A mixed-signal integrated circuit is any integrated circuit that has both analog
circuits and digital circuits on a single semiconductor die. In
real-life applications mixed-signal designs are everywhere, for example, a
smart mobile phone. However, it is more accurate to call them mixed-signal
systems. Mixed-signal ICs also process both analog and digital signals
together. For example, an analog-to-digital converter is a
mixed-signal circuit. Mixed-signal circuits or systems are typically
cost-effective solutions for building any modern consumer electronics
applications. Introduction
A mixed-signal system-on-a-chip can be a combination of analog circuits, digital
circuits, intrinsic mixed-signal circuits, and Embedded Software.
Integrated Circuits are generally classified as digital or analog.
Mixed-signal ICs are chips that contain both digital and analog circuits on the
same chip. This category of chip has grown dramatically with the increased
use of 3G cell phones and other portable technologies.
Mixed-signal ICs are often used to convert analog signals to digital
signals so that digital devices can process them. For example, mixed-signal
ICs are essential components for FM tuners in digital products such as media
players, which have digital amplifiers. Any analog signal can be digitized using
a very basic analog-to-digital converter, and the smallest and most
energy efficient of these would be in the form of mixed-signal ICs.
Mixed-signal ICs are more difficult to design and manufacture than analog-only
or digital-only integrated circuits. For example, an efficient mixed-signal IC
would have its digital and analog components share a common power supply.
However, as one can imagine, analog and digital components have very different
power needs and consumption characteristics that make this a
non-trivial goal in chip design. Examples
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 tools. Automated testing of the finished chips can also be
challenging. Teradyne, Agilent, and Texas Instruments 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 primitive 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, P+ guard-rings, differential topology, on-chip decoupling, and
triple-well isolation. Commercial examples
ICsense AnSem
Atari POKEY MOS Technology SID
PSoC - Cypress PSoC Programmable System on Chip
System to ASIC Texas Instruments' MSP430
Triad Semiconductor Wolfson Microelectronics
Most modern radio and communications use mixed signal circuits.
See also Analog front-end
References Further reading
Saraju Mohanty. Nanoelectronic Mixed-Signal System Design, ISBN
978-0071825719 and 0071825711, 1st Edition. McGraw-Hill.
R. Jacob Baker. CMOS Mixed-Signal Circuit Design, Second Edition.
http:CMOSedu.com/