AND8020D datasheet(11/18 Pages) AD

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AND8020/D

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SECTION 4. SERIES TERMINATION

Series Damping is a technique in which a termination

resistance is placed between the driver and the transmission

line with no termination resistance placed at the receiving

end of the line (Figure 13).

*T−Line Z0

Driver

VEE

* Optional

Receiver

Driver

VEE

* Optional

Receiver

Figure 13. Series Termination

or Twisted Pair

Differential

ECL

outputs

can

terminated

independent complimentary single−ended lines. Both sides

of any differential pair must be terminated as identically as

possible to minimize phase error and pulse width duty cycle

skew.

Series Termination is a special case of series damping in

which the sum of the termination resistor (RS) and the output

impedance of the Driver gate (RO) is equal to the line

characteristic impedance (Figure 14).

RS ) RO + Z0

(eq. 28)

Where:

RS = Series Termination Resistor

RO = Output Impedance

Z0 = Line Characteristic Impedance

*T−Line Z0

VEE

Receiver

Figure 14. Series Termination

Driver

Series termination techniques are useful when the

interconnect lengths are long or impedance discontinuities

exist on the line. Additionally, the signal travels down the

line at half amplitude minimizing problems associated with

crosstalk. Unfortunately, a drawback with this technique is

the possibility of a two−step signal appearing when the

driven inputs are far from the end of the transmission line.

To avoid this problem, the distance between the end of the

transmission line and input gates should adhere to the

guidelines specified from the section on unterminated lines.

Series Termination Theory

When the output of the series terminated driver gate

switches levels, this driver output voltage change,

DVO, is

impressed on the input to the transmission line (Point A) as

a change in voltage (

DVA) and propagates to the Receiver at

the output of the transmission line (Point B) as a change in

voltage (

DVB) in Figure 14.

DVA + DVO *

RS ) RO ) Z0

(eq. 29)

Where:

DVA = Input to the Transmission Line Voltage

Change

DVB = Receiver Input Voltage Change

DVO = Driver Output Voltage Change

Z0 = Line Characteristic Impedance

RO = Output Impedance of the Driver Gate

RS = Termination Resistance

Since Z0 = RS + RO, substitution into the above equations

yields:

DVA +

DVO

(eq. 30)

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