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| 型号： | AD9549A/PCBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 14/76页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION FOR AD9549A |
| 设计资源： | AD9549 Schematics AD9549 Gerber Files |
| 标准包装： | 1 |
| 主要目的： | 计时，时钟发生器 |
| 嵌入式： | 否 |
| 已用 IC / 零件： | AD9549A |
| 主要属性： | 8 kHz ~ 750 MHz 输入 |
| 次要属性： | 差分输出 |
| 已供物品： | 板 |
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AD9549
Rev. D | Page 21 of 76
In holdover mode, the AD9549 uses past tuning words when
the loop is closed to determine its output frequency. Therefore,
the loop must be successfully closed for holdover mode to work.
Switching in and out of holdover mode can be either automatic
or manual, depending on register settings.
Typically, the AD9549 operates in closed-loop mode. In closed-
loop mode, the FTW values come from the output of the digital
loop filter and vary with time. The DDS frequency is steered in
a manner similar to a conventional VCO-based PLL.
Note that in closed-loop mode, the DDS phase offset capability
is inoperative.
DAC Output
The output of the digital core of the DDS is a time series of
numbers representing a sinusoidal waveform. This series is
translated to an analog signal by means of a digital-to-analog
converter (DAC).
The DAC outputs its signal to two pins driven by a balanced
current source architecture (see the DAC output diagram in
Figure 26). The peak output current derives from the combi-
nation of two factors. The first is a reference current (IDAC_REF)
established at the DAC_RSET pin, and the second is a scale
factor programmed into the I/O register map.
06744-
026
SWITCH
CONTROL
CODE
IFS/2
AVDD3
AVSS
CURRENT
SWITCH
ARRAY
CURRENT
SWITCH
ARRAY
IOUT
IOUTB
50
50
IFS/2 + ICODE
IFS/2 – ICODE
IFS
49
50
51
52
Figure 26. DAC Output Pins
The value of IDAC_REF is set by connecting a resistor (RDAC_REF)
between the DAC_RSET pin and ground. The DAC_RSET pin
is internally connected to a virtual voltage reference of 1.2 V
nominal, so the reference current can be calculated by
REF
DAC
REF
DAC
R
I
_
2
.
1
=
Note that the recommended value of IDAC_REF is 120 μA, which
leads to a recommended value for RDAC_REF of 10 kΩ.
The scale factor consists of a 10-bit binary number (FSC)
programmed into the DAC full-scale current register (Address
0x040B and Address 0x040C) in the I/O register map. The full-
scale DAC output current (IDAC_FS) is given by
+
=
1024
192
72
_
FSC
I
REF
DAC
FS
DAC
Using the recommended value of RDAC_REF, the full-scale DAC
output current can be set with 10-bit granularity over a range of
approximately 8.6 mA to 31.7 mA. The default value is 20 mA.
PHASE DETECTOR
Coarse Phase Detector
The coarse phase detector uses the DAC sample rate (fS) to
determine the edge timing deviation between the REF signal
and the feedback signal generated by the DDS. Hence, fS sets
the timing resolution of the coarse phase detector. At the
recommended rate of fS = 1 GHz, the coarse phase detector
spans a range of over 131 μs (sufficient to accommodate REF
signal frequencies as low as 8 kHz).
The phase gain of the coarse phase detector is controlled via the
I/O registers by means of two numeric entries. The first is a
3-bit, power-of-2 scale factor, PDS. The second is a 6-bit linear
scale factor, PDG.
(
)
PDG
f
R
PhaseGain
PDS
R
S
CPD
6
2
+
=
Fine Phase Detector
The fine phase detector operates on a divided down version
of fS as its sampling time base. The sample rate of the fine phase
detector is set using a 4-bit word (PFD_Div) in the I/O register
map (Register 0x0023) and is given by
)
_
(
4
Div
PFD
f
Rate
Sample
Detector
Phase
Fine
S
=
The default value of PFD_Div is 5, so for fS = 1 GHz, the default
sample rate of the fine phase detector is 50 MHz. The upper
bound on the maximum allowable input frequency to the phase
detector (fPFD[MAX]) is 49% of the sample rate, or
)
_
(
8
]
[
Div
PFD
f
S
MAX
PFD
=
Therefore, fPFD[MAX] is 25 MHz in the preceding example.
The fine phase detector uses a proprietary technique to
determine the phase deviation between the REF signal and
feedback signal.
The phase gain of the fine phase detector is controlled by
an 8-bit scale factor (FPFD_Gain) in the I/O register map
(Register 0x0404). The nominal (default) value of FPFD_Gain
is 200 and establishes the phase gain as
R
FP
f
Gain
FPFD
R
D
PhaseGain
)
_
)(
10
2
(
7
10 ×
=
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