what is a mixed signal circuit
A mixed-signal integrated circuit combines analog circuits 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, is generally not considered a mixed-signal 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 especially using error detection and correction, digipot, 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.