(P)SpiceItUp: Optimizing Parasitic Capacitance in a PCB Design Using PSpice Advanced Analysis

The impact of parasitic capacitance is the primary cause of concern for high-frequency PCB designs. Parasitic capacitance is formed between two elements when a charge builds up between them because of being placed too closely in a circuit. This unintended capacitance can be troublesome for high-speed designs as it can directly impact the performance of a design in the following ways:

• Introduce crosstalk that can cause interference
• Cause EMI Noise that corrupts the signal quality
• Affect signal integrity and generate unwanted noise for a circuit

For advanced circuit boards, you can’t get rid of parasitic capacitance altogether. However, you can reduce its impact on the performance of a design.

In this blog, you will learn how to optimize parasitic capacitance by running the Sensitivity and Optimizer analysis on a design in PSpice® Advanced Analysis Option (PSpice® A/D).

To ensure that your PCB design does not experience failures due to parasitic capacitance, you first run PSpice® Advanced Sensitivity analysis to identify the component parameters that are critical to the measurement goals of your PCB design performance. After recognizing the sensitive components, perform PSpice® Advanced Optimizer analysis to find the best component values for your design specifications.

Simulating Design in PSpice A/D

We’ll use an example to showcase the impact of parasitic capacitance on an operational amplifier design. Let’s start by simulating the following design in PSpice A/D:

PSpice A/D supports transient analysis that can simulate the timing behavior of the circuit. Run transient analysis on the design and then add measurements for different parameters and evaluate the characteristics of the waveform displayed in PSpice A/D.

To run transient analysis perform the following steps in Allegro® System Capture:

1. Choose PSpice – New Simulation Profile.

2. In the Analysis tab of the Simulation Settings dialog box, select Time Domain (Transient) as the Analysis type.

3. Choose PSpice – Run.

To measure the maximum and minimum values of the circuit output voltage, add Max(V(out)) and Min(V(out)) as the measurement values in the Results window.

The values shown are not the expected results for Max(V(out)) and Min(V(out)). Let's try to optimize the parasitic capacitance to get results closer to the ideal output voltage.

The original value of the added measurement gives you a fair idea of what needs to be the ideal output voltage for your circuit. For the design used in this example, the ideal output voltage needs to be +/- 1.5V, approximately.

Now run Sensitivity analysis to identify the components that have the maximum effect on the yield. In the analysis results, note that the parasitic capacitance is included in the analysis.

• To perform sensitivity analysis, choose PSpice – Advanced Analysis – Sensitivity.

Now that you have identified the sensitive components run Optimizer analysis to tune the circuit to meet the precise requirements of the PCB design.

• To run Optimizer analysis for the design, choose PSpice – Advanced Analysis – Optimizer.

Note that the Optimizer analysis result also includes the parasitic capacitance.

Modify the operation amplifier design with the feedback resistor values and the parasitic capacitance values returned by Optimizer.

Run the transient simulation again. The transient analysis results show that Max(V(out)) and Min(V(out)) are now closer to the ideal output voltage.

This shows that keeping the parasitic capacitance under check is critical to high-frequency PCB designs.

Conclusion

PSpice Advanced Analysis options maximize design performance, reliability, and cost-effectiveness. Using PSpice Sensitivity Analysis Option along with PSpice A/D, Optimizer improves the performance of high-frequency PCB designs by providing the best values, including parasitic capacitance, for the identified sensitive components. This ensures that the design parameters are tuned for the best performance possible, and that the final product works as expected in the real world.

For more information on PSpice simulations, check out the articles, tutorials, and videos available on pspice.com

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About (P)SpiceItUp

The (P)SpiceItUp series provides solutions to analog and mixed-signal simulation and analysis-related tasks performed using PSpice A/D or PSpice Advanced Analysis. Our vision, as reflected by the logo and the title of this series, is to make the critical simulation and analysis activities easy and enjoyable. Our regular new blog posts cover every aspect of analysis and simulation, including modeling and schematic design preparation.