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參數(shù)資料
| 型號： | AD9517-2A/PCBZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 75/80頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVALUATION FOR AD9517-2A |
| 設(shè)計(jì)資源： | AD9517 Eval Brd Schematics AD9517 Gerber Files AD9517-2 BOM |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 主要目的： | 計(jì)時(shí)，時(shí)鐘發(fā)生器 |
| 嵌入式： | 否 |
| 已用 IC / 零件： | AD9517-2A |
| 主要屬性： | 2 輸入，12 輸出，2.2GHz VCO |
| 次要屬性： | CMOS，LVPECL 和 LVDS 兼容 |
| 已供物品： | 板 |
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Data Sheet
AD9517-2
Rev. E | Page 77 of 80
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
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 .
The resistor network is designed to match the transmission line
impedance (50 ) and the switching threshold (VS 1.3 V).
VS_LVPECL
LVPECL
50
SINGLE-ENDED
(NOT COUPLED)
VS
VS_DRV
LVPECL
127
83
06426-
145
Figure 71. DC-Coupled 3.3 V LVPECL Far-End Thevenin Termination
VS_LVPECL
LVPECL
Z0 = 50
VS = 3.3V
LVPECL
50
Z0 = 50
06426-
147
Figure 72. DC-Coupled 3.3 V LVPECL Y-Termination
VS_LVPECL
LVPECL
100 DIFFERENTIAL
(COUPLED)
TRANSMISSION LINE
VS
LVPECL
100
0.1nF
200
06426-
146
Figure 73. AC-Coupled LVPECL with Parallel Transmission Line
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
termination resistor that is connected to ground should be
changed to 19 .
Thevenin-equivalent termination uses a resistor network to
provide 50 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 and the pull-up resistor is 250 .
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 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.
VS
LVDS
100
DIFFERENTIAL (COUPLED)
VS
LVDS
100
06426-
047
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.
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