12.1 Introduction

In present-day information systems, signal processing is increasingly being carried out by digital VLSI integrated circuits. Broad fields of importance are ASICs and RAMs. Interface functions are required between the "real world" and the silicon system. The primary information acquired from the real world is usually in the form of time-continuous analog signals and must be interfaced to digital circuitry. The result of the digital processing must likewise be converted to back to analog form. In addition, the interfacing will frequently convert from the voltage domain outside the chip to current domain inside the chip. This is favourable for dynamic range in low supply voltage VLSI circuits and for high speed in the capacitance dominated chip interior.

The performance of an information system as a whole is determined to a large degree by the quality of the analog peripheral circuits. This is because digital processing can be performed to as high a resolution as is desired by simply increasing the word length. In addition, the speed of digital processing is steadily increasing by the evolution of submicron VLSI technology. The analog parts generally form the weak link in the system thus defining performance limits. The required interface functions are increasingly being realized on the same chip as the digital processing, i.e. in VLSI technology. The latter is determined by the digital demands. Submicron CMOS and, to a lesser degree, BiCMOS, will be technologies of choice. Pure bipolar will remain for niche applications.

A typical system interface...