AND8020 Datasheet, PDF (12/18 Pages) Analog Devices – Termination of ECL Logic Devices with EF (Emitter Follower) OUTPUT Structure

AND8020/D
From this relationship, DV_A = DV_O / 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 R_t
The Emitter Pull−Down Resistor, R_t, functions to
establish V_OH and V_OL levels. Voltage transitions imposed
on R_t propagate through R_S and Z₀ to a receiver. Negative
voltage transition are current limited by R_t, R_S, and Z₀ when
the driver output switches to the low state. The R_t value must
maximize the negative voltage transition and prevent the
output transistor from entering the cutoff operating region in
a low state (Figure 15).
R_O
Driver
R_S
R_t
*T−Line Z₀
Receiver
V_EE
Figure 15. Equivalent Circuit for RE Determination
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 V_OH. The
model becomes a simple series resistive network as shown
in Figure 16.
R_S
R_t
*T−Line Z₀
V_OH
V_EE
V_EE
Figure 16. Equivalent Circuit with Output Cutoff
The maximum current, I_max, occurs at the instant the
switch opens and is calculated by:
I
max
+
( VOH *
( Rt ) RS
VEE )
) Z0
)
(eq. 31)
An initial current, I_init, 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, I_init is defined as:
ǒ Ǔ _{1.25 *} ^Vpp
Iinit +
2
Z0
(eq. 32)
To satisfy the initial constraints of I_max > I_init:
( VOH * VEE )
( Rt ) RS ) Z0 )
u
ǒ Ǔ _1.25
*
VSWING
2
Z0
(eq. 33)
Solving for R_t, gives the inequality:
Rt v ( KZ0 ) Z0 * RS
Where:
(eq. 34)
Z₀ = Line Characteristic Impedance
R_O = Output Impedance of the Driver Gate
R_S = 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 V_OH = 2.4 V, V_SWING = 0.8 V, and V_EE = 0.0 V:
(2.4 *
(Rt ) RS
0.0)
) Z0)
y
0.5
Z0
(eq. 35)
4.0 * Z0 * RS y Rt
For the 100LVEL series (LVPECL mode operation),
where: V_OH = 2.345 V, V_SWING = 0.750 V, V_EE = 0.0 V:
(2.345 * 0.0)
(Rt ) RS ) Z0)
y
0.468
Z0
(eq. 36)
4.01 * Z0 * RS y Rt
For the 10EL series (PECL mode operation),
where: V_OH = 4.185 V, V_SWING = 0.958 V, V_EE = 0.0 V:
(4.185 * 0.0)
(Rt ) RS ) Z0)
y
0.599
Z0
(eq. 37)
5.99 * Z0 * RS y Rt
http://onsemi.com
12