EMC Question of the Week: August 9, 2021

Illustration of a single-ended source driving a twisted-wire pair

In well-designed systems, conductors labeled "ground" never carry

  1. differential signal currents
  2. single-ended signal currents
  3. power currents
  4. noise currents


The best answer is “a.” Of course, giving an electrical conductor in a system a particular name won't affect the currents that flow in that conductor. And while it's generally important for EMC engineers to recognize the difference between conductors that perform a "grounding" function and conductors that perform a "current-return" function, many well-designed systems don't make that distinction when labeling circuit board nets or wires in a cable. For example, the "ground" plane in a typical circuit board primarily functions as a power and signal current-return conductor. It also carries noise currents.

Nevertheless, differential signals should always be routed on a pair of conductors that are electrically balanced. By definition, balanced signal conductors have the same impedance to "ground," so one of those conductors cannot be "ground." Labeling a differential signal conductor "ground" would be like naming your capacitors R1, R2, R3, etc. It wouldn't change the way the circuit works, but it would be misleading and possibly result in poor design decisions.

It's not uncommon to see twisted-wire pairs with one conductor connected to circuit ground or differential signals routed on coaxial cables. Both situations convert half of the differential-signal voltage to a common-mode voltage that drives the cable relative to circuit ground. These problems are easily avoided by recognizing that differential-signal currents should never be routed on a conductor labeled "ground." 

Note: It is often recommended that NONE of the conductors carrying intentional signal or power currents be labeled "ground." Putting the word "Return" or "RTN" in the net name can help designers distinguish between current returns and actual safety or EMC grounds.

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