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What Is a Mixed-Signal Integrated Circuit?

A mixed-signal integrated circuit combines analog and digital components onto a single semiconductor chip. While conventional analog or digital circuit designs rely on the advantages of each individually, a mixed-signal integrated circuit uses the best of both to enable optimal chip performance. With the proliferation of smartphones and portable electronic devices, mixed-signal ICs have become increasingly popular.

Analog Signals vs Digital Signals

In its most basic form, a signal describes information transmitted from one system to another through a medium such as air or light. Speech, for example, is transmitted through air.

Electrical input and output signals are used in a wide variety of engineering applications — batteriessensors, motors, actuators, converters, circuits, and more — allowing an almost endless manipulation of information.

What Are Analog Signals?

Analog signals are continuous, time-varying signals that can take on an infinite number of values. They convey information by utilizing variations in a physical property, such as voltage, frequency, or current. Our eyes, for instance, receive information through variations in light (a continuous, waveform analog signal) to make sense of the world around us.

Frequency modulation (FM) and amplitude modulation (AM) are the two types of analog transmission. As their names suggest, FM manipulates frequency to transmit information, while AM manipulates amplitude.

Analog signals work well for audio and video transmission or for transmitting physical, “real world” information, such as temperature, light, and sound. Analog signals are commonly transmitted over radio, water, or cable (twisted pair, coaxial, or optical). Devices that capture analog signals include phones, voice recorders, temperature sensors, and control systems.

Analog signals:

  • Have a higher density (i.e., carry more information)
  • Are less bandwidth-intensive
  • Are cheaper and easier to process

Analog components on an integrated circuit board include operational amplifiers, resistors, capacitors, and transistors.

What Are Digital Signals?

Digital signals, in contrast, are discrete, only taking up a finite number of values. Digital signals are, in fact, a subset of analog signals, carrying only a portion of the information in analog signals. They are typically encoded in binary format (0s and 1s), representing on and off states of quantities such as voltage, polarization, or magnetization.

Digital signals are commonly used in wireless communications, computer buses, storage media, networking, and data communications.

Digital signals are:

  • Easily encrypted
  • Easier to upgrade and configure
  • Ideal for long-distance signal processing, preserving signal integrity

Typical components on a digital circuit board include microcontroller units (MCUs) and digital signal processors (DSPs). Digital circuits are also synchronous, meaning a reference clock coordinates their operation. This contrasts with asynchronous analog circuits, where information is processed at the input.

Types of Mixed-Signal Integrated Circuits

Many integrated circuits (ICs) combine small electronic components onto a single miniature chip, supporting various applications. These include digital ICs, analog ICs, mixed-signal ICs, and application-specific integrated circuits (ASICs).

In practical applications, however, these ICs are frequently combined to achieve desired outcomes. For example, ASICs and microcontrollers incorporate both digital and analog circuitry and are, in effect, mixed-signal integrated circuits.

Mixed-signal integrated circuits combine analog and digital circuitry to provide sophistication and flexibility in the design and development of semiconductor chip designs.

A mixed-signal integrated circuit aims for an optimal balance between accuracy and performance, combining passive elements (such as capacitors) with active elements (such as the high-voltage transistors used in power management).

Analog Mixed Signal (AMS) Chips

Analog mixed-signal chips seamlessly integrate analog with digital signals on a single chip design, ensuring smooth communication between analog sensors and digital processors, underpinning the next generation of electronic devices — powering IoT networks and more. These chips include:

  • Radio frequency integrated circuits: RFICs combine high-frequency  analog design approaches with microwave circuit design methodologies, incorporating modulators/demodulators, amplifiers, oscillators, filters, and mixers on a single chip. They power a wide range of wireless communication systems, from cellular to wireless to navigation systems.
  • Memory chips: Memory chips are mixed-signal integrated circuits incorporating millions of capacitors and transistors that store information temporarily (Random Access Memory) or permanently (Read Only Memory).
  • Voltage regulators: Voltage regulators are integrated circuits incorporating three pins (or more) that maintain a constant voltage level in a wider integrated circuit. Components in a switching voltage regulator include a transistor switch, a diode, a capacitor, and an inductor.
  • Power management integrated circuits: PMICs are highly efficient power supply devices that integrate multiple voltage regulators and control circuits onto a single chip. They power a range of applications, including edge computing, the Internet of Things, autonomous vehicles, and electric vehicles.

Analog-to-Digital Converter (ADCs)

An important application of a mixed-signal integrated circuit is converting physical, real-world (analog) signals into a machine-readable (digital) format.

Therefore, an ADC is commonly used in video and audio equipment, temperature, pressure, motion sensors, medical devices, communication systems, or any other digital device that needs to process analog inputs. Here are a few facts to remember about ADCs:

  • There are three steps in analog-to-digital conversion: sampling, quantizing, and encoding.
  • The sampling rate describes the number of samples taken per second of a continuous, analog signal. It’s an important quantity that influences the quality of the resulting digital signal. The sampling rate differs with the medium being sampled — for example, 8 KHz (8,000 samples per second) for telephones and 16 kHz for voice over internet (VoIP).
  • During quantizing, the signal amplitudes measured are rounded off for representation in binary format. Consequently, there might be slight differences between actual analog signal values and output digital signal values, something called quantization error.

