Analog Devices Inc v. Linear Tech Corp

I. Executive Summary and Procedural Information

• Parties & Counsel:
  • Plaintiff: Analog Devices, Inc. (Delaware)
  • Defendant: Linear Technology Corp. (California)
  • Plaintiff’s Counsel: Hale and Dorr LLP
• Case Identification: 1:00-cv-10841, D. Mass., 03/16/2004
• Venue Allegations: Venue is alleged to be proper based on Defendant having transacted business, committed acts of infringement, and maintained a regular and established place of business within the district.
• Core Dispute: Plaintiff alleges that a series of Defendant’s integrated circuits infringe a patent related to digital-to-analog converter (DAC) drive circuits capable of producing a "rail-to-rail" output voltage.
• Technical Context: The technology concerns high-performance integrated circuits that convert digital signals to analog voltages, specifically circuits that maximize the output voltage range to equal the full span of the power supply, a key feature in modern low-voltage electronics.
• Key Procedural History: The patent-in-suit, RE38,083, is a reissue of U.S. Patent No. 5,684,481. The act of reissuing a patent can involve amending the claims, which may raise the possibility of an intervening rights defense for the defendant depending on the nature of any amendments and the timing of the alleged infringement.

Case Timeline

Date	Event
1994-03-18	Priority Date for U.S. Patent No. RE38,083
1997-11-04	U.S. Patent No. 5,684,481 (Original Patent) Issued
2003-04-22	U.S. Reissue Patent No. RE38,083 (Patent-in-Suit) Issued
2004-03-16	First Amended Complaint Filed

II. Technology and Patent(s)-in-Suit Analysis

U.S. Reissue Patent No. RE38,083 - "Rail-To-Rail DAC Drive Circuit" (issued Apr. 22, 2003)

The Invention Explained

• Problem Addressed: The patent describes a problem in conventional voltage-mode digital-to-analog converters (DACs) that are paired with operational amplifiers (op amps) for buffering. The op amp often cannot handle an input voltage that spans the full power supply range (the "rails"). This limitation on the op amp's input restricts the entire circuit's output, preventing it from achieving a true "rail-to-rail" voltage swing (RE'083 Patent, col. 2:51-62).
• The Patented Solution: The invention addresses this by first reducing, or dividing, the DAC's output voltage to a level that is within the op amp's permissible input range. The op amp is then configured as a multiplier to amplify this reduced-swing signal, restoring the output to a full rail-to-rail range (RE'083 Patent, Abstract; col. 2:1-8). This division is preferably implemented by adding "dummy" most-significant-bits to the DAC's internal resistor ladder and holding them permanently off, creating an impedance-matched attenuation network (RE'083 Patent, col. 2:8-12; Fig. 4).
• Technical Importance: This circuit architecture enables the design of DACs that can operate on a single power supply while delivering an output signal that uses the maximum available voltage range, a crucial capability for improving dynamic range in battery-powered and other low-voltage electronic systems (RE'083 Patent, col. 2:34-39; col. 2:56-62).

Key Claims at a Glance

• The complaint does not identify specific asserted claims, alleging infringement of the patent generally (Compl. ¶8). Representative independent claims include 1, 12, and 22. Claim 12 is analyzed here as a representative example.
• Essential elements of Independent Claim 12:
  • A DAC that produces an analog output signal with a predetermined swing range.
  • An operational amplifier with a permissible input signal range that is less than the DAC's output swing range.
  • A "divider" connected to reduce the DAC's analog output swing to a range within the amplifier's permissible input range.
  • The amplifier receives its input from the DAC through the divider and provides an output with "greater than unity amplification."
  • The amplifier's feedback circuit has an "input impedance that matches the output impedance of said DAC."
• The complaint does not explicitly reserve the right to assert dependent claims.

III. The Accused Instrumentality

Product Identification

The accused instrumentalities are a series of integrated circuits identified as "LTC 1448, LTC 1658, LTC 1659, LTC 1654, LTC 1665/LTC 1660, LTC 1661, LTC 1662, LTC 1663 and LTC 1664 products" (Compl. ¶8).

