How does series termination work

The answer is an impedance mismatch. The key value to be aware of is the characteristic impedance of the transmission line. Reflection occurs when a signal travels down the length of a conductive medium and encounters an impedance mismatch. This can occur due to natural imperfections within a trace, line, or cable. However, even if there were no imperfections throughout the length of a transmission line, the ends would still represent a huge impedance mismatch, because the physical line itself comes to an abrupt end.

The amount of energy of your signal that gets reflected can be calculated from the impedance mismatch using a reflection coefficient. This is where the termination resistor comes in. Much of routing in PCB design can be considered an exercise in impedance matching, where you try to make sure the input impedance of an electrical load matches the output impedance of its signal source.

The easiest way to maintain these impedances is through the careful placement of termination resistors. Parallel termination is easy to implement because the value of the resistor is easy to obtain, you only need one extra component, and it perfrorms well with distributed loads.

The only major con with parallel termination is power dissipation via a continuous DC current path to ground. The power dissipation can add up across a circuit when you start terminating multiple nets. You must evaluate if the timing impact causes a performance degradation of the interface.

The series termination scheme works by introducing a resistor placed in series between the driver and receiver. The driver impedance and series resistance become the total effective driver impedance.

The transmission line impedance has to match the driver impedance to minimize reflection and manage overshoot. Figure 4. Series Termination Scheme. This example shows how to determine the value of the series termination resistor to manage the voltage overshoot effectively. You can explore other appropriate methods via simulation to determine a suitable series resistor value for your interface. Figure 5. Set up the desired interface in the Schematic Editor as represented in this figure and run the terminator wizard.

Figure 6. Figure 7. Figure 8. The new setup in this example is evaluated at different allowable conditions to ensure that DC and AC specifications are met and to identify the impact of introducing the resistor in the interface. Related reference Driver Selection Reference. The output characteristics of a driver determines how much overshoot voltage is seen at the receiver when the interface is not terminated.

You can address signal integrity concerns in your interface by selecting the appropriate driver. A driver can drive a receiver without termination even if it produces overshoot, undershoot, and ringing in the interface as long as it meets two key specifications: voltage threshold and maximum input voltage of the receiver device.

Table 2. This table lists the current limits that are the maximum allowable driver current values at the V OH level. Table 3. Take the measurement at the driver maximum allowable operating condition, which is at a low temperature and high supply voltage, to account for the worst possible overshoot condition. For this combination, disabling the diode offers the driver slightly more margin. For other combinations, enabling the diode offers the driver better margin. Input Setup with 2. Figure 9.

Input Setup with 3. Figure A dialog box appears with multiple tabs for each data characteristic available for the model. In the figure, the measured current is Based on the driver selection reference, the V OH for the 3.

There should be no more than two terminating resistors in the network, regardless of how many nodes are connected, because additional terminations place extra load on the CAN transceivers. Eric R. The calculations in the attached document indicate that the resistor needs to be at least a 7. The bus line is a twisted pair wire with a termination resistor Ohm on each side. Both wires are needed for proper communication.

In CANopen, there are unique addresses available for up to nodes on the bus. However the practical physical limit of nodes is about units per bus. The CAN bus is used not only to interchange information between devices connected thereto, but also to enable an OBD standard connector to be used so that that parameters of particular systems and information on errors can be read by means of external diagnostics interfaces.

How does series termination work? Which termination has highest power consumption? Does rs need termination? What is AC termination? Ignoring these options and using a series termination resistor will create a tradeoff between the three points listed above. If you analyze the equivalent RLC model that defines a transmission line with a series termination resistor, you can quickly determine the level of damping provided by the presence of a series termination resistor. In high-speed design involving transmission lines, you can safely ignore the parasitic inductance in the substrate and the DC resistance of the copper trace in the transmission line.

As a digital driver switches, the sudden change in signal level will induce a transient oscillation along the transmission line that is superimposed on top of the ON or OFF signal level. This transient is responsible for overshoot at the receiver, thus overshoot should be suppressed where possible. When the transmission line is critically damped, a transient oscillation will be completely suppressed and while still having the fastest rise time.

If you calculate the transient oscillation frequency and damping in this standard model, you can determine the value of the series termination resistor required to produce critical damping:. Series termination resistor required for critical damping. One can immediately see that the source output impedance and series termination resistor needs to be double the characteristic transmission line impedance in order to reach critical damping.

This illustrates the tradeoff between damping and impedance matching: one cannot critically damp the response and perfectly match impedance simultaneously. If you match the source impedance exactly to the transmission line impedance, then you will produce an underdamped oscillation when the driver switches. Determining the right compromise between damping and impedance matching really requires considering the noise margin at the receiver. If the noise margin is narrow, then you may need to allow a slight mismatch and power transfer reduction from the source and use a larger resistor, which will bring the response closer to critical damping.

Although this reduces the amplitude of the transient oscillation, it also increases the rise time somewhat, which will limit the maximum data rate. Because of the issue mentioned earlier, where the output impedance of a driver can be different in the ON and OFF states, you might be able to critically damp one edge of the pulse, while the other edge exhibits some ringing during switching.

If the load is a high-Z receiver that does not require termination, then you can even have a reflection that produces a stair-step response on one or both edges of the pulse. Always test transmission lines in your prototype for signal integrity. The signaling standard your components use may carry their own termination requirements.