Digital-to-Analog Converters (DACs)

Digital signals often need to be converted into a physical format. A DAC is used for this purpose, converting digital signals into analog light in TVs and screens, or sounds in speakers.

Here are a few facts to remember about DACs:

  • Resolution, conversion time, and reference value are key factors influencing the quality of the converted signal.
  • Resolution represents a DAC’s smallest output increment.
  • Conversion time is the elapsed time between input and output signals.
  • Reference value points to the highest voltage achievable in the DAC. A low-resolution/high-frequency DAC is used for image, video, and visual output, while a high-resolution/low-frequency DAC is used for audio output.

A major advantage of integrating analog and digital components into single-unit ADCs and DACs on a semiconductor chip is reduced power consumption, bandwidth, and signal distortion.

Considerations of Mixed-Signal Integrated Circuit Design

Integrated circuit design flow describes the process of designing circuits until they’re ready for production (such as in a foundry). Integrated circuit design employs various tools, software (including computer-aided design), processes (including electronic design automation), and devices to simulate and optimize processes, and eliminate errors. In integrated circuit design:

  • Digital integrated circuit design integrates transistor switches and logic circuits.
  • Analog design integrates analog signals from capacitors, transistors, amplifiers, diodes, and others that haven't been digitized.
  • Radio frequency integrated circuit design — often considered a subset of analog design — integrates signals above several hundred kilohertz where RF phenomena dominate.

Mixed-signal integrated design may amalgamate any of the above designs. Modern approaches, such as system-on-chip (SoC) and system-in-package (SiP) technologies, integrate designs from each domain into a single chip.

These are increasingly used in multifunctional devices that perform various functions, including communications, sensing, processing, and storage.

As rapidly advancing wireless communication technologies (including 5G, LoRa , and Wi-Fi), IoT, and sensing technologies spawn increasingly complex mixed-signal ICs, mixed-signal design requires the cooperation of multidisciplinary teams — working with an electronic design automation (EDA) tool — to reach design goals.

A mixed-signal design flow typically includes:

  • Domain-specific design, including digital and analog (or RF) behavioral simulation
  • Mixed-signal analysis
  • Layout, including the physical layout in an analog design or the place-and-route in a digital design
  • Assembly and physical verification
  •  Mixed-signal functional verification
  • Tape-out

Challenges of Integrating Mixed-Signal ICs

Mixed-signal integrated circuits are more difficult to design than analog or digital circuits. For instance, analog and digital components may share a power supply in a mixed-signal circuit. However, because each has very different power consumption characteristics, this presents a significant challenge in mixed-signal design. Hence, mixed-signal circuit design aims to minimize interconnects between digital and analog circuitry, with the added benefit of reduced weight and size.

Moreover, mixed-signal semiconductor chips typically operate within a larger assembly (such as a radio subsystem in a smartphone), often incorporating a SoC and sometimes incorporating on-chip memory blocks. This further complicates the manufacturing of mixed-signal chips.

There are other complications involved in manufacturing mixed-signal integrated circuits:

  • Design methodologies are far more advanced for digital circuits than analog counterparts. While digital circuitry design can be automated to a large extent, this is far less true for analog circuits, which introduces limitations.
  • Fast-changing digital signals generate noise at sensitive analog inputs, which may result in substrate coupling.
  • Complementary metal oxide semiconductor (CMOS) transistor technology integrates well with digital circuits, while bipolar transistor technology is better adapted to analog circuitry. This has posed a problem in mixed-signal integrated circuit design until the development of technologies such as BiCMOS (Bipolar CMOS) in recent years.
  • Testing mixed-signal integrated ICs remains challenging as they’re often built for specific use cases, which makes testing them more time-consuming and expensive.

Importance of Testing Mixed-Signal ICs

Electromigration and voltage drop are the main causes of failure in mixed-signal design. Therefore, designers must understand these additional complexities.

Also, because automated testing presents a challenge in mixed-signal IC design, engineers rely on specialized computer-aided design (CAD) software, such as Totem-SC, to test their designs.

Totem-SC is the cloud-native version of Totem, the semiconductor industry’s gold-standard multiphysics sign-off solution for transistor-level and mixed-signal designs. Foundry-certified — that is all finFET processes down to 3nm — silicon-correlated simulation results provide engineers with total confidence in the optimal performance of their designs.

Applications of Mixed-Signal Integrated Circuits

While modern semiconductor technology has reached dizzying heights in power, performance, and area (PPA), it has also introduced a great deal of complexity into the chip design process, where the integration of digital and analog specifications is increasingly required.

Consequently, mixed-signal integrated circuits power an increasingly diverse array of devices in sensors, imaging equipment, industrial control and power management, automotive applications, IoT, medicine, and others.

Devices that integrate mixed-signal integrated circuits include:

  • Serializers/deserializers (SerDes)
  • Analog-to-digital converters
  • Digital-to-analog converters
  • Power management integrated circuits
  • High bandwidth memory (HBM)
  • Dynamic random access memory (DRAM)
  • Embedded memory systems
  • Field programmable gate arrays (FPGAs)
  • Voice processors in cellular communications
  • Temperature, pressure, and other sensors in Internet of Things (IoT) networks

To see a real-life working design that solves electrical, thermal, and mechanical challenges on an integrated circuit board, read our free report “Electrothermal Mechanical Stress Reference Design Flow for Printed Circuit Boards and Electronic Packages.”

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