Terminations methods in a transmission line

Terminations methods in a transmission line

There are different types of termination techniques:

1. Near End (source) Termination  
2. Far End Termination Near End (Source) termination
   

Near End (source) Termination  

This is normally used by the single ended lines where the output driver has lower impedance than the transmission line. In this case the series termination is added to the output of the driver to match the line impedance.

Near-End (Source) Termination for Single Ended Lines  

• Place the resistor very close to the source driver.  
• Use source termination in cases of point-to-point connection.  
• Do not use daisy-chain topology.  
• All loads must be at the end of the line.

Far End termination

The most common type of Far End termination for HSSI is Parallel or AC Termination.

Parallel and AC Termination for Single Ended Lines  

• Parallel termination resistance should be matched to the characteristic impedance of the line.  
• Parallel termination causes power consumption due to the DC path at the end of the line for high signal level. The thermal treatment should be done by selection of the parallel termination resistor.  
• The power consumption can be avoided with AC termination, where a capacitor is inserted in series to the termination resistor.  
• The Capacitor value in AC termination should be in the range of 100-200pF
• This termination technique increases the rising and falling times at load due to the capacitive load. Timing considerations should be made by selection of the value of C.  
• Place the components close to the receiver pins so that no long stubs are required.

Termination of Differential lines

Special attention should be given to the termination of the differential lines. The differential lines require more than 50 Ohm line impedance and 100 Ohm differential impedance between the lines. So the termination must be matched to the 100 Ohm

Terminations for Differential Lines  

Place termination after the connection of the trace to the pin of receiver so that no stubs are created before the receiver. This avoids reflections generated by the impedance discontinuity.  Do not use layer change or a via for the placement of the termination.