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| EMC Tip |
| "When laying out a printed circuit board, route the ground and power traces first, then I/O and high speed traces, then the rest of the remaining signal traces." |
| One of the essential
elements of an effective high-speed circuit is a low-impedance power supply. A
low-impedance power supply is required for the high-speed circuits to deliver rapid
changes in current without a resulting voltage drop at the power supply rails. The ability
of digital circuits to switch quickly, especially in the nano-second range is greatly
dependent on the power supply inductance. Low power supply inductance is essential to
reducing power supply noise voltages. In order to achieve low inductance power or ground connections, a useful and common technique is to separate power and ground on different planes in a multilayer circuit board. The separate power and ground planes provide large conducting surfaces that effectively reduce self inductance. In other situations where separate power or ground planes are not feasible, then routing of power and ground demands a great deal of real estate on the board where power and ground must share the same layer as signal traces. In order to minimize power and ground resistance, the power and ground traces must be as wide as practical while still leaving sufficient room on the circuit board for routing other signal traces. The effect of self inductance in power and ground connections can be mitigated by careful selection and placement of decoupling capacitors. For this reason, routing of power and ground traces should be given first priority, especially when no separate power or ground planes are used. After routing the power and ground traces, the I/O traces should be routed next. I/O traces carry signals which directly interface with external components (boxes) or devices via long interconnecting cables. Generally it is good practice to route I/O signals in a way so the I/O internal traces on the circuit board do not venture too far away from the equipment casing. When I/O traces are routed well into the circuit board away from their entry/exit point at the connector, the likelihood of inductive or capacitive coupling to other circuits is greatly increased. High frequency signals may be induced or coupled onto the I/O lines that can cause the I/O conductors to radiate energy much like a good antenna would. Keeping the I/O traces close to their connectors by not routing them far onto the circuit board also minimizes the possibility that EMI picked up by the I/O conductors from outside the equipment casing can penetrate circuits inside the casing. The next traces to be routed after I/O traces are high-speed traces. These traces may carry clock or data signals that can be sensitive to very small increases in inductance. Inductance is the electrical property which opposes any sudden changes in current along the trace thus potentially limiting the speed at which a circuit can switch states. This is bad news from a signal integrity perspective. In order to keep inductance as low as possible in high-speed traces, the traces must be kept as short as possible. When routing high-speed traces, the shortest route is usually preferred. Another advantage to keeping high-speed traces short, is that effective loop areas are reduced. Loop areas are formed by the physical region bounded by the signal carrying trace and the signal return trace. Reducing effective loop area and trace length in high-speed circuits should lessen the chance of radiation and pickup due to antenna effects. In order to keep the high-speed traces short, they must be routed early on, so the trace path length and route geometry can be optimized.
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