Intel CHIPSET 820E - Manual (Page 117)

Intel ® 820E Chipset
R
Design Guide 117
Figure 73. Critical Dimensions for Component Placement
Intel®
82562ET /
82562EM
B
A
ICH2
EEPROM
Magnetics
Module
Line
RJ45
crit_dim_comp_plac
Distance Priority Guideline
A 1 <1 inch
B 2 <1 inch
2.22.4.4.1. Distance from Magnetics Module to RJ45
Distance ‘A,’ in the previous figure, should be given the highest priority during board layout. The
separation between the magnetics module and the RJ45 connector should be kept to less than 1 inch. The
following trace characteristics are important and should be observed:
• Differential impedance: The differential impedance should be 100 Ω . The single-ended trace
impedance is approximately 50 Ω . However, the differential impedance also can be affected by the
spacing between traces.
• Trace symmetry: Differential pairs (e.g., TDP and TDN) should be routed with consistent separation
and with exactly the same lengths and physical dimensions (e.g., width).
Caution: Asymmetric and unequal-length traces in the differential pairs contribute to common-mode noise. This
can degrade the receive circuit’s performance and contribute to radiated emissions from the transmit
circuit. If the Intel
82562ET component must be placed farther than a couple of inches from the RJ45
connector, distance B can be sacrificed. It should be a priority to minimize the total distance between the
Intel
82562ET component and RJ-45.
Note: The measured trace impedance for layout designs targeting 100
Ω often yields a lower actual impedance.
OEMs should verify the actual trace impedance and adjust their layout accordingly. If the actual
impedance is consistently low, a target of 105 Ω –110 Ω should compensate for second-order effects.