source or load via the power leads and these currents are independent of ground. Consequently no differential mode current flows through ground. Conducted EMI emissions are measured up to 30 MHz. Currents at frequencies below 5MHz are mostly differential mode, while those above 5MHz are usually common mode.