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An example of the kind ot PCB components overlay which can be produced using OrCAD/SDT, printed on an Epson FX80. (Courtesy Prometheus Software)

at Berkeley, and will also accept net It’s also interesting that bccause of design tools are now virtually essential lists produced by other CAD packages.    the Circuit design complications pro-    for the design of most ICs. The ICs

Both OrCAD/VST and PSpice are    duced by parasitic components and    themselves are becoming morę and

availabic from Prometheus Software    other solid-state electronics effects    morę complex, while commercial pres-

Devclopments, in Melbourne.    within an IC chip, designing even a    sures are making it necessary for design-

Perhaps the other main application of    “digital” chip tends to involve a lot of    ers to produce them in proportionally

CAD in electronics, apart from Circuit    work using analog design tools. So the    shorter times. It just wouldn¹t bc possi-

and PCB design/drafting, is in IC chip    designer of a new logie chip often has a    ble to meet these requirements without

design. In many ways this is a similar    need to use an analog Circuit simulation    the powerful help providcd by CAD.

I    kind of application, with the products tool like SPICE, to look at the perform- For simple one-off or short run Circuit

just “scaled down”'a couple of orders ance likely to be obtained. When you boards, or knocking out the odd Circuit _n    of magnitude. From a basie design point    get down to the IC chip level, the dif-    schematic, traditional manuał tech-

\    of view, an IC is essentially a very smali    ferences between analog and digital    niqucs are still probably the best ap-

PCB and set of components, all inte- tend to become very blurred!    proach. But in industry, CAD design

grated inside a chip of Silicon. Of Generally it’s true to say that CAD tools are now almost essential. <2> course it’s not quite that simple. Be-cause the components are integrated to-gether inside the Silicon, there are all sorts of possible interactions and design complications. In fact almost every monolithic circuit component — i whether it’s an “active” part like a tran-j    sistor or FET, or a nominally “passive”

part like a resistor — tends to bc una¹ voidably accompanicd by at least one additional parasitic component (usually another transistor). These parasitic com-, ponents are inherent in a monolithic ICs structure, and can have a crucial effect on circuit operation.

So designing an IC does tend to be rather morę complex than designing a discrcte circuit or PCB, and to involve a great deal morę cxpcrtise. In practice this tends to mean that CAD systems for IC design have to be rather morę powerful, and need to run on large mini and mainframe computers. Because of the vcry Fine detail required, it is also

necessary to use extremely high resolu-    The display produced by OrCAD/VST, the new digital simulation package

tion colour graphics displays and plot-    which allows you to “breadboardⁿ and check the operation of a logie circuit

t_ers    before it is actually built. (Courtesy Prometheus Software)

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   •    ELECTRONICS Australia. September 1987    101