AD9751_15 Datasheet, PDF (11/28 Pages) Analog Devices – 10-Bit, 300 MSPS

AD9751
OPTIONAL
EXTERNAL
REFERENCE
BUFFER
ADDITIONAL
EXTERNAL
LOAD
AD9751
REFERENCE
SECTION
AVDD
1.2V REF
0.1␮F
REFIO
FSADJ
CURRENT
SOURCE
ARRAY
I_REF 2k⍀
Figure 4. Internal Reference Configuration
AVDD
EXTERNAL
REFERENCE
I_REF
AD9751
REFERENCE
SECTION
AVDD
1.2V REF
REFIO
FSADJ
CURRENT
SOURCE
ARRAY
2k⍀
PORT 1
DATA IN
PORT 2
t_S
t_H
DATA X
DATA Y
CLK
I_OUTA OR I_OUTB
t _LPW
t _PD
DATA X
DATA Y
1/2 CYCLE + t_PD
7a. DAC Input Timing Requirements with
PLL Active, Single Clock Cycle
PORT 1
DATA IN
PORT 2
DATA W
DATA X
DATA Y
DATA Z
Figure 5. External Reference Configuration
PLL CLOCK MULTIPLIER OPERATION
The Phase-Locked Loop (PLL) is intrinsic to the operation of the
AD9751 in that it produces the necessary internally synchronized
2× clock for the edge-triggered latches, multiplexer, and DAC.
With PLLVDD connected to its supply voltage, the AD9751 is
in PLL active mode. Figure 6 shows a functional block diagram
of the AD9751 clock control circuitry with PLL active. The
circuitry consists of a phase detector, charge pump, voltage
controlled oscillator (VCO), input data rate range control, clock
logic circuitry, and control input/outputs. The ÷ 2 logic in the
feedback loop allows the PLL to generate the 2× clock needed
for the DAC output latch.
Figure 7 defines the input and output timing for the AD9751
with the PLL active. CLK in Figure 7 represents the clock that
is generated external to the AD9751. The input data at both
Ports 1 and 2 is latched on the same CLK rising edge. CLK
may be applied as a single-ended signal by tying CLK– to
midsupply and applying CLK to CLK+, or as a differential
signal applied to CLK+ and CLK–.
RESET has no purpose when using the internal PLL and
should be grounded. When the AD9751 is in PLL active
mode, PLLLOCK is the output of the internal phase detector.
When locked, the lock output in this mode is Logic 1.
CLKVDD
(3.0V TO 3.6V) PLLLOCK
392⍀ ^1.0␮F
LPF PLLVDD
3.0V TO
3.6V
CLK+
CLK–
DIFFERENTIAL
TO
SINGLE-ENDED
AMP
PHASE
DETECTOR
TO INPUT
LATCHES
AD9751
CHARGE
PUMP
VCO
RANGE
CONTROL
(،1, 2, 4, 8)
،2
TO DAC
LATCH
CLKCOM
DIV0
DIV1
Figure 6. Clock Circuitry with PLL Active
CLK
I_OUTA OR I_OUTB
XXX
DATA W DATA X
DATA Y DATA Z
Figure 7b. DAC Input Timing Requirements with
PLL Active, Multiple Clock Cycles
Typically, the VCO can generate outputs of 100 MHz to 400 MHz.
The range control is used to keep the VCO operating within its
designed range while allowing input clocks as low as 6.25 MHz.
With the PLL active, logic levels at DIV0 and DIV1 determine
the divide (prescaler) ratio of the range controller. Table I gives
the frequency range of the input clock for the different states of
DIV0 and DIV1.
Table I. CLK Rates for DIV0, DIV1 Levels with PLL Active
CLK Frequency
50 MHz–150 MHz
25 MHz–100 MHz
12.5 MHz–50 MHz
6.25 MHz–25 MHz
DIV1
0
0
1
1
DIV0
0
1
0
1
Range Controller
÷1
÷2
÷4
÷8
A 392 Ω resistor and 1.0 µF capacitor connected in series from
LPF to PLLVDD are required to optimize the phase noise versus
settling/acquisition time characteristics of the PLL. To obtain
optimum noise and distortion performance, PLLVDD should
be set to a voltage level similar to DVDD and CLKVDD.
In general, the best phase noise performance for any PLL range
control setting is achieved with the VCO operating near its
maximum output frequency of 400 MHz.
As stated earlier, applications requiring input data rates below
6.25 MSPS must disable the PLL clock multiplier and provide an
external 2× reference clock. At higher data rates however, applica-
tions already containing a low phase noise (i.e., jitter) reference
clock that is twice the input data rate should consider disabling the
PLL clock multiplier to achieve the best SNR performance from the
AD9751. Note that the SFDR performance of the AD9751 remains
unaffected with or without the PLL clock multiplier enabled.
REV. C
–11–