Functionality and Market Context

The complaint alleges these are integrated circuits designed, manufactured, and sold by the Defendant (Compl. ¶3, ¶8). The complaint does not provide any further technical detail, schematics, datasheets, or description of the specific functionality or internal architecture of the accused products.

IV. Analysis of Infringement Allegations

The complaint provides a list of accused products and makes a general allegation of infringement without providing a claim chart or a narrative infringement theory that maps specific features of the accused products to the elements of any asserted patent claim (Compl. ¶8). Therefore, a detailed analysis of the infringement allegations is not possible based on the complaint alone.

No probative visual evidence provided in complaint.

V. Key Claim Terms for Construction

• The Term: "divider" (Claim 12)

  • Context and Importance: The presence and nature of a "divider" is a core concept of the invention. Whether the accused circuits contain a structure that meets the legal definition of this term will be a central point of the infringement analysis.
  • Intrinsic Evidence for a Broader Interpretation: The specification describes the element functionally as a "divide block" (RE'083 Patent, col. 2:47) and an "attenuation network" (RE'083 Patent, col. 2:9), which may support a construction covering any circuit that attenuates the DAC's output voltage before it reaches the amplifier.
  • Intrinsic Evidence for a Narrower Interpretation: The patent's preferred embodiment implements the divider structurally as "m dummy bits in the most significant bit positions of said DAC" which are held "continually held OFF" (RE'083 Patent, col. 5:20-24). This language could support a narrower construction limited to this specific integrated implementation rather than any generic, external voltage divider.

• The Term: "input impedance that matches the output impedance of said DAC" (Claim 12)

  • Context and Importance: This limitation requires a specific relationship between the amplifier's feedback network and the DAC itself. The meaning of "matches"—whether it requires numerical identity or functional correspondence—will be critical for determining infringement. Practitioners may focus on this term because it appears to be a specific design constraint required by the claim.
  • Intrinsic Evidence for a Broader Interpretation: The specification states that it is "desirable" for the impedances to be equal to achieve "input bias current cancellation" (RE'083 Patent, col. 3:55-61). The use of "desirable" may suggest that perfect matching is a goal of a preferred embodiment, not a strict requirement for all implementations covered by the claim.
  • Intrinsic Evidence for a Narrower Interpretation: The detailed description states that the described feedback network results in a "net input impedance of R; this is the same as the DAC's R-2R output impedance" (RE'083 Patent, col. 3:65-67). The phrase "the same as" provides strong evidence for a construction requiring the impedance values to be substantially identical.

VI. Other Allegations

• Indirect Infringement: The complaint makes a conclusory allegation of induced and contributory infringement without pleading any specific supporting facts, such as the existence of instructional materials or knowledge of direct infringement by third parties (Compl. ¶8).
• Willful Infringement: The complaint alleges that infringement "has been and continues to be willful and deliberate" but does not allege any facts to support this claim, such as pre-suit notice or knowledge of the patent (Compl. ¶9).

VII. Analyst’s Conclusion: Key Questions for the Case

• A central issue will be one of claim construction: can the term "divider," which is described functionally in the patent, be limited to the specific structural embodiment of "dummy bits" integrated within the DAC's resistor ladder, or can it be read more broadly to cover other forms of voltage attenuation circuits? The outcome of this question will significantly impact the scope of the patent.
• A key evidentiary question will be establishing the internal operation of the accused integrated circuits. As the complaint lacks any technical details, discovery will be required to determine if the accused products practice the claimed two-step method of attenuating a DAC voltage and then amplifying it with a gain greater than one using an impedance-matched feedback loop.
• A third question may concern intervening rights: because the patent-in-suit is a reissue, the court may need to determine if the asserted claims were substantively broadened from the original patent. If so, Defendant may be shielded from damages for any infringing activity that occurred before the reissue date, depending on the specific facts.