Digital Circuit Boards: Mach 1 GHz

5.10 Characteristic Impedance and the Error Budget

A part of the error budget must include errors caused by variations in characteristic impedance. When series terminations are used, the signal level and the termination resistor must match the line impedance. A mismatch produces an error signal. There are many other factors that influence the characteristic impedance of a trace. The obvious factors are variations in trace width, thickness, and spacing to the nearest conducting plane(s). The secondary effects that should be considered include the presence of nearby parallel traces. Nearby traces include side-by-side traces or traces on a nearby layer. Another source of error is the variation in dielectric constant across the board. Circuit board manufacturers are able to control the characteristic impedance of traces to within ± 10%. This may only be the impedance of a test strip and not the characteristic impedance of all traces. Test strips can also be applied to inner layers. It is up to the designer to request these strips. It is also up to the designer to interpret the manufacturer's claims correctly.

Terminating resistors may have a tolerance of about ± 10%. The driver source resistance and its tolerance must also be considered. When the errors related to resistor tolerance and characteristic impedance are equal, the logic level at the load can vary a maximum of ± 20%. Resistors that have a 20% tolerance will obviously use up more of the error budget. The effect of parasitic ...