Electronic Design

Bus-Termination Scheme Offers Space And Layout Advantages

In terminating high-speed parallel data buses, designers face several challenges. Signals with fast edge rates demand terminations with very low parasitics. Currently, active bus devices produce edge rates as low as 0.3 ns with the slew rate on existing generations of SCSI at 1 ns or less. Consequently, laying out termination resistors on a system board can take several weeks. Adding to this burden is limited board space. Approaches such as chip-scale packaging may help, but they could conflict with established layout design rules, forcing a move to smaller pitches.

A new alternative in high-speed bus terminations is now being offered by CTS Corp., Ekhart, Ind. The method is being used in the company's ClearONE terminators, which are BGA-packaged devices that address the demands for high-frequency performance and easier board layout (Fig. 1). These components provide bandwidths of over 1 GHz and a component density of up to 750 resistors/in.2 That translates to a space savings of 25% to 40% versus chip-array solutions. Despite their compact size, ClearONE terminators come in 1- and 1.27-mm ball pitches, letting designers stay with the 5-mil trace widths and spacings that are popular in the industry.

The terminators also help with board layout. "With this technology, there will be a great reduction in design time," says Rick DeMars, corporate marketing manager at CTS. "Our part allows board designers to line up terminators with connectors and route them straight through." In addition, CTS will supply customers with Gerber files to facilitate the autorouting of their terminations.

The ClearONE devices consist of thick-film resistors on ceramic substrates. For interconnect, they rely on copper conductors and filled vias. The company exploits its proprietary formulations for via-fill and ball-attach materials to obtain very low levels of lead inductance. DeMars notes that "while most packaging techniques have lead inductances of 1 to 8 nH, this technology allows us to get down to 0.05 nH."

Another innovation is a patented crosstalk barrier consisting of conductive strips that are grounded by the user, as would be done with a conventional microstrip connector. Keep in mind that the crosstalk barrier affords additional crosstalk protection beyond that provided by ClearONE's inherently low coupling capacitance. In the RT1300, the first ClearONE product, the coupling capacitance between resistors is just 0.12 pF. Compare that against plastic-encapsulated resistor networks, which may carry 1.5 to 3 pF of capacitance between pins (Fig. 2).

Even with these advantages, ClearONE's prices are similar to plastic-packaged components, thanks in part to ClearONE's use of glass passivation rather than the costlier full encapsulation.

For more information on ClearONE, contact Bob Reinhard at CTS Corp. at (219) 589-3111. CTS may also be found on the Internet at www.ctscorp.com.

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