Cirrus Logic AN83 - Manual (Page 44)

AN83
44 AN83REV3
mit signal traces should be at least 100 mil. This
will provide a good impedance matching for the
transmit and receive circuitry inside the CS8900A.
A ground trace should be run parallel to the trans-
mit traces. Also, a ground plane should run under-
neath the transmit and receive traces on the solder
side of a two layered PCB. Please refer to the Fig-
ures 33 and 34 for illustration of the above guide
lines.
Routing of the AUI signals: The CS8900A has
three pairs of differential signals connecting it to an
Auxiliary Unit Interface (AUI). An isolation trans-
former separates the three signal pairs and the AUI
connector (a 15 pin sub-D connector). The isola-
tion transformer should be placed as close as possi-
ble to the AUI connector. Signal traces of each
differential pair should be in parallel with equal
length and impedance. Thus minimizing differen-
tial noise due to impedance mismatch. Place the
AUI signal traces on the component side.
RECOMMENDED MAGNETICS FOR
THE CS8900A
The CS8900A is has two types of Ethernet interfac-
es 10BASE-T and AUI. For both the interfaces,
analog filters are on the chip. The Figures 10 and
16 show typical connection required for these inter-
faces.
For an AUI interface, an isolation transformer
without a common mode choke (CMC) is used.
For the 10BASE-T interface, choice between isola-
tion transformer and isolation transformer with a
common mode choke (CMC) depends on the com-
mon mode noise that exists on the 10BASE-T lines
in a particular system. A common mode choke re-
duces common mode noise emitted by the
10BASE-T lines. A CMC may be required in cer-
tain applications to meet EMI requirements and to
meet 10BASE-T common mode output voltage
noise specification. The physical dimensions of the
isolation transformer and the isolation transformer
with a CMC are the same. Both are typically avail-
Figure 33. 10BASE-T Transit Layout Details