Definition: A termination resistor is a type of resistor used at the ends of electrical transmission lines or signal traces to match the impedance of the line and prevent signal reflections.
How Does a Termination Resistor Work?
A termination resistor is placed at the end of a transmission line to prevent signals from reflecting back. To fully understand its working, it’s essential to understand the behavior of twisted pair transmission lines.
Twisted pair cables have inductance and capacitance distributed along their length. These characteristics can be visualized as numerous capacitors and inductors connected in parallel and series. The characteristic impedance of a twisted pair cable is not arbitrary but is determined by the cable’s physical properties. For example, in RS485 systems, the typical characteristic impedance of twisted pair cables is 120 ohms.
By placing a 120-ohm resistor at the end of an RS485 transmission line, the resistor absorbs the signal energy instead of being reflected in the communication bus. However, if the resistor’s value deviates from the cable’s characteristic impedance, the signal reflects and causes interference and potential data corruption.
Why Are Termination Resistors Used?
- Prevent Signal Reflections: When a signal travels along a transmission line, it may reflected toward the source if the line’s impedance does not match the load or open-end impedance. The reflected signal interferes with the original signal and causes noise, data corruption, and even complete signal loss. Termination resistors are used to match the impedance of a transmission line. A resistor equal to the line’s characteristic impedance is placed at the end. This helps the signal get absorbed instead of bouncing back.
- Maintain Signal Integrity: In high-speed digital circuits, it is important to keep the signal clear. Termination resistors improve signal quality by reducing reflections, ringing, and other interference that can cause distortion.
- Reduce Electromagnetic Interference (EMI): Termination resistors reduce reflections and ringing in a transmission line. This helps lower electromagnetic interference (EMI) from the line. It is especially useful in places where many electronic devices are close to each other.
Types of Termination Resistors
- Parallel Termination: A resistor is placed between the signal line and the ground at the end of the transmission line. When the signal reaches the end, the resistor absorbs it. and stops the signal from bouncing back, which can cause interference and distortion. This method is common and matches the characteristic impedance of the line. The resistor’s value is set to match the impedance of the transmission line, such as 50Ω, 75Ω, or 120Ω.
- Series Termination: A resistor is placed in series with the signal line near the source. This type of termination reduces the initial amplitude of the signal. The resistor matches impedance, limits the current flowing into the transmission line, and minimizes reflections. This ensures that the combined impedance of the driver and resistor matches the line’s characteristic impedance. As a result, it prevents signals from bouncing back to the source and causing interference. Series termination is commonly used in point-to-point connections like clocks and control signal lines.
- Thevenin Termination: Thevenin termination uses two resistors to match the impedance of a transmission line. One resistor connects the signal line to a positive voltage (e.g., Vcc), while the other connects the signal line to the ground. These resistors create a voltage divider that matches the line’s characteristic impedance. This helps absorb the signal at the end of the line and prevents reflections that could cause interference. This method balances power consumption and signal integrity effectively.
Placement of Termination Resistors in RS485 Systems
In most RS485 configurations, two termination resistors are used:
- One at each end of the twisted pair cable.
When only one device sends data and is at one end of the cable, only one termination resistor is needed at the other end. This stops signals from bouncing back and causing interference.
Applications of Termination Resistors
- High-Speed Data Buses: Termination resistors are commonly used in high-speed data buses, such as PCIe, DDR memory interfaces, and other digital communication systems, to ensure the signal is transmitted correctly.
- Communication Networks: Termination resistors are used in networks like CAN bus or RS-485. They stop reflections in long cables and help data transmit reliably.
- Printed Circuit Boards (PCBs): On PCBs, termination resistors are placed at the end of traces carrying high-speed signals to stop reflections and keep the signal clear.
Conclusion
Termination resistors are important in systems that send high-speed signals over long distances or through traces with high impedance. They ensure the impedance of the load matches the transmission line’s impedance. This prevents signal reflections and keeps the system reliable and intact.