AND8020 Datasheet, PDF (7/18 Pages) Analog Devices – Termination of ECL Logic Devices with EF (Emitter Follower) OUTPUT Structure

Table C. Y Term Table
|V_CC−V_EE| = 5.0 V
Z₀
R_t1
R_t2
R_t3
50
50
50
112
70
70
70
156
75
75
75
166
80
80
80
179
90
90
90
201
100
100
100
223
120
120
120
268
150
150
150
335
AND8020/D
|V_CC−V_EE| = 3.3 V
Z₀
R_t1
R_t2
R_t3
50
50
50
46
70
70
70
64
75
75
75
68
80
80
80
72
90
90
90
82
100
100
100
91
120
120
120
109
150
150
150
136
|V_CC−V_EE| = 2.5 V
Z₀
R_t1
R_t2
R_t3
50
50
50
21.2
70
70
70
29.7
75
75
75
31.8
80
80
80
33.9
90
90
90
38.1
100
100
100
42.4
120
120
120
50.8
150
150
150
63.6
*T−Line Z₀
R_t
*T−Line Z₀
R_E R_E ^{(*or twisted pair)}
V_EE
Figure 7. Standard Pair with External Parallel
Near DC Current Return − Standard Pair
Termination
The standard pair termination scheme uses a pull−down
resistor, R_E, located at each driver pin to return the output
transistor bias current near the driver, and an impedance
matching parallel resistor, R_T, located at the receiver input
pins (see Figure 7, standard pair with external parallel, and
Figure 8, standard pair termination with internal
termination, and Figure 9, standard pair termination with
singulated internal termination resistors). The impedance
matching parallel resistor may be internal or external
depending on the receiver device. If internal to the receiver,
the resistor may be singulated or combined (“combo”) for
external pinout.
The diagram of Figure 7 shows a Standard Pair
Termination with an R_E resistor for DC output current bias
located nearby each driver pin: refer to Table B, for values
of R_E. The differential transmission line AC impedance
matching resistance, R_t, is located externally near the
receiver input pins.
As a variation of a Standard Pair Termination, a receiver
may provide the differential transmission line AC
impedance matching resistance, R_t, internally. This internal
impedance matching termination may be pinned out either
combined into a Combo V_t pin or each resistor may be
singulated and pinned out, such as V_t1 and V_t2.
When left open, the Combo Pin still provides a passive
100 W termination across the nearby receiver’s differential
signal line pair. This can compliment a pull−down resistor, R_E,
located on each line of a differential at the driver pins. This is
illustrated in Figure 8.
Driver
R_E
*T−Line Z₀
*T−Line Z₀
(*or twisted pair)
R_E
V_EE
Open V_t Pin
Receiver
RR
Internal Termination Combo Pin
Figure 8. Standard Pair Termination with
Internal Termination
When the Internal Termination resistors are singulated,
the two V_t pins must be shorted to create the 100 W value as
shown in Figure 9.
Driver
*T−Line Z₀
*T−Line Z₀
(*or twisted pair)
R_E
R_E
Vt1
V_EE ^Shorted
Vt2
Receiver
RR
Internal Termination Singulated Pins
Figure 9. Standard Pair Termination with
Singulated Internal Termination Resistors
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