A mixed-signal integrated circuit combines analog circuits, used for analog signal processing, with digital signal processing (DSP) circuits on a single semiconductor die. An integrated circuit that includes both analog and digital circuitry (but no DSP), like a 555 timer or a pipeline ADC, is generally not considered a mixed-signal integrated circuit but rather a mixed-mode or mixed analog/digital integrated circuit.

Typically, mixed-signal chips perform some whole function or sub-function in a larger assembly such as the radio subsystem of a cell phone, or the read data path and laser sled control logic of a DVD player. They often contain an entire system-on-a-chip.

Examples of mixed-signal integrated circuits include data converters using delta-sigma modulation, analog-to-digital converter/digital-to-analog converter using error detection and correction, and digital radio chips. Digitally controlled sound chips are also mixed-signal circuits. With the advent of cellular technology and network technology this category now includes cellular telephone, software radio, LAN and WAN router integrated circuits.

Because of the use of both digital signal processing and analog circuitry, mixed-signal ICs are usually designed for a very specific purpose and their design requires a high level of expertise and careful use of computer aided design (CAD) tools. Automated testing of the finished chips can also be challenging. Teradyne and Agilent are the major suppliers of the test equipment for mixed-signal chips.

The particular challenges of mixed signal include:

• CMOS technology is usually optimal for digital performance and scaling while bipolar transistors are usually optimal for analog performance, yet until the last decade it has been difficult to either combine these cost-effectively or to design both analog and digital in a single technology without serious performance compromises. The advent of technologies like high performance CMOS, BiCMOS, CMOS SOI and SiGe have removed many of the compromises that previously had to be made.

• Testing functional operation of mixed-signal ICs remains complex, expensive and often a "one-off" implementation task

• Systematic design methodologies comparable to digital design methods are far more primiitve in the analog and mixed-signal arena. Analog circuit design can not generally be automated to nearly the extent that digital circuit design can. Combining the two technologies multiplies this complication.

• Fast-changing digital signals send noise to sensitive analog inputs. One path for this noise is substrate coupling. A variety of techniques are used to attempt to block or cancel this noise coupling, such as fully differential amplifiers.

Commercial examples

• Atari POKEY
• Integrated Device Technology
• MOS Technology SID
• Triad Semiconductor Via Configurable ASICs

Further reading

• http://CMOSedu.com/

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