2. The working principle of single-ended signals is that the received signal equals the difference between the signal and the power supply or ground. Consequently, noise on the signal or power supply system cannot be effectively canceled.
3. This stark contrast explains why differential signaling is highly effective for high-speed signaling and is extensively used in fast serial buses and double-data-rate memory.
1. In a differential pair, both positive and negative signals must consistently travel together along the transmission path within the same environment. Maintaining proximity between these signals ensures coupling via electromagnetic fields at corresponding points. Since differential pairs are symmetrical, their environment must also be symmetrical, though perfect symmetry is unattainable due to dimensional tolerances. Designers can achieve nearly ideal differential signaling by adhering to basic rules: ensure signals appear simultaneously on each line, maintain equal trace lengths (as denoted by the same letters in the diagram), and symmetrically connect series termination resistors or common-mode filters to the positive and negative pins of the differential driver.
2. When routing, maintain point-to-point connections and ensure any stub or branch (C in the diagram) remains within 0.6Tr inches, where Tr is the driver output rise time. Attempt to apply consistent length limits (as with A and E in the figure) wherever feasible. Use a field solver for designing trace spacing to easily obtain even-mode and odd-mode impedance values. Note that a 50 ohm board does not imply that even-mode, odd-mode, or differential characteristic impedance is also 50 ohms.
3. Consider the effects of environmental noise and odd-mode impedance when terminating a differential signal to ground or a reference voltage. Also, contemplate terminating even-mode or half of the even-mode value to suppress unwanted noise. For differential mode termination between two wires, consider twice the odd mode impedance. Effective suppression of radiated noise from the same source relies on tightly coupled differential pairs, as this ensures nearly identical surrounding electromagnetic fields, achieved when traces are closely aligned.
4. Compensate for any skew between complementary output signals by extending trace lengths close to the driver, maintaining differential extension whenever possible. Balance the number and style of left and right bends. Replace odd- and even-mode impedances with single-ended characteristic impedances as termination impedances. Ensure that the total trace length is equal, rather than every segment being identical.
5. Route differential pairs across gaps in power or ground planes to prevent autorouting tools from mistakenly routing them as single-ended. Add test pads at various locations on each side of the differential pair for testing purposes. Avoid routing other signals too closely or parallel to the differential pair, or on adjacent layers, to prevent crosstalk that can disrupt the differential signal balance.
6. When analyzing the target circuit with simulations, model connectors, cables, and differential topologies on other boards in the system to account for parasitic inductance and capacitance. Avoid misleading measurements and false failures during PCB testing caused by unbalancing the differential pair with probes or test equipment placed on one side of the differential pair.
