Calculating Series Termination Resistance Values in Altium Designer

 In the world of high-speed digital design, signal integrity is of paramount importance. As data rates increase and signal edges become steeper, the effects of transmission line reflections can significantly degrade signal quality, leading to timing errors and data corruption. To mitigate these issues, proper termination techniques must be employed. One common termination strategy is the use of series termination resistors, which can be calculated and implemented within Altium Designer, a powerful PCB design software.

Understanding Transmission Lines

Before delving into the calculation of series termination resistance values, it is essential to understand the concept of transmission lines. In high-speed digital circuits, the traces on printed circuit boards (PCBs) behave like transmission lines, where the signals propagate as electromagnetic waves. These waves are subject to reflections at the end of the transmission line, which can cause signal degradation and interference.

Characteristic Impedance

The characteristic impedance (Z0) of a transmission line is a fundamental parameter that determines the behavior of the signal propagation. It is a function of the physical properties of the transmission line, such as the trace width, trace thickness, dielectric constant of the PCB material, and the spacing between the trace and the reference plane. Achieving the desired characteristic impedance is crucial for proper signal integrity and termination.

Series Termination

Series termination is a technique used to minimize signal reflections at the receiver end of a transmission line. By placing a termination resistor in series with the receiver, the impedance mismatch between the transmission line and the receiver is mitigated, reducing reflections and improving signal quality.

Calculating Series Termination Resistance

The value of the series termination resistor (RTERM) is calculated based on the characteristic impedance of the transmission line (Z0) and the input impedance of the receiver (ZRCVR). The formula for calculating the series termination resistance is as follows:

RTERM = Z0 x (ZRCVR / (Z0 + ZRCVR))

In this equation, Z0 represents the characteristic impedance of the transmission line, and ZRCVR is the input impedance of the receiver. The calculation ensures that the total impedance seen by the transmission line is matched to its characteristic impedance, minimizing reflections.

Example Calculation

Let's consider an example where the characteristic impedance of the transmission line (Z0) is 50 Ohms, and the input impedance of the receiver (ZRCVR) is 100 Ohms. To calculate the series termination resistance, we substitute the values into the formula:

RTERM = 50 Ohms x (100 Ohms / (50 Ohms + 100 Ohms)) RTERM = 50 Ohms x (100 Ohms / 150 Ohms) RTERM = 33.33 Ohms

In this scenario, a series termination resistor of 33.33 Ohms should be placed in series with the receiver to minimize reflections and maintain signal integrity.

Implementing Series Termination in Altium Designer

Altium Designer provides a range of tools and features to assist in the design and implementation of series termination resistors. Here's a general workflow for calculating and implementing series termination in Altium Designer:

1. Define Transmission Line Properties: In the PCB design editor, specify the desired characteristic impedance for your transmission line by setting the appropriate trace width, thickness, and dielectric properties.
2. Determine Receiver Input Impedance: Obtain the input impedance of the receiver from the device datasheet or manufacturer specifications.
3. Calculate Series Termination Resistance: Use the formula provided earlier to calculate the required series termination resistance based on the characteristic impedance of the transmission line and the receiver input impedance.
4. Place Termination Resistor: In the schematic capture environment, place a resistor component with the calculated value in series with the receiver input.
5. PCB Layout and Routing: When transitioning to the PCB layout stage, ensure that the series termination resistor is placed as close as possible to the receiver input to minimize any additional trace length and maintain signal integrity.
6. Signal Integrity Analysis: Altium Designer offers advanced signal integrity analysis tools, such as the Impedance Profile and Crosstalk tools, which can be used to verify the effectiveness of the series termination implementation and identify any potential issues.

Example Table: Termination Resistor Values

Here's an example table illustrating various series termination resistor values calculated for different combinations of characteristic impedance (Z0) and receiver input impedance (ZRCVR):

Z0 (Ohms)
ZRCVR (Ohms)
RTERM (Ohms)
50
100
33.33
60
120
40.00
75
150
50.00
90
180
60.00
100
200
66.67

Note that these values are calculated using the formula provided earlier and serve as examples. In practical applications, the specific values should be calculated based on the actual characteristic impedance of the transmission line and the receiver input impedance.

FAQ (Frequently Asked Questions)

1. Why is series termination important in high-speed digital design? Series termination is crucial in high-speed digital design to minimize signal reflections and ensure proper signal integrity. As data rates increase and signal edges become steeper, reflections can cause significant signal degradation, leading to timing errors and data corruption.
2. How does series termination work? Series termination involves placing a resistor in series with the receiver input to match the total impedance seen by the transmission line to its characteristic impedance. By doing so, reflections at the receiver end are minimized, improving signal quality.
3. Can I use a different termination technique instead of series termination? Yes, there are other termination techniques available, such as parallel termination and AC termination. The choice of termination technique depends on factors like the transmission line impedance, receiver input impedance, and specific design requirements.
4. How do I determine the characteristic impedance of my transmission line? The characteristic impedance of a transmission line depends on various factors, including the trace width, trace thickness, dielectric constant of the PCB material, and the spacing between the trace and the reference plane. Altium Designer provides tools to calculate and set the desired characteristic impedance during the PCB design process.
5. What happens if I don't implement proper termination? If proper termination techniques are not implemented, signal reflections can occur at the receiver end of the transmission line. These reflections can cause signal degradation, timing errors, and potentially lead to data corruption or complete signal loss, especially in high-speed digital circuits.