Capacitive Technology Filters And Decouples With Fewer Parts

Signal-integrity challenges at the system level continue to grow as signal speeds increase, signal levels drop, and component densities on the pc board rise. Designers must ensure that their circuits function properly and meet various EMC requirements while staying within cost and pc-board real-estate budgets. The decoupling and filtering circuits needed to suppress noise can add many passive components.

These components can be combined into integrated passive devices (IPDs), but these are often application-specific solutions available from a single source. Even when multiple sources do exist, differences in IPD characteristics from vendor to vendor may affect their performance. Discrete solutions pose challenges, too, because the capacitors used in decoupling and filter circuits can distort signals by introducing droop, ringing, skew, and delay. To address these problems, designers may resort to slower switching speeds or shortened signal paths, or use paired and matched passives to obtain the necessary filtering and decoupling.

A new capacitive technology introduced by X2Y Attenuators LLC, Erie, Pa., can overcome the limitations of currently available signal-integrity solutions by reducing parts count while enhancing performance. It also opens the door to multisourced solutions. The X2Y technology is not a capacitor per se, but rather an architecture that can be used to manufacture a variety of devices, including capacitors, decouplers, transient voltage suppressors, and filters.

Intended to perform both decoupling and filtering, X2Y devices are application-specific in the sense that capacitance value, package style, and dielectric type are optimized for the application. Nevertheless, it should be possible for multiple vendors to produce X2Y devices with equivalent performance, which alleviates the problem of single-sourced solutions. This benefit results from having inherently low parasitics in X2Y designs.

According to the company, an X2Y device has characteristics approaching those of an ideal capacitor with high self-resonant frequency and extremely low ESR and ESL. Functionally, an X2Y device is a three-terminal part that can be represented by a single "X" decoupling capacitor that connects line to line and two "Y" bypass capacitors that are balanced from line to ground (Fig. 1). The X2Y's structure consists of layers of hot electrodes and ground shields, which act as Faraday shields for each hot electrode (Fig. 2). The balanced structure of electrodes and ground acts to cancel the device's internal inductance. And because the device operates in the bypass mode, it isn't current-limited.

The Y capacitors are formed between the hot electrodes and the ground shields. The X capacitor is formed between the hot electrodes, and it is shielded by the grounds. In operation, common-mode noise currents are shunted through the Y capacitors to the board ground—when one Y is shunting noise to ground, the other is blocking noise from ground to line. Differential-mode noise currents meet and cancel each other on the ground plane, eliminating loop noise. Considered dielectric independent, an X2Y device may be constructed with film, ferrite, or MOV dielectrics.

The X2Y can be contrasted with a typical discrete or hybrid design that requires five components to prevent signal errors on a pair of balanced signal lines. In the typical case, on each line there is a three-terminal feed-through capacitor in series with a wound inductor. A single decoupling capacitor is connected from line to line. The same five-piece circuit can be replaced by a single X2Y device.

This comparison is primarily a functional analogy. In actual applications, the X2Y's performance with respect to resonant frequency, ESR, and ESL may allow it to replace a significantly greater number of passive components—as many as 18 or more. As with other solutions, though, performance depends on proper termination of the circuit, component placement, and layout.

The company says that its architecture disproves two assumptions made by capacitor manufacturers when discussing existing decoupling technology. One is that reducing ESR and ESL in one plane increases these parameters in the opposite plane. The other is that phase current (power plane noise) cancellation can't be achieved between planes.

These performance claims are supported by by a few examples of actual X2Y components. A 100-nF 1206-size device built with an X7R dielectric exhibited an ESR of 2.4 mΩ. In the case of a 0.01-µF two-hole feedthrough component, the ESL was determined to be less than 25 pH. In a third case, a two-hole discoidal capacitor offered capacitive insertion loss well beyond 1 GHz, well above the normally expected self-resonant frequency of 275 MHz.

For more information, contact James Muccioli, v.p. of technology at X2Y Attenuators, at (248) 489-0007. Or go to www.x2y.com, or www.syfer.com.