Tools for design and teaching analogue integrated circuits

Date:	22, May 2014
Authors:	Jacky Porte
Contact:	<Jacky.Porte@lip6.fr>
Version:	12 February 2024 (mml)

Contents

Analogue Design

Analogue Design is compulsory

While in most electronic systems, signal processing is carried out digitally by a digital processor (DSP), mainly for reasons of precision and noise immunity, analogue nature and continuous time of the physical signals to be processed, makes it impossible to completely eliminate the analogue part. Interfacing circuits are therefore required. Thus, in an environment physically heterogeneous, the signal to be processed must first be handled by a sensor which will transform acoustic (microphone), light (cell), speed (accelerometer), electromagnetic (antenna), ..., information into an electrical signal. Then, before going through an analogue to digital (ADC) converter, the electrical signal must be processed by an analogue processor (ASP) which will adapt the signal to be compatible with the characteristics of the converter thanks to analogue continuous-time or discrete-time operations such as transduction, filtering, amplification, sampling-blocking, extrapolation, etc.

If we consider the generic signal processing chain of the figure below, all of these elementary operations, with a possible digital preprocessing (extrapolation, filtering, gain control, etc.) constitutes the signal acquisition chain.

On the other hand, the digitally processed signal must be restored in an analogue continuous-time form to drive the immediate physical environment. So, after digital to analogue conversion and conformation by an analogue processor performing the continuous-time and discrete-time operations similar (or dual) to those carried out in acquisition (interpolation, filtering, transduction, etc.), the adapted signal can be applied to the actuator (earphone, camera, control unit, etc.).

When the integration of analogue and digital (AMS) functions is on the same substrate, the AMS "single-chip" constitutes an ASIC (Application-Specific Integrated Circuits) called AMS SoC (Analogue and Mixed Signal System on Chip).

Analogue EDA is compulsory

Today, the complexity of integrated circuits is still increasing, resulting from the always decreasing dimensions of the CMOS transistors, the evolution of technological processes, the strong demand for new features integrating numerous analogue functions (radio-communication, etc.) and above all the economic pressure which increasingly shortens the deadlines for time-to-market product.

However, unlike the digital domain where the use of standard cell libraries and Electronic Design Automation software (EDA) decreases the design time, in the analogue domain, it is practically impossible to store all the functions matching the entire possible spectrum of applications with all associated electrical specifications. On the other hand, taking into account the fact that such library would very quickly become obsolete due to the rapid technology change, it is obvious, that without the help of appropriate CAD tools, allowing the automatic generation of parameterized modules, in particular by technology, analogue part of the AMS SoC is becoming the bottleneck in terms of design time and cost.

Software are compulsory to teach Analogue Design

Analogue micro-electronics is an applied science requiring multi-disciplinary background: solid-state physics, theory of electrical networks, mathematics, etc. This is why it is interesting, but also difficult. Thus, becoming an analogue designer implies an accumulation of knowledge difficult to acquire in a straightforward manner during a "traditional" school curriculum. Self-training using dedicated tutorials is certainly an essential way to deepen one's knowledge, and possibly fill one's gaps.

Specific features of Analogue Design

The art of trade-offs.

Analogue design is by nature "the art of making trade-offs", to handle a large number of constraints most of the time conflicting ones. It is thus commonly accepted that sizing high-performance analogue devices is not a purely algorithmic operation but it requires the use of heuristics, i.e. a set of rules based on expertise.
Multiple toplogies (or circuit structures).

For implementing the same function, there are very often several circuit topologies likely to meet the set of explicitly provided quantitative specifications in an optimal way, and in a more or less suboptimal way, some other qualitative performance indicators.
Models are everywhere.

To meet circuit's specifications at system level implies a translation of these specifications in terms of performance indicators at each functional block level, which in turn must be translated into performance indicators at each elementary function, then, at physical level depending on the target technological process, and, last but not least, at each primitive level. The design verification requires numerous dedicated simulations using a large range of models, from functional to physical level, going through different behavioral levels. On the other hand, analogue design involves a precise modeling of the different active and passive components, as well as good knowledge of their behavior based on different types of stimuli (temperature, large signal, small signal, noise, etc.) and possible idiosyncrasies related to modeling problems (discontinuity of certain models, etc.).
Statistical characterization.

To design an industrial-grade analogue integrated circuit, you must not only size the different elementary components in order to fulfil the nominal target specifications, but it must also be ensured that these specifications are met taking into account the inevitable dispersions on the manufacturing tolerances affecting the components. Analogue design is a statistical process which involves centering performance indicators in order to guarantee optimal production output.
Reuse.

An analogue circuit behaviour depends on the technology process used, thus, the expertise accumulated during the design of an analogue device must, as much as possible, directly be reusable in case of technological process migration or during the design of a similar circuit.

Oceane software

To meet the specific features of the integrated analogue design, OCEANE (Tools for design and teaching analogue integrated circuits) is designed as a "toolbox" consisting of top-level tools called compilers addressing devices of the same hierarchical level and nature and environments addressing the tools specific to the realization of a functional block. It is designed to take as input a set of high-level specifications for a function block or elementary function, and to output a netlist in standard format SPICE, corresponding to a topological description of the primitive components, such as transistors, resistors, capacitors and inductors, with their sizes and their layout style settings for the generation of the corresponding layout. Interfacing with other tools (simulators, automatic layout generators) is thus directly feasible.

To download Oceane with online English documentation (and also French pdf), send a mail to:

marie-minerve.louerat (at) lip6.fr