What is Analog Design?

Definition

Analog design in the context of integrated circuit (IC) design is a discipline that focuses on the creation of circuits that operate in and are optimized for continuous time-domain behavior. Typical objectives of analog design include: 

• Signal fidelity
• Amplification
• Filtering

When the term “integrated circuit design” is mentioned, most people think of the design of complex microprocessors. These circuits are designed using digital design techniques, which focus on the propagation of discrete values, i.e., “ones and zeros.” It’s important to understand that this model of propagating “ones and zeros” is used to simplify the analysis of huge networks. The actual devices in any circuit are responding to continuously varying stimulus, so analog circuit design is really the foundation of the design of digital circuits. 

Importance of Analog Design

Since all the basic devices in an IC respond to continuous time stimulus, analog design forms the foundation for all IC design. Modern IC technology presents many design challenges. There is significant variability in the manufacturing process for advanced technology nodes. The actual operation of the high number of devices on advanced ICs also causes variability. This variability manifests as changes in operating voltage, operating temperature, and  in performance. Densely packed devices can also interact with each other and with the silicon substrate, package, and board to cause signal distortions. All of these effects can occur between devices and within a single IC  as well.

Analog design must compensate for all of these effects to ensure three basic qualities: fidelity/precision, consistency, and performance. Reliability analysis and signal integrity analysis are some of the activities that are used to model and mitigate these effects. Examples of the importance of these three items with regard to IC applications are as follows:

• Fidelity/precision. Many analog designs form the foundation for circuits that sense the external conditions of an IC. Sensing ambient temperature, air pressure, motion, and light are fundamental parts of many internet of things (IoT) devices. Sensing light forms the basis of machine vision, for example. Accurately sensing these continuous time effects requires precise measurements, which translates into the need for excellent fidelity and precision for the analog circuit performing these measurements. Analog design ensures fidelity/precision.
• Consistency. Digital circuit design models the propagation of discrete “ones and zeros” to simplify the analysis of large numbers of devices. A “one” is typically the primary supply voltage and a “zero” is the absence of any voltage. For this model to work effectively, the performance of the circuit elements must be consistent across all the previously mentioned variability conditions. Consistency ensures voltages are at one of the reference levels of “one” or “zero.” Analog design ensures these conditions are met.
• Performance. This parameter takes two basic forms: speed and power. All ICs have stringent requirements for speed to meet overall system throughput. They must also stay within a particular range of power dissipation. This item is driven by the need for systems to remain in acceptable thermal envelopes to ensure effective heat dissipation. The need for lower power is also driven by financial operating constraints. Analog design ensures power and speed are within acceptable bounds.

Analog Design vs. Digital Design

The primary difference between analog design and digital design is the type of underlying analysis that is used.

In analog design, circuit stimulus is treated as a continuously varying signal over time. The behavior of the circuit is modeled in the time and frequency domains with attention focused on the fidelity/precision, consistency, and performance of the resultant waveforms. Circuit variability, both manufacturing and design induced, must be modeled and compensated for as well.

Digital design treats circuit stimulus as a series of discrete logic “ones” and logic “zeros” over time. A logic “one” is typically represented by the presence of the supply voltage for the IC and a logic “zero” is represented by the absence of this voltage (i.e., zero volts). The devices in digital circuits must spend most of their time at either logic “one” or logic “zero”. As long as the circuits processing these signals are consistent in their response to these logic levels, digital design works well. Analog design is responsible to deliver these qualities.

This allows the behavior of the circuit to be analyzed using combinatorial and sequential models, only considering two voltages (“one” and “zero”), which substantially simplifies the design and verification process.

How to design analog circuits

Analog IC design typically involves a top-down design and implementation process followed by a bottom-up verification process. There are many variations on this overall approach. Here are the basic steps:

• Develop a high-level specification for the design. What functions will it perform?  What are the performance, power, and area (i.e., cost) targets for the design?
• Develop a top-level design to achieve the required results using macro-functions such as filters, comparators, and amplifiers
• Create the device-level circuit descriptions to support the top-level design using components such as transistors, resistors, and capacitors. This step often draws from a library of pre-defined functions which will need to be customized for the specific requirements of each unique design.
• Verify that the design delivers on all its specifications using simulation. The software used here will typically model the circuit using linear and non-linear elements that have been tuned for the target fabrication process. It is during this step that manufacturing process and operational variability will be modeled to ensure the device design remains robust in the face of these uncertainties.
• Implement a physical layout of the design by assembling the pre-defined layouts of all components. During this step, the density of the layout is optimized to minimize cost. There are many placement rules that must be followed to ensure that the design is optimized for manufacturability and signal integrity. Validation that these rules are followed occurs during this step, which is called physical verification.
• The equivalent circuit is then extracted from the layout. Parasitic effects such as crosstalk and wiring resistance are now present in the design description, and the design is re-simulated to ensure it still operates as intended with these new effects added.  The extracted design is also compared to the original design to ensure the correct devices were used and connected correctly. This process is called logic versus schematic, or LVS checking.
• Any structures required for testing the circuit are added during this phase as well. Once complete, the design is ready for either manufacturing or integration into a larger digital design. Integrating analog designs into a larger digital design is referred to as AMS, or analog/mixed signal design.