New SPICE Convergence Parameters Conquer High-Gain Feedback (Intusoft)

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Gardena, California - August 9, 2004 - Intusoft announced today a revolutionary technique for improving DC convergence in SPICE simulation. A new stepping algorithm (ICSTEP) has been designed that primarily targets circuitry employing analog feedback that uses cascaded high-gain amplifiers, notably with active regions that are offset.

Problem:
DC Convergence in SPICE simulators cannot be achieved for analog feedback circuits where high gain amplifiers are cascaded, and with active regions that are offset. These circuits work perfectly in practice, however, today's standard SPICE convergence algorithms tend to oscillate and find a solution only by chance. This is due to the chaotic nature of SPICE's numerical iteration process. In the real world, such circuits do not oscillate in the time domain because capacitors are used to dampen the oscillation in accordance with control system theory.

Intusoft's Solution:
A new convergence stepping algorithm was implemented by Larry Meares, president of Intusoft, to solve the above problem. In his algorithm, each capacitor that has an Initial Condition (IC)=xxx will have a conductor in parallel with a current source placed across it. The current (ceq) is the product of the IC voltage and the conductance (geq). The conductance starts off very high and is stepped down toward zero for each successful iteration. At the final iteration, ceq and geq are set to zero. Meares commented, "It's probably easier to visualize the algorithm's operation using a Thevenin Equivalent consisting of a resistor in series with a voltage source." He added that the Norton version, however, slips directly into the SPICE admittance matrix.

This kind of initialization is typically handled in SPICE using the .NODESET option to initialize node voltages. However, in the case of op-amps, it becomes necessary to initialize voltage at the op-amp inputs such that the difference between the inverting and non-inverting inputs, multiplied by the amplifier's DC gain, results in the correct output. If the DC gain is 1meg and the desired output accuracy is .1%, then one would have to input the 2 voltages with 9-digit precision. Even then, the DC gain would still not be precisely known!

Op-amps have low gain at high frequency in order to be free from oscillation. Capacitors are generally used to control high-frequency gain. Importantly, ICSTEP starts at the "final value" by shorting out critical capacitors, thus making the circuit gain very low. In addition, it starts with the right answer at the output and lets SPICE calculate the input voltages. If the op-amps are being used as integrators, all that is needed is to initialize the feedback capacitor. Then, ICSTEP gradually removes the forced initial condition, allowing SPICE to find the correct answer.

The newly assigned convergence parameter has been proven effective. Meares demonstrated his new algorithm using a coupled 2-stage feedback loop design employing high-gain amplifiers. The "SPICE-buster" converged in 6 iterations as opposed to failed convergence after 445 trials without the new convergence options. Meares also commented that the ICSTEP option can also be used to correctly initialize digital behavioral circuits for DC operating point and AC analyses.

Intusoft's ICAP/4 design simulation product suite features IsSpice4, a 4th generation enhancement of Berkeley SPICE 3F5 and XSPICE based technology. ICSTEP forms another addition to a collection of innovations unique to IsSpice4, including a VSECTOL option for optimizing DC convergence accuracy with a notable increase in speed.

Established in 1985, Intusoft designs, develops and markets award-winning software to electronics industries worldwide. Its products target a breadth of applications across all industrial, technological, military and consumer industries, by providing leading tools for analog and mixed-signal circuit design, verification and automated test synthesis. Information on Intusoft and its products is provided at: www.intusoft.com.

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