AD9520-4 Analog Devices, Inc., AD9520-4 Datasheet - Page 36
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AD9520-4
Manufacturer Part Number
AD9520-4
Description
12 Lvpecl/24 Cmos Output Clock Generator With Integrated 1.6 Ghz Vco
Manufacturer
Analog Devices, Inc.
Datasheet
1.AD9520-4.pdf
(84 pages)
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AD9520-4
In differential mode, the reference input pins are internally self-
biased so that they can be ac-coupled via capacitors. It is possible to
dc couple to these inputs. If the differential REFIN is driven by
a single-ended signal, the unused side ( REFIN ) should be
decoupled via a suitable capacitor to a quiet ground.
shows the equivalent circuit of REFIN.
Crystal mode is nearly identical to differential mode. The user
enables a maintaining amplifier by setting the Enable XTAL
OSC bit, and putting a series resonant, AT fundamental cut
crystal across the REFIN pins.
Reference Switchover
The AD9520 supports dual single-ended CMOS inputs, as well
as a single differential reference input. In the dual single-ended
reference mode, the AD9520 supports automatic and manual
PLL reference clock switching between REF1 (on Pin REFIN)
and REF2 (on Pin REFIN ). This feature supports networking
and other applications that require redundant references.
The AD9520 features an optional dc offset option in single-
ended mode. This option is designed to eliminate the risk of the
reference inputs chattering when they are ac-coupled and the
reference clock disappears. When using the reference switchover,
the single-ended reference inputs should be dc-coupled CMOS
levels (with the AD9520 dc offset feature disabled). Alternatively,
the inputs can be ac-coupled and dc offset feature enabled. The
user should keep in mind, however, that the minimum input
amplitude for the reference inputs is greater when the dc offset
is turned on.
REFIN
REFIN
REF1
REF2
Figure 40. REFIN Equivalent Circuit for NonXTAL Mode
10kΩ
10kΩ
85kΩ
85kΩ
12kΩ
10kΩ
VS
VS
150Ω
150Ω
Figure 40
VS
Rev. 0 | Page 36 of 84
There are several configurable modes of reference switchover.
The switchover can be performed manually or automatically.
The manual switchover is done either through a register setting
(0x01C) or by using the REF_SEL pin. The automatic switchover
occurs when REF1 disappears. There is also a switchover deglitch
feature, which ensures that the PLL does not receive rising edges
that are far out of alignment with the newly selected reference.
There are two reference automatic switchover modes (0x01C):
• Prefer REF1. Switch from REF1 to REF2 when REF1 disappears.
• Stay on REF2. Automatically switch to REF2 if REF1 disappears,
In automatic mode, REF1 is monitored by REF2. If REF1
disappears (two consecutive falling edges of REF2 without an
edge transition on REF1), REF1 is considered missing. On the
next subsequent rising edge of REF2, REF2 is used as the reference
clock to the PLL. If 0x01C[3] = 0b (default), when REF1 returns
(four rising edges of REF1 without two falling edges of REF2
between the REF1 edges), the PLL reference switches back to
REF1. If 0x01C[3] = 1b, the user can control when to switch
back to REF1. This is done by programming the part to manual
reference select mode (0x01C[4] = 0b) and by ensuring that the
registers and/or REF_SEL pin are set to select the desired
reference. Automatic mode can be reenabled once REF1 is
reselected.
Manual switchover requires the presence of a clock on the reference
input that is being switched to or that the deglitching feature be
disabled (0x01C[7]).
Reference Divider R
The reference inputs are routed to the reference divider, R. R (a
14-bit counter) can be set to any value from 0 to 16,383 by writing
to 0x011 and 0x012. (Both R = 0 and R = 1 give divide-by-1.) The
output of the R divider goes to one of the PFD inputs to be
compared with the VCO frequency divided by the N divider.
The frequency applied to the PFD must not exceed the maximum
allowable frequency, which depends on the antibacklash pulse
setting (see Table 2).
The R divider has its own reset. R divider can be reset using the
shared reset bit of the R, A, and B counters. It can also be reset
by a SYNC operation.
VCXO/VCO Feedback Divider N: P, A, B, R
The N divider is a combination of a prescaler (P) and two counters,
A and B. The total divider value is
where P can be 2, 4, 8, 16, or 32.
Return to REF1 from REF2 when REF1 returns.
but do not switch back to REF1 if it reappears. The reference
can be set back to REF1 manually at an appropriate time.
N = (P × B) + A