
LXT9860/9880
—
Advanced 10/100 Repeater with Integrated Management
66
Datasheet
Document #: 248987
Revision#: 003
Rev Date: 08/07/01
Place magnetics as close as possible to the LXT98x0.
Keep transmit pair traces short.
Do not route transmit pair adjacent to a ground plane. Eliminate planes under the transmit
traces completely. Otherwise, keep planes 3-4 layers away.
Improve EMI performance by filtering the output center tap supply. A single ferrite bead may
be used in the center tap supply to all ports. A ferrite bead with a total maximum current rating
of 1.5Amp is recommended.
Place 270pF 5% capacitors at TPIP and TPIN to improve signal-to-noise immunity at the
receiver, especially for long line lengths.
In addition, follow all the standard guidelines for a twisted-pair interface:
Route the signal pairs differentially, close together. Allow nothing to come between them.
Keep distances as short as possible; both traces should have the same length.
Avoid vias and layer changes.
Keep the transmit and receive pairs apart to avoid cross-talk.
To provide maximum isolation, place entire receive termination network on one side and
transmit on the other side of the PCB.
Bypass common-mode noise to ground on the in-board side of the magnetics using 0.01
μ
F
capacitors.
Keep termination circuits grouped closely together and on the same side of the board.
Always put termination circuits close to the source end of any circuit.
4.3.3.1
Magnetics Information
The LXT98x0 requires a 1:1 ratio for the receive transformers and a 1:1 ratio for the transmit
transformers. The transformer isolation voltage should be rated at 2 kV to protect the circuitry from
static voltages across the connectors and cables. Refer to
Table 25
for magnetics specifications.
4.3.4
Clock
A stable, external 25 MHz system clock source (CMOS) is required. See
Table 26
.
Table 25. LXT98x0 Magnetics Specifications
Parameter
Min
Nom
Max
Units
Test Condition
Rx turns ratio
–
1 : 1
–
–
Tx turns ratio
–
1 : 1
–
–
Insertion loss
0.0
–
1.1
dB
80 MHz
Primary inductance
350
–
–
μ
H
Transformer isolation
–
2
–
kV
Differential to common mode rejection
-40
–
dB
.1 to 60 MHz
-35
–
dB
60 to 100 MHz
Return Loss - standard
-16
–
dB
30 MHz
-10
–
dB
80 MHz