AM7968-125DKC Advanced Micro Devices, AM7968-125DKC Datasheet - Page 72

• Manufacturer Part Number: AM7968-125DKC
• Description: TAXIchip Integrated Circuits(Transparent Asynchronous Xmitter-Receiver Interface)
• Manufacturer: Advanced Micro Devices

Figure 5-9
68
TAXI
TAXI
RX
TX
AMD
RE
R3
R4
+
–
+
–
+5 V
+5 V
C
5.5.2 AC-Coupled TAXl-Fiber Optic Transceiver Interface
Some applications will require the TAXIchip set to optical transceiver interconnection to
be AC-coupled. AC coupling should be used in the following situations: a) when the
TAXIchip set and optical components are driven by a common power supply but the
supply pins are decoupled using the scheme recommended in Chapter 6, and b) when
the TAXIchip set and optical components operate on different power supplies.
AC coupling via capacitors along with the necessary design equations is shown in
Figure 5-9. In this configuration, RE is the ECL output pull-down resistor, C provides the
AC coupling, the connection is made with a transmission line (coax, twisted pair,
microstrip) of length L and characteristic impedance, Z0, and R1 and R2 provide a
matched line termination and voltage bias to the midpoint of the optical component’s
logic swing (V
The configuration shown in Figure 5-9 is recommended for any line length, L, which
separates the TAXIchip set and the optical module. Although the matched line termina-
tion is not necessary for L<3 , the V
links. Therefore, even for line lengths where matched line terminations are not neces-
sary (less than 3 ), the resistors R1 and R2 can be chosen to give a matched load
without any added complexity.
AC-Coupled TAXl-Fiber Optical Interface
5.6 Interfacing to Coaxial Cable
In many applications, system cost can be reduced by using coaxial cable as the serial
media. Unlike optical fiber, which requires optical components between the fiber and the
TAXIchip set, coaxial cable can be connected directly to the TAXI SEROUT pins, giving
lower system costs.
RE
R4
R3
C
L, Z0
L, Z0
bb
).
TAXIchip Integrated Circuits Technical Manual
C
C
R2
RE
R1
V
bb
V
EE
V
V
R2
RE
CC
R1
Optical Fiber
V
EE
CC
+
–
+
–
ODL
ODL
RX
TX
bb
bias voltage is always needed for AC-coupled
V
C = Largest RF Capacitor Available
R1+ R2
bb
R1R2
186 < RE < 7.2 (Z0)
R3+ R4
= Midpoint of ODL
5R4
Signal Swing
=
=
R3+ R4
R3R4
Midpoint of Pseudo-
ECL Signal Swing
= Z0
=
V
IH
2
+ V
IL
= 3.7 V
= V
12330E-16
EE
+
(V
CC
R1 + R2
– V
EE
) R2