AND8020D datasheet(12/18 Pages) AD

12 / 18 page

AND8020/D

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From this relationship,

DVA = DVO / 2, an incident wave

of half amplitude propagates down the transmission line. At

the Receivers input Point B, typically high impedance, the

transmission line sees an unterminated open line and the

signal reflection coefficient at the Receiver load is

approximately unity. The reflection causes the voltage to

double at the receiving end. When the reflected wave arrives

back at the source end, its energy is dissipated by the series

resistor. When the sum of the source and series impedance

is equal to the characteristic impedance of the line, no further

reflections occur.

Calculation of Rt

The Emitter Pull−Down Resistor, Rt, functions to

establish VOH and VOL levels. Voltage transitions imposed

on Rt propagate through RS and Z0 to a receiver. Negative

voltage transition are current limited by Rt, RS, and Z0 when

the driver output switches to the low state. The Rt value must

maximize the negative voltage transition and prevent the

output transistor from entering the cutoff operating region in

a low state (Figure 15).

*T−Line Z0

VEE

Receiver

Figure 15. Equivalent Circuit for RE Determination

Driver

The worst case scenario occurs when the driver output

emitter follower enters into cutoff during a negative going

transition. When this happens, the driver can be considered

opened and, at the instant it opens, the line characteristic

impedance behaves as a linear resistor returned to VOH. The

model becomes a simple series resistive network as shown

in Figure 16.

*T−Line Z0

VEE

Figure 16. Equivalent Circuit with Output Cutoff

VEE

VOH

The maximum current, Imax, occurs at the instant the

switch opens and is calculated by:

I max +

(VOH * VEE )

(Rt ) RS ) Z0 )

(eq. 31)

An initial current, Iinit, must be sufficient to generate a

transient voltage equal to half of the logic swing since the

voltage at the receiver will double due the reflection

coefficient approaching 1.0 for series termination. To

accommodate reflections caused by discontinuities and load

capacitances the transient voltage should be increased by

25%. Thus, Iinit is defined as:

Iinit +

1.25 *

Vpp

(eq. 32)

To satisfy the initial constraints of Imax > Iinit:

( VOH * VEE )

( Rt ) RS ) Z0 )

1.25 *

VSWING

(eq. 33)

Solving for Rt, gives the inequality:

Rt v (KZ0 )Z0 * RS

(eq. 34)

Where:

Z0 = Line Characteristic Impedance

RO = Output Impedance of the Driver Gate

RS = Termination Resistance

KZ0 = Coefficient to Z0

For various series, the coefficient to Z0, KZ0, is presented

in Table E: Coefficient to Z0.

Table E. Coefficient to Z0

Series

KZ0

10EP

4.0

100LVEL

4.01

10EL

5.99

10E

7.10

100E

6.57

For the 10EP series (LVPECL mode operation),

where VOH = 2.4 V, VSWING = 0.8 V, and VEE = 0.0 V:

(2.4

* 0.0)

(Rt ) RS ) Z0)

y 0.5

*Z0 * RS y Rt

4.0

(eq. 35)

For the 100LVEL series (LVPECL mode operation),

where: VOH = 2.345 V, VSWING = 0.750 V, VEE = 0.0 V:

(2.345

* 0.0)

(Rt ) RS ) Z0)

y 0.468

*Z0 * RS y Rt

4.01

(eq. 36)

For the 10EL series (PECL mode operation),

where: VOH = 4.185 V, VSWING = 0.958 V, VEE = 0.0 V:

(4.185

* 0.0)

(Rt ) RS ) Z0)

y 0.599

*Z0 * RS y Rt

5.99

(eq. 37)

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