Introduction to Analog IC Design
Interested in learning about analog integrated circuits? This article explores the design metrics and applications of these essential but underappreciated ICs.
Integrated circuits (ICs) have ruled the electronics industry since they were introduced in the late 1950s. All signs suggest that these little black boxes will continue to dominate the market, and analog IC design, in particular, has become increasingly important over the years.
Despite this, when most people think of ICs, they think of digital circuits like computer processors or microcontrollers. This article should help correct that. We’ll review the nature of analog ICs, go over some of these circuits’ areas of application, and finally examine the unique requirements and challenges of designing them.
What is an Analog IC?
Before we jump into discussing analog IC design, we need to define ‘analog’.
Analog signals are continuous in time and have an infinite range of values. All signals found in nature, from sound waves to brain waves, are analog. Digital signals, on the other hand, are discrete over time and can only be one of a certain number of states at any given time. Figure 1 provides a comparison of the two signal types.
Figure 1. Analog (top) vs. digital (bottom) signal. Image used courtesy of Monolithic Power Systems
An analog IC is an integrated circuit that produces or amplifies analog, rather than digital, signals.
Applications of Analog ICs
Now that we know what analog ICs are, Figure 2 shows the fields in which they’re used.
Figure 2. Applications of analog electronics. Image used courtesy of Semiconductor Research Corporation
As you can see, analog ICs have diverse applications. Let’s look at them by category.
To control circuit operation, analog circuits create signals based on sensor values. Analog circuits extensively use a technique called feedback, in which the circuit output value feeds back within the circuit to adjust the output based on its own value.
Some applications, such as playing or recording audio, require the conversion of digital signals to or from analog. The circuits used to accomplish this must incorporate analog circuitry for the purpose of measuring analog signals.
Let’s say you want to play some music on your phone. To generate the audio that plays through your phone’s speaker, the digital data representing the sound needs to be converted to actual sound waves. Since sound waves are analog signals, this means the signal switches from the digital to the analog domain.
This is carried out via a circuit known as a digital-to-analog converter, or DAC. Conversely, recording audio requires changing an analog signal (the audio being recorded) into a digital one, so an analog-to-digital converter (ADC) would be used. Both DACs and ADCs require special circuitry, both digital and analog, to enable high performance while keeping power consumption reasonable.
All circuits require power regulation to ensure they receive the correct amount of voltage and current for proper operation. Power sources can be noisy or faulty, which can hamper or even destroy electrical systems. That’s why it’s necessary to include power electronics that, by providing safety features such as automatic shutoff and/or isolation from the source, keep the circuits from being harmed.
These power regulation circuits can convert AC electricity from a wall outlet to low-voltage DC. They can also perform DC-to-DC conversion to generate voltages of different values. Additionally, they can utilize energy harvesting to convert solar, RF, or radiant energy to a DC power supply voltage.
While data traveling over a cable is commonly thought of as digital, it’s actually analog in nature. Analog circuits provide the driving capability to send data across the cable with enough juice to be readable on the other side. The receiving end must also have sensing amplifiers that can read the data from the transmitter and convert it back to a full-scale digital signal.
Data transmission applications for analog circuits aren’t limited to wireline communication—the transceivers used in wireless data transmission contain many analog circuits. These function to generate and measure electromagnetic waves, allowing the transceiver to transmit the required data over distances and convert the RF signals to full-scale digital ones.
Instrumentation and Physical World Interfaces
To sense real-world signals or generate signals that must interact with the real world, it’s necessary to produce continuous-time signals. By definition, this means using analog circuitry.
Precise testing and measurement equipment requires high-performance analog circuits that can convert sensor output signals to readable voltages with a high level of accuracy. This allows the voltages to be converted to digital signals. Analog ICs are used as components of instruments ranging from digital multimeters, which sense input voltages and currents, to accelerometers, which sense the change of value in a sensing capacitor.
Digital circuits still rule this domain, but analog circuits can perform highly efficient computation as well. The analog circuit with the highest computing performance is your brain, and electronic circuits that perform analog computing are usually trying to replicate the human brain’s operation. This is a big topic of research at present.
Analog IC Design Requirements
When engineers design digital circuits, they’re mainly concerned about two design features: power consumption and processing speed. These two variables usually work against each other—designers have to find an optimum balance between the two for their specific application. For analog circuit designers, there are many more considerations to take into account, as we’ll see in this section.
The diagram in Figure 3, commonly known as the analog design octagon, illustrates the metrics that analog designers must take into account when creating a specification or assessing an already-designed circuit’s performance.
Figure 3. The analog design octagon. Image used courtesy of All About Circuits
Because power dissipation and speed are shared metrics between analog and digital circuits, we won’t discuss them here. We will, however, examine the other points on the design octagon.
Noise—either produced by circuitry, or in the form of unwanted signals from the natural world that can’t be mitigated—is a big issue for analog circuits. Analog circuit components can produce non-negligible noise values that add to the true signal of interest, hampering the signal integrity.
Due to their high-speed switching, digital circuits can provide an amount of noise large enough to destroy an analog circuit signal output. However, analog circuits can be designed in a way to “filter out” the unwanted noise. Designers should take note of this.
Linearity and Gain
Analog circuits are primarily composed of amplifiers, including the operational amplifiers we all know and love. An ideal op-amp is assumed to have an infinite gain—to get op-amp operation as close to the ideal as possible, the amplifier gain (output over input) must be maximized.
Meanwhile, linearity is defined as the ability of a circuit’s transfer function to remain as linear as possible across the entire operating range. A perfectly linear circuit will operate the same way at the bottom of the input range as at the top. This is desirable because it allows for simple processing of the data. If the circuit is non-linear, complicated post-processing needs to be done or the data will be read incorrectly.
Supply Voltage and Voltage Swing
Analog circuit designers must also ensure that the supply voltage has an adequate value for the required output voltage swing. The circuit must also be capable of sensing and producing defined levels of input and output voltage. It’s the supply voltage that allows this.
Input and output impedance are particularly important in data transmission applications. Communication lines must have matched impedances to minimize reflected signals, which could otherwise destroy the transmitted data signal.
Challenges of Analog IC Design
Because so many metrics must be taken into account, analog circuits are still mostly created by hand. Digital circuit design is highly automated, making use of hardware description languages that can produce physical circuit layouts from code.
Furthermore, while transistor size has continuously decreased in accordance with Moore’s law, analog circuits have not been able to take advantage of these advanced nodes to the same degree as digital circuits. Today’s digital circuits are made at 3 nm, and are likely to become smaller still. Analog circuits, by contrast, are currently designed in the 65 nm to 28 nm range.
The shrinking of transistors causes high-order parasitic effects that are a plague on analog circuits, though they don’t hamper digital performance greatly. For this reason, it takes more time for analog ICs to go from one technology node to another.
We started off this article with a review of what qualifies an integrated circuit as ‘analog’, before moving on to describe ways in which analog circuit design is essential to the electronic systems of today and tomorrow. Finally, we examined the complex performance metrics used by analog circuit designers. Next time, we’ll move from a broad focus to a more specific one—the structure and operation of MOSFET transistors, which are essential to the function of modern analog ICs.