Energy coupling; capacitive coupling and remedies

Principle of energy coupling

Every noise problem needs a source, a coupling path and a receiver. Energy coupling is the transfer of electrical energy from the noise source to the receiver through the coupling path. The path can be a conductor (conductive), an electric field (capacitive), a magnetic field (inductive) or a radiated wave (electromagnetic). The noise can be eliminated by reducing the source, breaking the path, or making the receiver immune.

Capacitive coupling

Any two conductors at different potentials with a dielectric (air) between them form a stray capacitance C. A changing voltage on the source conductor passes a displacement current into the victim conductor:

I_noise = C · dV/dt

The noise voltage across the victim impedance R is V_n = R · C · dV/dt (for low frequencies). It is more serious when dV/dt is high (fast edges, high voltages), the receiver impedance is high, conductors run parallel for a long way and the spacing is small.

  Source ----+----------
             | C (stray)
  Victim ----+----[R]--- GND

Remedies

1. Increase spacing and reduce parallel run length; cross traces at right angles.
2. Electrostatic (Faraday) shield grounded, around the victim or the source; use shielded cables.
3. Reduce the receiver impedance (lower R, e.g. a pull-down resistor or buffer).
4. Reduce dV/dt of the source: slow the edges using series resistors, snubbers or RC filters.
5. Use balanced/differential lines so the induced noise appears as common-mode.
6. Place a ground trace/guard between the lines on a PCB; use ground planes.