AD9572
Rev. B | Page 18 of 20
termination network should match the PCB trace impedance
and provide the desired switching point. The reduced signal
swing may still meet receiver input requirements in some
applications. This can be useful when driving long trace lengths
on less critical nets.
50Ω
10Ω
PULLUP
= 3.3
CMOS
5pF
100Ω
100Ω
7498-018
Figure 21. CMOS Output with Far-End Termination
LVPECL CLOCK DISTRIBUTION
The LVPECL outputs, which are open emitter, require a dc
termination to bias the output transistors. The simplified
equivalent circuit in Figure 19 shows the LVPECL output stage.
In most applications, a standard LVPECL far-end termination is
recommended, as shown in Figure 22. The resistor network is
designed to match the transmission line impedance (50 Ω) and
establish a dc bias of (V
CC
– 2 V). An alternative dc-coupled
LVPECL termination network with a reduced number of
components is also possible as shown in Figure 23.
50Ω
50Ω
3.3V
SINGLE-ENDED
(NOT COUPLED)
3.3V
3.3
LVPECL
127Ω127Ω
83Ω83Ω
07498-029
V
T
= V
CC
– 2.0V
V
CC
= 3.3V
LVPECL
Figure 22. LVPECL Far-End Termination
50Ω
50Ω
LVPECL
50Ω
50Ω
50Ω
07498-030
LVPECL
Figure 23. LVPECL Y Termination
An ac- coupled LVPECL termination scheme is shown in
Figure 24.
50Ω
50Ω
LVPECL
50Ω50Ω
200Ω200Ω
07498-031
LVPECL
TERM
0.1µF
0.1µF
Figure 24. LVPECL AC- Coupled Termination
LVDS CLOCK DISTRIBUTION
The AD9572 is also available with low voltage differential
signaling (LVDS) outputs. LVDS uses a current mode output
stage with a factory programmed current level. The normal
value (default) for this current is 3.5 mA, which yields a 350 mV
output swing across a 100 Ω resistor. The LVDS outputs meet or
exceed all ANSI/TIA/EIA-644 specifications.
A recommended termination circuit for the LVDS outputs is
shown in Figure 25.
50Ω
50Ω
LVDS
100Ω
07498-032
LVDS
Figure 25. LVDS Output Termination
See the AN-586 Application Note on the Analog Devices
website at www.analog.com for more information about LVDS.
REFERENCE INPUT
By default, the crystal oscillator is enabled and used as the
reference source, which requires the connection of an external
25 MHz crystal cut to resonate in fundamental mode. The total
load capacitance presented to the oscillator should sum to 14 pF.
In the example shown in Figure 26, parasitic trace capacitance
of 1.5 pF, and an AD9572 input pin capacitance of 1.5 pF are
assumed, with the series combination of the two 22 pF
capacitances providing a further 11 pF. The REFSEL pin is
pulled high internally by about 30 k to support default
operation.
07498-033
XTAL
OSC
TO PLLs
REFCLK
REFSEL
22pF
22pF
Figure 26. Reference Input section
When REFSEL is tied low, the crystal oscillator is powered down,
and the REFCLK pin must provide a good quality 25 MHz
reference clock instead. This single-ended input can be driven
by either a dc-coupled LVCMOS level signal or an ac-coupled
