Routing rules for crosstalk and impedance matching

Minimising crosstalk during routing

• Keep spacing at least 3W (centre to centre) between traces; the larger the spacing the lower the coupling capacitance and inductance.
• Keep parallel runs short; route adjacent layers at right angles.
• Keep traces close to the ground plane (small dielectric height) so the field is confined.
• Use ground guard traces with stitching vias between critical signals.
• Slow the edge rates with series resistors where timing allows; use differential signalling for noisy lines.
• Separate analog and digital and clock circuits; keep sensitive lines short.

Trace impedance matching techniques

Impedance matching prevents reflection on high-speed traces.

1. Controlled-impedance routing: characteristic impedance Z₀ depends on the trace width (w), copper thickness (t), dielectric height (h) and dielectric constant (εr). For a microstrip, Z₀ ≈ 87/√(εr + 1.41) · ln(5.98h/(0.8w + t)). Choose w and h in the stack-up to obtain the target (e.g. 50 ohm single-ended, 100 ohm differential), and keep these constant along the trace.
2. Termination:
   • Series (source) termination: a resistor R_s = Z₀ - R_out in series near the driver; the line load is high-impedance. Low power, suitable for a single point-to-point load.
   • Parallel (end) termination: resistor R = Z₀ from the receiver end to ground (or Vcc); good for multiple loads but dissipates DC power.
   • Thevenin termination: two resistors (R1 ∥ R2 = Z₀) to Vcc and ground.
   • AC termination: resistor R = Z₀ in series with a capacitor to ground, which reduces power.
3. Avoid discontinuities: no stubs, minimal vias, smooth bends, uniform width, no plane splits and matched connector impedance.
4. Differential pairs: keep constant spacing and equal lengths for matched impedance.

 Series:   Drv --[Rs]------------------- Rcv
 Parallel: Drv ------------------+------ Rcv
                                 [R=Z0]
                                  GND