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File Name: Mixed-Signal Integration: Functional Verification in the Presence of Linear Analog Components

Mixed-signal integration is the verification step that ensures a combination of digital and analog blocks operates correctly as a system. While mixed-signal simulation can be part of the process, it is important to understand the characteristics of analog blocks and how they interact with digital logic to ensure a successful verification process. This paper exposes some common pitfalls and proposes modeling and verification techniques for use in the integration phase of mixed-signal design.

A PHY mixed-signal design flow combines block-level analog design and verification, block-level logic design and verification, and a system-level integration process. The integration step is used to check logical system properties, as well as to check interconnections at the digital/analog boundaries inside the design. Borrowing standard techniques from digital systems, it is customary to assume that system level logic verification will detect integration errors as well as it does for digital-only systems.

Unfortunately, this assumption does not hold for mixed-signal systems. Various types of A/D converters can hide the very information that requires verifying. This insight suggests that a theory describing the verification of systems containing digital and analog blocks remains to be developed.

This paper examines the properties of a mixed-signal design pattern that occurs in many varieties in real designs. The example combines digital logic with variable linear analog components in a feedback configuration. For this example, we illustrate common errors that go undetected. We then propose modeling and verification techniques appropriate for use in the integration phase of mixed-signal design. While this example uses only linear analog components, we note that the insights gained by studying this system apply to non-linear systems as well.

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