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Apache spices up analog circuit simulator

Posted: 13 Nov 2002 ?? ?Print Version ?Bookmark and Share

Keywords:apache design solutions? nspice? spice ciruict simulator? analog circuit simulator? hspice?

Apache Design Solutions is introducing a new version of the popular Spice circuit simulator as it seeks to broaden its share of the analog circuit simulator business. NSpice - written from scratch in C++ - is netlist- and model-compatible to the HSpice device modeling tool, Apache claimed, but with dramatic improvements in capacity, speed, and convergence.

The tool approaches Nassda's Hsim in capacity and HSpice for accuracy, said Keith Mueller, Apache's VP for sales and marketing. The simulator can model more than a million elements in one run.

It effectively "bridges" simulation between chips, and packages, boards and backplanes - allowing a variety of nonlinear and high-frequency elements to be simulated in one run, he said. S-parameters can be read directly, for example, without approximation. "We are more accurate than HSpice on high-speed I/O," Mueller said.

Since its founding, the company has been involved with circuit simulation. Apache CEO Andrew Yang founded Anagram and built the ADM Spice-like simulator. CTO Shen Lin and investment partner Norman Chang worked on inductance and parasitic extraction at HP Labs.

While Apache's recently-introduced "Tomahawk-S" power analyzer looks at steady-state conditions, the focus of NSpice is entirely "dynamic" issues, Mueller said. System behaviors that depend on frequency shifts - such as S-parameters, energy dissipation patterns that show up in high-speed backplane simulations - can be simulated with NSpice. "S-parameters are traditionally more accurate than lumped RLC models," Mueller insisted. "This appeals to 'systems guys' like Cisco."

Convergence issues

NSpice claims to avoid the convergence issues of Spice, a curve tracing routine that can force a simulation to take an excessive amount of run-time. Lack of convergence need not necessarily be a deficiency of Spice, reminded Yu Lin, Apache's director for simulation products.

Spice will fail to converge with (1) faulty input data [such as a bad model parameter, like a current source with an unusually large resistor]; with (2) inappropriate initial values; or with (3) incorrect usage of Spice node definitions [like a zero-value resistor]. On bias simulations, for example, all nodes should start from zero, for repeated iterations to promote proper convergence.

But his NSpice product, Lin suggested, faster convergence for otherwise nonlinear models and transfer functions. "We have a better time-step control algorithms," he insisted.

Spice modeling typically assumes good circuit partitioning with perfect Vcc and grounds, he said. And clock tree networks are typically modeled without the effects of inductance and capacitance. BSIM3 models yield good results for normal (low-frequency) operating regions. "If you are simulating a limited span of frequencies, you are okay," said Lin. "But, over a wide span, you will have a lot of difficulty."

NSpice, in contrast, simulates circuit behavior at high frequencies. It easily generates eye diagrams for communications systems analysis, Mueller said.

Apache offers two versions of the NSpice simulator: One with limited capacity for Windows and another with higher capacity for Unix. A one-year license for Unix is priced at $12,000. The software is written on Linux but then ported to Sun, HP, and Windows platforms.

- Stephan Ohr

EE Times





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