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EMBEDDED TERMINATION RESISTORS WHICH AUTOMATICALL Y COMPENSATE FOR PROCESS-INDUCED IMPEDANCE VARIATIONS IN TRANSMISSION LINES

IP.com Disclosure Number: IPCOM000009745D
Original Publication Date: 2000-May-01
Included in the Prior Art Database: 2002-Sep-16
Document File: 4 page(s) / 219K

Publishing Venue

Motorola

Related People

Gregory Dunn: AUTHOR [+4]

Abstract

For high speed digital applications, chip, packaging, and board interconnections can exhibit electrical resonances and reflections which degrade signal integrity and electrical system reliability. The problem increases with higher frequencies (becanse a larger number of traces in a given board design exhibit such behavior with shorter rise times) and lower power supply voltages (because lower noise is needed). To prevent this problem, transmission lines are typically terminated with discrete surfacemounted resistors, either in series with the line (series termination) or connecting to ground (paralleI termination; a capacitor is sometimes placed between the resistor and ground to reduce static power dissipation).

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MOTOROLA

Technical Developments

EMBEDDED TERMINATION RESISTORS WHICH AUTOMATICALL Y COMPENSATE FOR PROCESS-INDUCED IMPEDANCE VARIATIONS IN TRANSMISSION LINES

by Gregory Dunn, Lawrence Lach, Jovica Savic and Allyson Beuhler

PROBLEM

For high speed digital applications, chip, packaging, and board interconnections can exhibit electrical resonances and reflections which degrade signal integrity and electrical system reliability. The problem increases with higher frequencies (becanse a larger number of traces in a given board design exhibit such behavior with shorter rise times) and lower power supply voltages (because lower noise is needed). To prevent this problem, transmission lines are typically terminated with discrete surfacemounted resistors, either in series with the line (series termination) or connecting to ground (paralleI termination; a capacitor is sometimes placed between the resistor and ground to reduce static power dissipation).

Optimal performance is achieved by matching the termination resistor value to the characteristic line impedance (typically 50 Q). However, process variations typically canse actUal line impedances to vary significantly from the design value. For exampIe, an overetch of the copper will increase the impedance, while a thinner than intended dielectric underlying the line will decrease the impedance.

This results in mismatch between the termination resistor value and the line impedance, and thus imperfect damping of signal resonances and reflections.

SOLUTION

The solution to this problem is an embedded termination resistor the value of which depends upon

Motorola. Inv. 2000

processing variation in a similar fashion to the line impedances. Thus, matched line and resistor design values will guarantee a reasonable match even when actUal values vary due to processing variations such as linewidth variation and dielectric thickness variation. In this way, optimal circuit performance is achieved.

An embedded resistor construction which fulfills these criteria is illustrated in Figure I. A polymer thick film (PTF) resistor ink is deposited in a well underlying the transmission line, forming a vertical resistor between the line and a ground plane.

Variations in transmission line width (x) cause variations in line impedance and resistor width.

Variations in dielectric thickness (L) cause variations in line impedance and resistor length. Thus, both line impedance and resistor value are affected by processing variations.

A critical element of the resistor construction is that the length (L) of the resistor, viz., the vertical distance between copper terminations, be precisely equal to the dielectric thickness, and vary in synchrony with it from location to location, board to board, and lot to lot. In this way, automatic compensation for variations in transmission line impedance is achieved: a thicker dielectric reduces the capacitance per unit length between line and ground, thereby increasing the line impedance; if the...