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.

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SIGNALS TERMINATIONS

SIGNAL-BUS-CLOCK TERMINATIONS In Embedded HARDWARE DESIGNS

There are two type of terminations

1.  Source Termination -Near Source
2.  End Termination -Near Receiver

Source termination works differently than end termination, but the net effect is the same. It works by controlling signal reflections, not preventing the reflections in the first place. Because of this, it only works if a driver output is feeding a single load.

The huge benefit of source termination is that it does not load down the driver like end termination does. 

SERIES TERMINATION :

Most drivers have some resistance on it's output. That resistance is usually in the 20-30 ohm range. The sum of the output resistance of the driver and your resistor must equal your trace impedance in series termination.Source Termination may effect the rise time of the signal because of the RC time constant ie. Resistance of termination and capacitance of cable . 

SERIES TERMINATION

Reducing rise time will reduce the demand at the driver side which reducing the Instantaneous current demand on the driver [reducing ground bounce and EMI].A series terminated line does not stop reflections but reduces the amplitude of the ringing. If a source resistor is used with multiple loads, only the last device will see a cleaner edge.

OUTPUT RESISTANCE + TERMINATION RESISTANCE = TRACK IMPEDANCE

A reflection will occur when the termination resistor does not match the trace impedance 

Parallel Trace Termination :

PARALLEL TERMINATION

In  parallel termination a resistor is connected to the ground or VCC parallel to I/O.Power dissipation is high during the operations at lower frequency ie. at lower switching rates .

TERMINATION RESISTANCE = TRACK IMPEDANCE

A reflection will occur when the termination resistor does not match the trace impedance .

INSTRUCTION WHILE PROBING THE SIGNAL

When checking signals on Scope you must probe at the receiver. Probing anywhere else will likely give you a distorted waveform and trick you into thinking that things are worse than they really are. Also, make sure that your ground clip is as short as possible.

Will include Differential pair terminations in next articles..

GooD LucK - AJ

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