AD9517-4_15 Datasheet, PDF (77/80 Pages) Analog Devices – 12-Output Clock Generator with Integrated 1.6 GHz VCO

Data Sheet
LVPECL CLOCK DISTRIBUTION
The LVPECL outputs of the AD9517 provide the lowest jitter
clock signals that are available from the AD9517. The LVPECL
outputs (because they are open emitter) require a dc termination
to bias the output transistors. The simplified equivalent circuit in
Figure 59 shows the LVPECL output stage.
In most applications, an LVPECL far-end Thevenin termination
(see Figure 71) or Y-termination (see Figure 72) is recommended.
In each case, the VS of the receiving buffer should match the
VS_LVPECL voltage. If it does not, ac coupling is recommended (see
Figure 73). In the case of Figure 73, pull-down resistors of <150 Ω
are not recommended when VS_LVEPCL = 3.3 V; if used, damage to
the LVPECL drivers may result. The minimum recommended
pull-down resistor size for VS_LVPECL = 2.5 V is 100 Ω.
The resistor network is designed to match the transmission line
impedance (50 Ω) and the switching threshold (VS − 1.3 V).
V_{S_DRV}
V_{S_LVPECL}
50Ω
127Ω
V_S
127Ω
LVPECL
SINGLE-ENDED
(NOT COUPLED)
LVPECL
50Ω
83Ω
83Ω
Figure 71. DC-Coupled 3.3 V LVPECL Far-End Thevenin Termination
V_{S_LVPECL}
LVPECL
Z₀ = 50Ω
Z₀ = 50Ω
V_S = 3.3V
50Ω ^50Ω
LVPECL
50Ω
Figure 72. DC-Coupled 3.3 V LVPECL Y-Termination
V_{S_LVPECL}
LVPECL
200Ω
0.1nF
100Ω DIFFERENTIAL
(COUPLED)
100Ω
0.1nF TRANSMISSION LINE
200Ω
V_S
LVPECL
Figure 73. AC-Coupled LVPECL with Parallel Transmission Line
AD9517-4
LVPECL Y-termination is an elegant termination scheme that
uses the fewest components and offers both odd- and even-mode
impedance matching. Even-mode impedance matching is an
important consideration for closely coupled transmission lines
at high frequencies. Its main drawback is that it offers limited
flexibility for varying the drive strength of the emitter-follower
LVPECL driver. This can be an important consideration when
driving long trace lengths but is usually not an issue. In the case
shown in Figure 72, where VS_LVPECL = 2.5 V, the 50 Ω
termination resistor that is connected to ground should be
changed to 19 Ω.
Thevenin-equivalent termination uses a resistor network to
provide 50 Ω termination to a dc voltage that is below VOL of
the LVPECL driver. In this case, VS_LVPECL on the AD9517
should equal VS of the receiving buffer. Although the resistor
combination shown in Figure 72 results in a dc bias point of
VS_LVPECL − 2 V, the actual common-mode voltage is
VS_LVPECL − 1.3 V because additional current flows from the
AD9517 LVPECL driver through the pull-down resistor.
The circuit is identical when VS_LVPECL = 2.5 V, except that the
pull-down resistor is 62.5 Ω and the pull-up resistor is 250 Ω.
LVDS CLOCK DISTRIBUTION
The AD9517 provides four clock outputs (OUT4 to OUT7) that
are selectable as either CMOS or LVDS level outputs. LVDS is a
differential output option that uses a current mode output stage.
The nominal current is 3.5 mA, which yields 350 mV output swing
across a 100 Ω resistor. An output current of 7 mA is also available
in cases where a larger output swing is required. The LVDS
output meets or exceeds all ANSI/TIA/EIA-644 specifications.
A recommended termination circuit for the LVDS outputs is
shown in Figure 74.
V_S
V_S
LVDS
100Ω
DIFFERENTIAL (COUPLED)
100Ω
LVDS
Figure 74. LVDS Output Termination
See the AN-586 Application Note, LVDS Data Outputs for High-
Speed Analog-to-Digital Converters for more information on LVDS.
Rev. E | Page 77 of 80