Page 275 - The Mechatronics Handbook

process, with the lowest level being detailed transistor models and the highest a VHDL description of a
half adder.

13.3 Analog and Mixed-Signal Circuit Development: Modeling
and Simulating Systems with Micro- (or Nano-) Scale
Feature Sizes and Mixed Digital (Discrete) and Analog
(Continuous) Input, Output, and Signals

At the lowest level, digital circuits are in fact analog devices. A CMOS inverter, for example, does not
“switch” instantaneously from a voltage level representing binary 0 to a voltage level representing binary
1. However, by careful design of the inverter’s physical structures, it is possible to make the switching
time from the range of voltage outputs which are considered to be “0” to the range considered to be “1”
(or vice versa) acceptably short. In MOSFETs, for example, the two discrete signals of interest can be
identified with the transistor, modeled as a switch, being “open” or “closed,” and the “switching” from one
state to another can be ignored except at the very lowest levels of abstraction. In much design and simulation
work, the analog aspects of the digital circuit’s behavior can thus be ignored. Only at the lower levels of
abstraction will the analog properties of VLSI devices or the quantum effects occurring, e.g., in a MOSFET
need to be explicitly taken into account, ideally by powerful automated development tools supported by
detailed models. At higher levels this behavior can be encapsulated and expressed in terms of minimum
and maximum switching times with respect to a given capacitive load and given voltage levels. Even in
digital systems, however, as submicron feature sizes become more common, more attention must be paid
to analog effects. For example, at small feature sizes, wire delay due to RC effects and crosstalk in nearby
wires become more significant factors in obtaining good simulation results [20]. It is instructive to examine
how simulation support for digital systems can be extended to account for these factors.
Typically, analog circuit devices are much more likely to be “hand-crafted” than digital devices. SPICE
and SPICE-like simulations are commonly used to measure performance at the level of transistors, resistors,
capacitors, and inductors. For example, due to the growing importance of wireless and mobile computing,
a great deal of work in analog design is currently addressing the question of how to produce circuits
(digital, analog, and mixed-signal) that are “low-power,” and simulations for devices to be used in these
circuits are typically carried out at the SPICE level. Unless a new physical technology is to be employed,
the simulations will mostly rely on the commonly available models for transistors, transmission lines, etc.,
thus encapsulating the lowest level behaviors.
Let us examine the factors given above for the success of digital system simulation and development
to see how the analog domain compares. We assume a development cycle similar to that shown in Fig. 13.1.
• Is there a small set of basic circuit elements? In the analog domain it is possible to identify sets of
components, such as current mirrors, op-amps, etc. However, there is no “universal” gate or small
set of gates from which all other devices can be made, as is true in the digital domain. Another
complicating factor is that elementary analog circuit elements are usually defined in terms of
physical performance. There is no clean notion of 0/1 behavior. Because analog signals are con-
tinuous, it is often much more difficult to untangle complex circuit behaviors and to carry out
meaningful simulations where clean parameter separations give clear results. Once a preliminary
analog device or circuit design has been developed, the process of using simulations to decide on
exact parameter values is known as “exploring the design space.” This process necessarily exhibits
high computational complexity. Often heuristic methods such as simulated annealing, neural nets,
or a genetic algorithm can be used to perform the necessary search efficiently [21].
• Is there a small set of well-understood technologies? In this area, the analog and mixed signal
domain is similar to the digital domain. Much analog development activity focuses on a few
standard and well-parameterized technologies. In general, analog devices are much more sensi-
tive to variations in process parameters, and this must be accounted for in analog simulation.

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