Why You Need an Integrated Circuit Simulator

Integrated Circuit Simulation, Your Second-Look Assistant

I talk to electronics designers from all different stages of careers and skill levels. For over a decade, my involvement with EDA marketing and support has given me the opportunity to discuss designs of all types with seasoned professional PCB designers, fledgling engineers, PCB design specialists, university students, and even hobbyists.

On the subject of nonlinear circuits, it’s apparent to me that designers rely heavily on simulation software during the learning years of their careers, but tend to fall out of the simulation habit as their careers progress. You might say, “well, duh.”

In spite of this, there are good reasons not to forget or neglect to use integrated circuit simulation data. Let’s take a look at some here:

Save Time on PCB Prototyping

Typically, not every part of a design can be simulated. Moreover, in modern mixed-signal designs with large microcontrollers or digital sections mostly integrated circuit software-driven, the configuration of the test stimulus is not practical.

However, there are always a few smaller critical elements of the design - usually in the analog realm - where you need to make sure your circuit components will work. If you simulate just those elements of the design which involve analog signals and power, or which perform some new function purely in hardware, it’s going to save a lot of time simply because you can get a quick answer if the essential logic or analog behavior in your subcircuit does what you intend.

Save Money on Components and PCB Spins

Apart from the axiom, that time is money, using the integrated circuit simulator for testing and “virtual prototyping” will definitely save money in wasted components and PCB spins. That’s a given. 

One reason many engineers give up on simulating stuff is that they think they have to use a different integrated circuit software tool for simulation than they do for schematic design. This trap is easy to fall into because there are a number of “free” simulation tools available for download from semiconductor manufacturers, such as LTSpice and TI-Tina. But it’s a big hassle having to redraw everything in two different places, and typically, dedicated SPICE simulation tools come with lightweight schematic editors, which are not very good for PCB design.

Having a professional schematic design with unified component models means you can add SPICE, XSPICE, Digital Simcode (for mixed mode), and PSPICE models directly into the components in your design. Then, the SPICE simulation is ready to go from the same schematic editor - no need to redraw anything.

So now you know, by Integrated Circuit Simulation, I mean an integrated circuit simulator. But you might also be thinking of simulating ICs. ;-)

Take a Second Look, Friend

The esoteric nature of electronics can lead you to believe an integrated circuit simulator will behave one way when in reality it’s different, and before you go to the trouble of building it, simulation can be a quick sanity check.

When I am designing analog circuits in particular (usually audio related; I am a musician and an engineer after all), I like to use the simulator to check:

• DC biases and conditions around the circuit (particularly needed with discrete transistor and JFET-based designs).

• Frequency responses (A.K.A. AC small signal analysis).

• Clipping levels and non-linearity characteristics (using transient analysis).

• Device power dissipation, voltages, and currents (DC analysis and Transient).

• The effects of component manufacturing variances (monte-carlo analysis).

More than anything, running these allows me to quickly see if something doesn’t work as I had intended. If the simulator provides me with an unexpected result, it prompts me to check everything and look for circuit design errors, incorrect component values, or bad part choices.

This may seem obvious - but it’s worth discussing. Many people, once they’ve gained more experience, see simulators as a learning tool they used in school. But really, they are here to help with all designs.

Designing with Simulation

Integrated Circuit simulation is a very powerful tool for design as well. Not just for testing your theoretically calculated component values, but also for trying out an array of “what-if” scenarios.

For example, let’s say I’m designing a second-order high-pass filter for a speaker crossover:

I want to figure out the best range of resistor and capacitor values to use while maintaining a Butterworth (maximally flat) pass-band characteristic, and enabling user-selectable cutoff frequency with a switch. I can use Global Parameters in the XSPICE simulation setup section within CircuitStudio^® to create multiple parameter sweeps with component values calculated from the Butterworth formulas:

From here, using the Parameter Sweep option allows me to vary the global parameters CUTOFF_FREQ and DAMING_COEFF which the component values for Butterworth response are calculated from. The result is a series of response curves:

My personal preference for the waveform editor is to have a dark background. I used the document options to swap the foreground and background colors. I also formatted the waveforms to display magnitude in dB and phase in degrees. Once I apply these settings to the document and save it, they are stored perpetually; thus I can perform any number of simulations with different variations and it remembers my preferences for the display.

Taking this parameter sweep and Global Parameters capability a step further helped me analyze and “virtually prototype” the classic “Boogie” graphic equalizer. In this case, I have a multi-channel design with an inductor-capacitor series resonant used for each of the filter frequency bands between 32Hz and 16KHz. I wanted to simulate the effect of varying each gain potentiometer between flat, full cut, and full boost. Each position for each band is simulated with the potentiometer positions adjusted as Global Parameter formulas:

In this case, I have a bunch of PSPICE-style “IF” statements. When the Parameter Sweep varies the “POTS” parameter, it causes each of the potentiometer resistance values at different specific points to represent the extreme and center positions in turn. The resulting AC analysis is shown below:

Now I can go ahead and prototype this equalizer build knowing that it will sound awesome with my electric guitar.

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