Using switched capacitors to create programmable analog logic blocks in mixed-signal designs

by Sachin Gupta, Cypress Semiconductor , TechOnline India - September 07, 2010

Switched capacitor blocks are the basic building blocks of a programmable analog solution. They enable the integration of both analog and digital functions onto a single chip and define today’s true system-on-chip (SoC) architectures. Conventional analog signal processing circuits use continuous time circuits consisting of resistors, capacitors and operational amplifiers.

Any physical system design needs both analog and digital functionality. Achieving a modular, programmable design is crucial for the demanding applications of future, which has led to more and more designs integrating subsystems and using mixed-signal architectures.

Scalability as well as dynamic changes in customer requirements are two of the challenges designers face when implementing a system using fixed-function components. A modular, programmable design helps overcome the issues associated with the porting of designs to different devices at a later stage in a product’s lifecycle.

For these kinds of applications, a programmable design allows a more flexible approach compared to fixed-function implementations. Achieving such flexibility in the analog domain, however, has been a challenge for developers. The use of switched capacitor circuits greatly helps resolve this issue.

Switched capacitor blocks are the basic building blocks of a programmable analog solution. They enable the integration of both analog and digital functions onto a single chip and define today’s true system-on-chip (SoC) architectures. Conventional analog signal processing circuits use continuous time circuits consisting of resistors, capacitors and operational amplifiers.

Continuous time analog circuits use the ratio of resistances, or magnitude of resistance or value of resistance and capacitor to set transfer functions. It is true that the absolute accuracy of resistance and capacitor using MOS technology is not good enough to perform analog signal processing function.

However, relatively speaking, the accuracy of a capacitor using MOS technology is acceptable. In addition, the fabrication of accurate small capacitors is relatively easier and occupies less space using MOS technology as compared to that of resistance. Thus, we find switched capacitor circuits replacing conventional continuous time circuits in many architectures.

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