Digital Circuit Boards: Mach 1 GHz

5.11 Resistor Networks

Multiple terminating resistors are often supplied as a single in-line package or SIP. When resistors are used for series terminations, separate connections must be made to each resistor. Note that on a SIP package, a separate lead goes from the top of the resistor to the board for each resistor. To reduce lead length in short rise-time circuits, a DIP or dual-in-line package is preferred. Any short transmission line between the signal source and the resistor can add to the signal propagation time. For a SIP package, the ends of the resistors nearest the board should connect directly to the logic. This is another case where a field solver may be required to determine if the performance is adequate. In general, high speed circuits are easier to manage if the resistors are mounted individually and not in SIP or DIP packages. There are solutions where the resistors are buried in the board as a part of vias. Another solution involves surface-mounted components. The approach used is a cost versus performance issue.

When parallel terminations are used, a group of resistors are often connected together at one end of the package. This common tie is then connected to the logic reference conductor, which is often the ground plane. The traces carrying the signals should terminate on their loads (logic gates) before reaching the resistor. If the termination is made in the reverse order, the result will be a short open-ended stub that connects to the load. Under these ...