Mixed-signal circuits refers to integrated circuits (ICs) which have both analog circuits and digital circuits combined on a single semiconductor die.

Until the mid-1990s, "mixed-signal" usually meant ADCs, DACs, modem and electronic power supply and digital buffer integrated circuits. With the advent of cellular technology and network technology this now includes cellular telephone, software radio, LAN and WAN router integrated circuits.

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, 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 primitve in the analog and mixed-signal arena. Analog circuit design can not generally be automated to nearly the extend that digital circuit design can. Combining the two technologies multiplies this complication.