Datasheet

DS_21x3_001 78Q2123/78Q2133 Data Sheet
4.8 Isolation Transformers
Two simple 1:1 isolation transformers are required at the line interface. Transformers with integrated
common-mode chokes are recommended for exceeding FCC requirements. This table gives the
recommended line transformer characteristics.
Name Value Condition
Turns Ratio
1 CT : 1 CT r5%
Open-Circuit Inductance
350 PH (min)
@ 10 mV, 10 kHz
Leakage Inductance
0.40 PH (max)
@ 1 MHz (min)
Inter-Winding Capacitance 25 pF (max)
D.C. Resistance
0.9 :(max)
Insertion Loss 0.4 dB (typ) 0 - 65 MHz
HIPOT 1500 Vrms
Note: The 100Base-TX amplitude specifications assume a transformer loss of 0.4 dB. For the transmit
line transformer with higher insertion losses, up to 1.2 dB of insertion loss can be compensated by
selecting the appropriate setting in the Transmit Amplitude Selection bits in register MR19.15:14.
4.9 ReferenceCrystal
If the internal crystal oscillator is to be used, a crystal with the following characteristics should be chosen.
Name Value Units
Frequency 25.00000 MHz
Load Capacitance* 4** pF
Frequency Tolerance
r50
PPM
Aging
r2
PPM/yr
Temperature Stability ( 0 - 70
o
C)
r5
PPM
Oscillation Mode Parallel Resonance, Fundamental Mode
Parameters at 25
o
C r2
o
C ; Drive Level = 0.5 mW
Drive Level (typ) 50-100
PW
Shunt Capacitance (max) 10 pF
Motional Capacitance (min) 10 pF
Series Resistance (max) 60
:
Spurious Response (max) > 5 dB below main within 500 kHz
* Equivalent differential capacitance across the XTLP/XTLN pins.
** If crystal with a larger load is used, external shunt capacitors to ground should be added to make up
the equivalent capacitance difference.
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