Signal Impedance
A resistor placed in series with a driver output minimizes waveform distortion caused by signal reflections along a transmission line. This series termination matches the source impedance to the characteristic impedance of the interconnecting trace. It slows the edge rate of the signal to prevent overshoot and undershoot that otherwise occur when energy hits an open load.
Proper sizing of the resistor depends on the output impedance of the integrated circuit and the impedance of the board layout traces.
Fabrication Requirement
Controlled trace width and substrate dielectric thickness define the impedance environment that dictates the selection of a resistor value. Design engineers calculate this value to eliminate ringing that degrades logic level stability at the receiver. A board fabricator holds tight tolerances on trace geometry to ensure the predicted line impedance remains consistent with the component choice.
Failure to balance these values leads to signal integrity errors that standard automated test equipment detects during final assembly verification.
Assembly Consequence
Soldering the resistor as close as possible to the driver pin reduces the parasitic inductance that renders the matching effect useless. High frequency switching demands this proximity to keep the path between the driver and the resistor short. A mismatch between the component and the PCB trace creates reflections that travel back to the driver and corrupt the voltage levels intended for subsequent logic gates.
Correct implementation ensures the output signal remains within the threshold specifications for reliable switching across the entire load range.
