HDMP-1012 Datasheet, PDF (39/42 Pages) Agilent(Hewlett-Packard) – 4Low Cost Gigabit Rate Transmit/Receive Chip Set

ECL
OUTPUT
I-ECL
O-ECL
ECLGND
800
R_T
16 K
V_TT = -2 V VIL = -1.9 V V_EE
V_BB = -1.3
45 µA
V_EE
Z_O > 100 Ω
L < 10 CM
R_T = 300
ECL
INPUT
V_TT = -2 V
Figure 21. I-ECL and O-ECL Simplified Circuit Schematic.
found in the data sheet under Tx
/ Rx I/O Definitions.
can be tied together and bypassed acceptable, but will reduce noise
to ground with a 0.1 uF capacitor. margins due to extra parasitics.
I-ECL and O-ECL
These I/O are designed to
interface directly to ECL-100K
family. A simplified schematic
diagram of I/O cell is shown in
Figure 21. Many of the ECL
interface theories and techniques
can be found in commercial ECL
data books.
The I-ECL inputs have internal
pull down resistors, such that if
left opened, the logic level will
float low. Also, the inputs can be
tied to ground directly to yield a
logic high. These inputs can be
driven from standard ECL buffers,
using conventional termination
techniques. Series terminations at
the source can also be used.
In most cases, the unconnected I-
ECL input will suffice for a logic
low. However, some I-ECL pins
with adjacent high speed lines,
such as the TCLKSEL pin in the
Rx, may be affected by crosstalk
dependent on the trace
separations in the board layout. In
this case, all pins with logic lows
The O-ECL can each drive a 50 Ω
line terminated with 50 Ω to Vtt
= -2 V. However, because of the
multiple outputs, driving all lines
into 50 Ω loads will cause
excessive power dissipation and
may lead to undesirable current
spikes in the power and ground
planes. Therefore, it is recom-
mended that the termination
resistor, which sets the output
bias current, be limited to 300 Ω.
To minimize reflections, it is
desirable to match the character-
istic impedance of the line to the
termination resistance. But
because of PC board limitations,
realistic values of 100 Ω trans-
mission lines are more
achievable. With 100 Ω lines, it is
recommended that the maximum
distance of the transmission lines
do not exceed 10 cm.
The preferred termination
resistors are individual surface-
mount resistors, commonly
connected to Vtt = -2 V which is
properly bypassed to ground.
Resistor packs such as SIPs are
Each of the O-ECL outputs has a
small trickle current which turns
on the output emitter follower
such that DC logic levels would
appear without external pull-down
terminations. This feature is
useful for testing and system
debugging.
High Speed Interface: I-H50 &
O-BLL
The simplified schematic
diagrams of I-H50 and O-BLL is
shown in Figure 22. The I-H50
input cell has internal 50 Ω
resistors built into the differential
input lines. The termination is
connected via HGND which
isolates the high speed ground
currents from the internal
grounds. The DC level for the
inputs is at 0 V. Since all of the
high speed inputs into G-LINK do
not have a DC component, it is
recommended that I-H50 inputs
be AC coupled with a 0.1 µF
capacitor. It is also recommended
that the unused differential inputs
be terminated with 50 Ω. The O-
BLL output cell is designed to
611