Digital Circuit Boards: Mach 1 GHz

3.5 Common Mode and Normal Mode

To appreciate the meaning of these terms, it is worth discussing the rejection of common-mode interference in analog circuits (below 100 kHz). The circuit geometry is very similar to the arrangements found in logic transport between different pieces of hardware. In analog signal processing, the signal of interest is often generated at a remote point and carried to conditioning electronics over a long cable. The zero or reference potential for the signal is often carried on a shield conductor G₁ that is grounded near the signal source. The circuits that amplify the signal are associated with a second ground G₂ located at the point of amplification.

The potential difference that can be measured between G₁ and G₂ at G₂ is generated by fields in the area of cable routing. Consider a cable routed between two pieces of hardware. Fields in the area cross the loop formed by this cable and nearby conducting structures (grounds). These fields are often related to utility power, but they can include fields from radio and television transmitters, as well as digital circuitry. Signals that are observed between G₁ and G₂ at the receiving end of the cable will also appear between G₁ and all the signal leads in the cable.⁴ This common signal must be attenuated or filtered so that the signal of interest (signal difference) can be amplified. This average or common signal is called a common-mode signal. For low frequency analog signals, these circuits are called