(P)SpiceITUp: The Power of Options in Managing Accuracy and Speed Using Relative and Absolute Tolerances

There is a powerful but usually overlooked feature of PSpice^® in the Options tab of the Simulations Settings dialog box. Although the default values provided in this tab take care of most situations, it is good to be aware of them for you never know when you might want to tinker with the values for that rare but not improbable instance. Anyway, tolerances that are controlled through many of these parameters are important in determining how accurate the results are, and since accuracy is your goal, knowing the nuances will help.

Let’s jump straight into the action with an easy example. If you briefly glance at the list of categories under Options, you will notice Auto Converge. The relaxed default values in this category will take care that your simulation converges. If you used a simulator from a decade ago, you will know that convergence failure was often an irritant that required knowledge of simulation options. Of course, modern solvers, like PSpice^® A/D, avoid convergence pitfalls by integrating intelligent heuristic decisions into their algorithms. But in the past, many simulations were rescued from non-convergence in most cases by just checking the AutoConverge option and opting to use the relaxed default values – just like the anti-brake system of modern cars that takes over in an extraordinary scenario. In a few rare cases, you might have to play around with the values just a bit. The option is there for you even now and it’s good to know in case you must use it.

To show the power of some of the parameters, in this post we will discuss the tolerance parameters in the General and Auto Converge categories of the Analog Simulation section, namely, RELTOL, VNTOL, and ABSTOL. While at it, do note the Restart option that is selected by default in the Auto Converge section, which pauses PSpice if convergence is not achieved at the end of the simulation time.

The tolerances are not unique to only PSpice or simulators, they are part of any problem requiring numerical methods of solutions involving reiterative steps that finally converge. For example, many data analysis applications will require ABSTOL and RELTOL. Let us then first try to understand what these tolerances exactly mean. Tolerance is the accuracy of each step in a simulation, meaning, it is the error that will be tolerated. 

In the case of PSpice as in other simulators, the following are the definition of the tolerances:

• RELTOL is the relative tolerance of voltage and current. It is the universal accuracy control. RELTOL directly affects the Newton convergence criteria and the time-step control algorithm. It specifies the upper limit on errors relative to the size of the signals present.
• ABSTOL is the current tolerance or the best accuracy of currents in a simulation run. It defines the smallest interesting current anywhere in the circuit. Currents smaller than ABSTOL are ignored when checking for convergence.
• VNTOL is the voltage tolerance or the best accuracy of voltages in a simulation run. It defines the smallest interesting voltage anywhere in the circuit. Voltages smaller than VNTOL are ignored when checking for convergence. 

VNTOL or ABSTOL supplements RELTOL and impacts the handling of very small voltages/currents by the simulator. These two values prevent the simulator from attempting to converge nano volt or pico ampere signals to pico volt or Femto ampere levels.

What do the tolerances mean when it comes to the accuracy and speed of a simulation? One given is that a very low tolerance will lead to a high level of accuracy but will inversely affect the speed of simulation. The good news is that PSpice comes with a set of default values that work most of the time. However, there are times when you want to relax the default values, say to speed up the simulation or, as stated previously, to ensure convergence. Although you can experiment with different values and achieve the optimum result through trial and error methods, here are some guidelines that you can use to maximize accuracy and speed:

RELTOL

Usually, you will run many iterations of simulations to fine-tune the design. Set RELTOL to a slightly higher value for the initial rounds to increase the speed. You can then change the RELTOL value to the default 0.001 for a more accurate solution.

ABSTOL and VNTOL

Reduce the accuracy of ABSTOL and VNTOL depending on the level of current and voltage, say in a power electronic circuit, using the following guide: Absolute tolerances, such as VNTOL, should be set 10E-6 times smaller than the largest signal of the same type present in the circuit. ABSTOL should be in the range of 1n to 1u for most practical circuits. A value of 1n is recommended for ABSTOL for most power switching circuits. This would have minimal impact on accuracy. You should increase VNTOL if the circuit is operating higher voltages, such as a few 100 volts or more.  

Scheduling and Changing Tolerance Values at Runtime

One thing that might interest you, now that you know the power of the tolerances, is that you can pause a simulation and change the tolerances along with a few other parameters in the Edit Runtime Settings dialog box.

You can also schedule to change these three parameters for a transient analysis while the simulation is running by using the SCHEDULE function in the .OPTIONS statement. Refer to PSpice A/D User Guide for more information scheduling as well as changing tolerances at the runtime.

Conclusion

There are many features in PSpice that give us that very important edge to increase productivity and accuracy. Many of these features escape our attention because the default settings are good for the maximum cases with which we deal. Knowing how they work and what can be done encourages us to experiment and enhance. The many parameters presented in the Options tab are one such feature. In this post, we touched a few important parameters related to tolerance. In future posts, we will discuss the other parameters. Watch this space for more in the coming days.

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