EMC Design Guide for Printed Circuit Boards
Frame 58 of 78
Printed copies are uncontrolled
27. Corresponding power and ground signals should always be routed in parallel (side-
by-side) or on top of each other (on adjacent layers) to minimize loop area thus
reducing loop impedance (Figure 511).
Figure 511. Power and Ground Routing
28. VCC (clean power) traces should never be routed parallel to unfiltered (dirty) traces
that carry battery, ignition, high-current, or fast switching signals.
29. Use the lowest power, slowest logic that satisfies circuit requirements.
30. Power, ground, and signal traces on the board should be kept short and as wide as
possible. The traces should be shorter than the diagonal dimension of the board, and
ideally their length-to-width ratio should be kept at 10:1.
31. Placing ferrite beads on power tracks may provide attenuation of unwanted signals
above 1 MHz. When properly sized, these beads can be very effective in damping
high-frequency switching transients or parasitic ringing due to line reflections without
causing a DC loss. CAUTION: using ferrites may impede AC current flow.
32. Devices sensing battery or ignition, such as sensing resistors, should be placed at the
power entry point to PCB (close to I/O connector).
33. Devices, such as Zener diodes, MOV's or transzorbs should be placed at the power
entry point to PCB as their function is to limit/clip transients and spikes. Assure low-
impedance connection to ground.
34. Provide enough current storage (capacitor) on the incoming battery line when
designing switching power supplies and/or other circuits drawing discontinuous
currents from the battery, so that these currents do not appear on the wiring harness
where they can be radiated or conducted to other circuits.