PCB Design Blogs

After completing a circuit, it’s time to run simulations. The first step is to define a simulation profile. A simulation profile controls which analysis is run and what resources are to be used, for example, the models which define the parts for the simulators. The simulator application takes your circuit and the simulation profile, and gives you the results. You can set up different profiles and test the same circuit for different environments.

In this post, we’ll go through the steps and related concepts for creating a simulation profile in PSpice^® A/D.

To create a simulation profile, simply choose PSpice – New Simulation Profile and then go on to give the profile a name. A meaningful profile name helps you recall what you have in the profile, such as trans if you are creating a profile for Transient analysis.

Now comes the second part, deciding the analysis type you want to run and why. Let’s take a look at the steps involved in ‘simulating’ the worst possible scenario in the circuit design process where you don’t know what you want (well, if you say that's the worst-case scenario for anything, I will agree).

Before moving ahead, I do want to make one thing clear. Most simulators, including PSpice, are mathematical tools. It is a result of the hard work of many developers and UX experts that we have a simple mechanism to perform some of the most complex tasks on the planet. But you can always use netlist and simulation files instead of the easier GUI method taken in this post. We will cover the file way of doing things in another post.

Step #1: Choosing the Right Analysis Type

The moment the new profile dialog appears, you will notice that Time Domain (Transient) analysis is selected by default.

If you are not sure of what analysis to perform, here is an overview to help you decide.

• Time Domain (Transient): Go for this if you want to track voltages, currents, and digital states over time.
• DC Sweep: Go for this if you want to calculate the bias point of a circuit or to sweep DC values by simulating the circuit many times.
• AC Sweep/Noise: Go for this if you are interested in a small-signal response of the circuit (linearized around the bias point) when sweeping one or more sources over a range of frequencies.
• Bias Point: Go for this if you want node voltages and currents through the devices in the circuit.

Depending on the analysis type you select, several options and parameters come into play. The default selection of options is usually good enough but when needed you can benefit from the added power of additional analysis options, such as running Monte Carlo analysis to determine yield. However, in this post, we will stay with the defaults. The options and alternatives, will be covered in future posts.

So, let’s recap the defaults. Analysis Type as Time Domain (Transient), and Run To Time is set at 100 microseconds - u standing for micro. This is the time for which the simulation will run. Also, notice that simulation data gets saved from the moment the simulation starts as the value is 0 for Start saving data after. In other words, a transient analysis will run for 100 microseconds and data will be saved as soon as the simulation starts, meaning, the resulting waveform will start from 0 time.

Step #2: Configuring Files for Simulation

When you select Configuration Files, you are presented with options under three categories: Stimulus, Library, and Include.

Selecting Stimulus lets you add analog or digital input signals or stimuli for use in simulation.

Selecting Library lets you add the libraries containing the PSpice models. To ensure that correct files are found and used during the simulation, verify the library paths set here. 

You can then select Include and add any PSpice commands that you want to be loaded before the circuit analysis.

Again, for now, just ensure that the Library is configured correctly and accept the defaults for the other two, namely, Stimulus and Include.

Step #3: Fine-tuning Options

When you select Options, you are presented with various selections under four categories. These options let you fine-tune your simulations. For example, you can specify default values for drain area, source area, length, and width for MOSFETs. Again, you will most often use the default values supplied with most of the parameters. However, you can always try out different combinations of values.

Step #4: Optimizing Simulation Data

In Data Collection, restrict the simulation data you capture or set the accuracy of data. For example, collect voltages only where a marker is specified instead of the default - All but Internal Subcircuits. In most cases, the defaults will work for your circuit.

Step #5: Setting Up Result Display

Probe Window lets you set up how you view the results. The options are self-explanatory so you can pick the ones that you require. For example, although the default is to display the probe window only when the simulation has completed running, what stops you from keeping the probe window open during simulation by choosing during Simulation option? Well, there is the option to do it if you want!

You are now all set to run the simulation, by choosing PSpice – Run. In a future post, we will see how to interpret the simulation results using the powerful Markers feature of PSpice. 

Conclusion

In this post, we saw how to set up a simulation profile. Creating a simulation profile is the first and most important thing to do to simulate a circuit. To keep things simple, we have stayed on the straight and narrow of the defaults. We will discuss more about simulation profiles, such as parts of a profile, output and input files, in our future posts.