Introduction to Series Termination Resistance

Series termination resistance is a crucial concept in high-speed PCB design. It helps to minimize signal reflections, reduce electromagnetic interference (EMI), and improve signal integrity. In this article, we will explore how to calculate series termination resistance values using Altium Designer, a powerful PCB design software.

What is Series Termination Resistance?

Series termination resistance is a technique used to match the impedance of a transmission line to the source impedance of a driver. By adding a resistor in series with the driver output, the effective source impedance can be increased to match the characteristic impedance of the transmission line. This matching helps to minimize signal reflections and ensure proper signal transmission.

Why is Series Termination Resistance Important?

When a signal travels along a transmission line, it encounters impedance discontinuities at various points, such as connectors, vias, and component pins. These discontinuities cause a portion of the signal to be reflected back towards the source, leading to signal integrity issues like ringing, overshoot, and undershoot. Series termination resistance helps to mitigate these problems by reducing the magnitude of the reflected signals.

Calculating Series Termination Resistance Values

To calculate the appropriate series termination resistance value, you need to consider several factors, including the driver output impedance, the characteristic impedance of the transmission line, and the desired signal rise time.

Driver Output Impedance

The driver output impedance is an important parameter in determining the series termination resistance value. It represents the internal resistance of the driver circuit. The driver output impedance can be found in the device datasheet or obtained from the manufacturer.

Characteristic Impedance of the Transmission Line

The characteristic impedance of a transmission line depends on its physical properties, such as the width and thickness of the trace, the dielectric constant of the substrate, and the distance between the trace and the reference plane. In Altium Designer, you can calculate the characteristic impedance using the built-in impedance calculator.

To access the impedance calculator in Altium Designer:

1. Open your PCB project.
2. Go to the “Tools” menu and select “Impedance Calculator.”
3. Enter the relevant parameters, such as trace width, trace thickness, dielectric constant, and trace-to-plane distance.
4. Click “Calculate” to obtain the characteristic impedance value.

Desired Signal Rise Time

The desired signal rise time is another factor to consider when calculating the series termination resistance value. The rise time determines how quickly the signal transitions from a low level to a high level. Faster rise times require lower series termination resistance values to maintain signal integrity.

Calculation Formula

The series termination resistance value can be calculated using the following formula:

R_term = Z_0 - R_driver

Where:
– R_term is the series termination resistance value
– Z_0 is the characteristic impedance of the transmission line
– R_driver is the driver output impedance

For example, if the characteristic impedance of the transmission line is 50 ohms and the driver output impedance is 10 ohms, the series termination resistance value would be:

R_term = 50 ohms - 10 ohms = 40 ohms

Implementing Series Termination Resistance in Altium Designer

Once you have calculated the appropriate series termination resistance value, you can implement it in your PCB design using Altium Designer.

Adding a Series termination resistor

To add a series termination resistor to your design:

1. Open your schematic in Altium Designer.
2. Place a resistor component near the driver output pin.
3. Connect one end of the resistor to the driver output pin and the other end to the transmission line.
4. Set the resistor value to the calculated series termination resistance value.

Simulating the Design

After adding the series termination resistor, it’s a good practice to simulate your design to verify its performance. Altium Designer provides simulation tools that allow you to analyze signal integrity and identify potential issues.

To simulate your design in Altium Designer:

1. Create a new simulation profile by going to the “Simulate” menu and selecting “Create Simulation Profile.”
2. Configure the simulation settings, such as the stimulus type, stimulus parameters, and analysis type.
3. Run the simulation and examine the results, such as signal waveforms and eye diagrams.
4. Make adjustments to your design if necessary based on the simulation results.

Best Practices for Series Termination Resistance

When implementing series termination resistance in your PCB design, consider the following best practices:

Keep Trace Lengths Short

Minimize the length of the traces between the driver and the series termination resistor. Long traces can introduce additional impedance discontinuities and degrade signal integrity.

Place Resistors Close to the Driver

Position the series termination resistor as close to the driver output pin as possible. This helps to minimize the stub length and reduce the impact of reflections.

Use Appropriate Resistor Packages

Select resistor packages that are suitable for high-speed designs, such as small outline packages (SOT) or chip resistors. These packages have low parasitic inductance and capacitance, which helps to maintain signal integrity.

Consider the Power Dissipation

Series termination resistors dissipate power when the driver is in the high state. Ensure that the chosen resistor has an adequate power rating to handle the expected current and voltage levels.

Validate the Design through Testing

After implementing series termination resistance, it’s crucial to validate your design through physical testing. Perform signal integrity measurements using oscilloscopes or network analyzers to ensure that the signals meet the required specifications.

Frequently Asked Questions (FAQ)

1. Q: What happens if I don’t use series termination resistance in my high-speed design?
   A: Without series termination resistance, signal reflections can occur at impedance discontinuities, leading to signal integrity issues like ringing, overshoot, and undershoot. These issues can cause data corruption, timing violations, and EMI problems.

2. Q: Can I use the same series termination resistance value for all the signals in my design?
   A: Not necessarily. The appropriate series termination resistance value depends on factors such as the driver output impedance and the characteristic impedance of each transmission line. It’s important to calculate the resistance value for each signal individually.

3. Q: How do I determine the characteristic impedance of a transmission line in Altium Designer?
   A: Altium Designer provides an impedance calculator tool that allows you to calculate the characteristic impedance based on the physical properties of the transmission line, such as trace width, trace thickness, dielectric constant, and trace-to-plane distance.

4. Q: What is the purpose of simulating my design after adding series termination resistance?
   A: Simulating your design helps to verify its performance and identify potential signal integrity issues before fabricating the PCB. It allows you to analyze signal waveforms, eye diagrams, and other parameters to ensure that the design meets the required specifications.

5. Q: Are there any drawbacks to using series termination resistance?
   A: One potential drawback is the power dissipation in the series termination resistors when the driver is in the high state. This can increase the overall power consumption of the system. Additionally, the resistors introduce a small delay in the signal propagation, which should be accounted for in timing-critical designs.

Conclusion

Calculating and implementing series termination resistance is a crucial aspect of high-speed PCB design. By matching the impedance of the transmission line to the source impedance of the driver, series termination resistance helps to minimize signal reflections and improve signal integrity.

Altium Designer provides tools and features that simplify the process of calculating and implementing series termination resistance values. By using the impedance calculator, adding series termination resistors to your schematic, and simulating your design, you can ensure optimal signal quality and reliability in your high-speed PCB designs.

Remember to consider best practices such as keeping trace lengths short, placing resistors close to the driver, using appropriate resistor packages, and validating your design through testing. By following these guidelines and leveraging the capabilities of Altium Designer, you can effectively manage series termination resistance and achieve robust and reliable high-speed PCB designs.

Parameter	Value
Driver Output Impedance (R_driver)	10 ohms
Transmission Line Impedance (Z_0)	50 ohms
Series Termination Resistance	40 ohms

Table 1: Example of series termination resistance calculation