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參數(shù)資料
| 型號： | AD9874ABSTZ |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 通信及網(wǎng)絡(luò) |
| 英文描述： | SPECIALTY TELECOM CIRCUIT, PQFP48 |
| 封裝： | PLASTIC, MS-026BBC, LQFP-48 |
| 文件頁數(shù)： | 18/40頁 |
| 文件大?。?/td> | 1682K |
| 代理商： | AD9874ABSTZ |
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REV. A
AD9874
–25–
The signal transfer function of the AD9874 possesses inherent
antialias filtering by virtue of the continuous-time portions of
the loop filter in the band-pass
-
modulator. Figure 13b
illustrates this property by plotting the nominal signal transfer
function of the ADC for frequencies up to 2fCLK. The notches
that naturally occur for all frequencies that alias to the fCLK/8
pass band are clearly visible. Even at the widest bandwidth setting,
the notches are deep enough to provide greater than 80 dB of
alias protection. Thus, the wideband IF filtering requirements
preceding the AD9874 will be determined mostly by the mixer’s
image band, which is offset from the desired IF input frequency
by fCLK/4 (i.e., 2 3 fCLK/8) rather than any aliasing associated
with the ADC.
0
–10
–20
–30
–50
dB –40
NORMALIZED FREQUENCY – RELATIVE TO
fOUT
–60
–70
–80
0.5
1.0
1.5
2.0
NOTCH AT ALL ALIAS FREQUENCIES
Figure 13b. Signal Transfer Function of the
Band-Pass
-
Modulator from 0 fCLK to 2 fCLK
Figure 13c shows the nominal signal transfer function magni-
tude for frequencies near the fCLK/8 pass band. The width of the
pass band determines the transfer function droop, but even at
the lowest oversampling ratio (48) where the pass band edges
are at
fCLK/192 (
0.005 fCLK), the gain variation is less than
0.5 dB. Note that the amount of attenuation offered by the
signal transfer function near fCLK/8 should also be considered
when determining the narrow-band IF filtering requirements
preceding the AD9874.
–0.10
0
–5
dB –10
NORMALIZED FREQUENCY – RELATIVE TO
fCLK
–15
–20
–0.05
0
0.05
0.10
Figure 13c. Magnitude of the ADC’s Signal
Transfer Function near fCLK/8
Tuning of the
-
modulator’s two continuous-time resonators
is essential in realizing the ADC’s full dynamic range and must
be performed upon system startup. To facilitate tuning of the
LC tank, a capacitor array is internally connected to the MXOP
and MXON pins. The capacitance of this array is program-
mable from 0 pF to 200 pF
20% and can be programmed
either automatically or manually via the SPI port. The capaci-
tors of the active RC resonator are similarly programmable.
Note that the AD9874 can be placed in and out of its standby
mode without retuning since the tuning codes are stored in the
SPI Registers.
When tuning the LC tank, the sampling clock frequency must
be stable and the LNA/mixer, LO synthesizer, and ADC must
all be placed in standby. Tuning is triggered when the ADC is
taken out of standby if the TUNE_LC bit of Register 0x1C has
been set. This bit will clear when the tuning operation is com-
plete (less than 6 ms). The tuning codes can be read from the
3-bit CAPL1 (0x1D) and the 6-bit CAPL0 (0x1E) registers.
In a similar manner, tuning of the RC resonator is activated if
the TUNE_RC bit of Register 0x1C is set when the ADC is
taken out of standby. This bit will clear when tuning is com-
plete. The tuning code can be read from the CAPR (0x1F)
register. Setting both the TUNE_LC and TUNE_RC bits tunes
the LC tank and the active RC resonator in succession. During
tuning, the ADC is not operational and neither data nor a clock
is available from the SSI port. Table X lists the recommended
sequence of the SPI commands for tuning the ADC, and Table XI
lists all of the SPI registers associated with band-pass -
ADC.
Table X. Tuning Sequence
Address Value
Comments
0x00
0x45
LO synthesizer, LNA/mixer, and ADC are
placed in standby.
*
0x1C
0x03
Set TUNE_LC and TUNE_RC. Wait for
CLK to stabilize if CLK synthesizer used.
0x00
0x44
Take the ADC out of standby. Wait for
0x1C to clear (<6 ms). LNA/mixer can now
be taken out of standby.
*If external CLK VCO or source used, the CLK oscillator must also be disabled.
Table XI. SPI Registers Associated with Band-Pass
-
ADC
Address
Bit
Default
(Hex)
Breakdown
Width
Value
Name
0x00
(7:0)
8
0xFF
STBY
0x1C
(1)
1
0
TUNE_LC
(0)
1
0
TUNE_RC
0x1D
(2:0)
3
0
CAPL1(2:0)
0x1E
(5:0)
6
0x00
CAPL1(5:0)
0x1F
(7:0)
8
0x00
CAPR
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| AD9874BST | 制造商:Rochester Electronics LLC 功能描述:- Tape and Reel |
| AD9874EB | 制造商:AD 制造商全稱:Analog Devices 功能描述:IF Digitizing Subsystem |
| AD9874-EB | 制造商:Analog Devices 功能描述: |
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