Datasheet

78Q2120C
10/100BASE-TX
Transceiver
Page: 31 of 35 © 2009 Teridian Semiconductor Corporation Rev 1.3
ISOLATION TRANSFORMERS
Two simple 1:1 isolation transformers are required at the line interface. Transformers with integrated common-
mode choke are recommended for exceeding FCC requirements. This table gives the recommended line
transformer characteristics:
NAME VALUE CONDITION
Turns Ratio
1 CT : 1 CT ± 5%
Open-Circuit Inductance
350 µH (min)
@ 10 mV, 10 kHz
Leakage Inductance
0.40 µH (max)
@ 1 MHz (min)
Inter-Winding Capacitance 25 pF (max)
D.C. Resistance
0.9 Ω (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 MR19.11:10.
REFERENCE CRYSTAL
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
±50
PPM
Aging
±2
PPM/yr
Temperature Stability ( 0 - 70
o
C)
±5
PPM
Oscillation Mode Parallel Resonance, Fundamental Mode
Parameters at 25
o
C ± 2
o
C ; Drive Level = 0.5 mW
Drive Level 50 - 100
µW
Shunt Capacitance (max) 8 pF
Motional Capacitance (min) 10 f F
Series Resistance (max) 60
Ω
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.