Increasing design complexity and time-to-market pressure is placing greater emphasis on simulation to confirm device selection and system-level design. Running a simulation can help engineers avoid fundamental design errors, confirm component selection, and reduce the prototype stage to a relatively easy process of design optimisation. A number of tools are available to designers to help accomplish this. These include online simulation tools provided by manufacturers, as well as new generations of cost-effective PC-based tools that take advantage of the Pentium-class computing power and large memory capacity residing on the majority of 21st-century design engineers' desktops.
The online tools available, usually free of charge, at manufacturers' websites provide considerable help with component selection and provide wide-ranging simulation capabilities. Some examples include the National Semiconductor WEBENCH' tool, the Texas Instruments' TINA-TI and NI Multisim Analog Devices' Edition from Analog Devices, which can usually be downloaded free of charge or run directly from the respective manufacturer's website.
Figure 1: Texas Instruments' TINA-TI online tool provides considerable help with component selection and wide-ranging simulation capabilities.
Fundamentally, these tools provide a graphical editing environment allowing engineers to build the schematic of a functional block such as a filter, amplifier, analog switch or power supply by selecting the manufacturer's key components as appropriate from a menu. Ancillary components such as resistors, capacitors or inductors can also be implemented, and standard values selected. The user interface provides a convenient environment in which designers can change component values and evaluate several alternative devices to optimise performance of the function.
Tools such as the National Semiconductor Webench, for example, provide well developed capabilities for power supply design and also support design and prototyping of signal-path functions by enabling engineers to select from recommended amplifiers and ADCs. In addition, designers can import ready-built reference, which not only saves drawing the circuit on screen but also eliminates numerous challenges when implementing commonly used functions by replacing circuit design with a relatively straightforward parameterisation task.
To help streamline development and ease challenges for designers, the tools can typically perform board-level autorouting for maximum space efficiency and generate CAD outputs such as a netlist or gerber file for direct import into downstream engineering tools. Other valuable aspects of the online design and simulation tools now available typically include the ability to generate output files such as an electronic BOM, and parts and kits can also be ordered automatically from the online versions. This can provide a fast and efficient route to completing a power supply design, for example, to meet a given set of input and output criteria.
Embedded analysis tools
Once a design has been entered, an engineer can take advantage of a number of embedded tools to analyse the circuit's behaviour and performance. Basic checking of error rules is a valuable first step, which automatically highlights circuit errors. This can save engineers many hours or even days at the hardware-debug stage, searching for obvious errors that stubbornly evade detection. By quickly eliminating potential hardware errors, engineers can move on swiftly to analyse the circuit using more sophisticated tools such as AC and DC analysis, transient analysis and noise performance. Depending on the type of function or circuit being designed, the engineer may wish to perform a number of DC sweeps by varying many applied voltage and current sources at the same time. Other facilities may include Monte Carlo analysis, which allows a number of parameters such as voltage levels and input-signal characteristics to be varied randomly within preset limits to highlight response to unpredictable operating conditions.
Depending on the application, engineers may also wish to verify noise performance over a range of operating temperatures. This may be particularly useful when designing automotive systems or other circuits destined for use in environments where operating temperatures may be particularly high or may vary widely. Where the types of components supported include power electronic devices, such as switching regulators, thermal analysis is frequently a priority. Features such as WebTHERM within the Webench suite allow engineers to acquire a graphical representation of the circuit's thermal performance and identify potential hotspots. This empowers engineers to optimise component selection, adjust operating limits or anticipate heatsink requirements, as appropriate.
Figure 1: National Semiconductor's WEBENCH online design tool.
A variety of virtual instruments are available to assess the simulation results. These may include tools such as an oscilloscope, AC/DC multimeter, function generator or X-Y recorder, implemented in software and configurable by the engineer to optimise parameters such as time and voltage range, and to set various thresholds and start/finish times for sampling.
To streamline design and accelerate completion of actual hardware, additional facilities include generation of test instructions and performance specifications, which help when verifying the performance of the target hardware against simulated results.
As far as learning how to use these online tools is concerned, some of these carry forward features and properties from existing online design assistance initiatives. For example, Analog Devices has preserved the look and feel of its amplifier parametric evaluation tool in the NI Multisim environment. This saves engineers having to learn a completely new set of tools and user interfaces.
The simulation capabilities of these tools take advantage of SPICE (Simulation Program with Integrated Circuit Emphasis) technology, utilising industry-standard simulation techniques and device models built using a common core syntax. SPICE was developed at the University of California at Berkeley, and has been universally adopted by the IC design community. Third-party tools developers have also sought to extend the capabilities of SPICE to create simulators for board-level and system-level designers. This has led to the emergence of a number of variations on the original SPICE proposals, including model formats such as PSPICE and MSPICE that are widely used within the CAE community, and the XSPICE simulator, which contains a number of extensions to enhance efficiency for system-level design.
This heritage provides advantages for engineers including access to the manufacturer's device-level knowledge acquired during the IC development program, since SPICE is the de facto standard simulation environment for IC design. The SPICE models available from manufacturer websites (or included with downloadable tools) are usually macromodels, optimised to produce accurate results for board-level or system-level design within typical desktop constraints on memory and processing power.
For more advanced capabilities, commercial simulation tools from specialist CAE and EDA companies incorporate large libraries of SPICE models from multiple IC manufacturers as well as independent developers of board-level IP and SPICE models. Engineers can use these to simulate a larger proportion of an entire system, such as a signal chain, comprising key components from a number of manufacturers. Engineers can also apply more advanced simulation and analysis capabilities. These can include support for IC programme scripting languages like VHDL, “smoke” analysis to verify that operating voltages and currents are within maximum device specifications, and MCU emulation tools to simulate the behaviour of peripherals such as ADCs or timers embedded in certain types of microcontrollers.
Importing models into the desktop environment may require some intervention from the engineer, particularly if using an older non-SPICE3 simulator. Fortunately, SPICE syntax is relatively straightforward. With only a little familiarity, engineers can adapt models to allow them to be smoothly imported into a desktop environment. Necessary modifications may be as simple as changing the filename extension.
Farnell's technical staff have made several studies into aspects such as SPICE model performance for a variety of commonly used components, as well as interoperability between formats such as SPICE3, XSPICE and PSPICE. They have also gathered information about other simulation packages and approaches such as I/O Buffer Information Specification (IBIS), Saber and Simulink. Each of these offers some unique features that may make it a more appropriate tool for a given application. This knowledge provides valuable assistance to help engineers make the most of sophisticated board-level simulation support that IC manufacturers are now offering electronic design engineers.
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