AND8020 Datasheet, PDF (11/18 Pages) Analog Devices – Termination of ECL Logic Devices with EF (Emitter Follower) OUTPUT Structure

AND8020/D
SECTION 4. SERIES TERMINATION
R
R
RR
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).
Driver
R_t
R_S
R_S
R_t
* Optional
*T−Line Z₀
*T−Line Z₀
or Twisted Pair
Receiver
V_EE
Driver
R_S
* Optional
*T−Line Z₀
Receiver
R_t
V_EE
Figure 13. Series Termination
Differential ECL outputs can be terminated as
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 (R_S) and the output
impedance of the Driver gate (R_O) is equal to the line
characteristic impedance (Figure 14).
Where:
RS ) RO + Z0
(eq. 28)
R_S = Series Termination Resistor
R_O = Output Impedance
Z₀ = Line Characteristic Impedance
V_O
A
B
R_O
Driver
R_S
R_t
*T−Line Z₀
Receiver
V_EE
Figure 14. Series Termination
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, DV_O, is
impressed on the input to the transmission line (Point A) as
a change in voltage (DV_A) and propagates to the Receiver at
the output of the transmission line (Point B) as a change in
voltage (DV_B) in Figure 14.
ǒ Ǔ DVA + DVO *
Z0
RS ) RO ) Z0
(eq. 29)
Where:
DV_A = Input to the Transmission Line Voltage
Change
DV_B = Receiver Input Voltage Change
DV_O = Driver Output Voltage Change
Z₀ = Line Characteristic Impedance
R_O = Output Impedance of the Driver Gate
R_S = Termination Resistance
Since Z₀ = R_S + R_O, substitution into the above equations
yields:
DVA
+
DVO
2
(eq. 30)
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