Digital Circuit Boards: Mach 1 GHz

5.5 Logic Level Variation

A part of the noise budget is taken up by the inability of a logic driver to provide a voltage that reaches the limits of the power supply. The limits of voltage swing are called out in the manufacturer's specifications. For a logic 1, the highest loaded voltage is specified as VOH_MAX. The lowest maximum loaded voltage is VOH_MIN. The highest logic 0 is VOL_MAX, which defines the other limit to the logic voltage swing. The difference between VOH_MIN and VOL_MAX defines an envelope that must be considered in handling a noise budget. When parallel terminations are used the terminating resistor can be made intentionally high to compensate for a lack of voltage swing. In series terminations the resistor can be lowered to accommodate for the lack of voltage swing.

Series line terminations must take into account that the driver may have a finite source impedance. The source impedance plus the series resistor must match the line impedance. If the sum of the resistances is high then the logic level at the open end of the line will be low. If the sum of the resistances is low then the logic level at the open end of the line will be high. These adjustments are sometimes needed to keep errors within the assigned budget. In fast circuits, the location of the termination resistors is critical. Ideally, series resistors should be placed at the driver. Any lead length ahead of the resistor is a short transmission line that causes reflections that in effect adds to the rise ...